| Message ID | 20241205160925.230119-3-mic@digikod.net (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Script execution control (was O_MAYEXEC) | expand |
On Thu, Dec 05, 2024 at 05:09:19PM +0100, Mickaël Salaün wrote: > The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and > their *_LOCKED counterparts are designed to be set by processes setting > up an execution environment, such as a user session, a container, or a > security sandbox. Unlike other securebits, these ones can be set by > unprivileged processes. Like seccomp filters or Landlock domains, the > securebits are inherited across processes. > > When SECBIT_EXEC_RESTRICT_FILE is set, programs interpreting code should > control executable resources according to execveat(2) + AT_EXECVE_CHECK > (see previous commit). > > When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should deny > execution of user interactive commands (which excludes executable > regular files). > > Being able to configure each of these securebits enables system > administrators or owner of image containers to gradually validate the > related changes and to identify potential issues (e.g. with interpreter > or audit logs). > > It should be noted that unlike other security bits, the > SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE bits are > dedicated to user space willing to restrict itself. Because of that, > they only make sense in the context of a trusted environment (e.g. > sandbox, container, user session, full system) where the process > changing its behavior (according to these bits) and all its parent > processes are trusted. Otherwise, any parent process could just execute > its own malicious code (interpreting a script or not), or even enforce a > seccomp filter to mask these bits. > > Such a secure environment can be achieved with an appropriate access > control (e.g. mount's noexec option, file access rights, LSM policy) and > an enlighten ld.so checking that libraries are allowed for execution > e.g., to protect against illegitimate use of LD_PRELOAD. > > Ptrace restrictions according to these securebits would not make sense > because of the processes' trust assumption. > > Scripts may need some changes to deal with untrusted data (e.g. stdin, > environment variables), but that is outside the scope of the kernel. > > See chromeOS's documentation about script execution control and the > related threat model: > https://www.chromium.org/chromium-os/developer-library/guides/security/noexec-shell-scripts/ > > Cc: Al Viro <viro@zeniv.linux.org.uk> > Cc: Andy Lutomirski <luto@amacapital.net> > Cc: Christian Brauner <brauner@kernel.org> > Cc: Kees Cook <keescook@chromium.org> > Cc: Paul Moore <paul@paul-moore.com> > Reviewed-by: Serge Hallyn <serge@hallyn.com> > Signed-off-by: Mickaël Salaün <mic@digikod.net> > Link: https://lore.kernel.org/r/20241205160925.230119-3-mic@digikod.net > --- > > Changes since v21: > * Extend user documentation with exception regarding tailored execution > environments (e.g. chromeOS's libc) as discussed with Jeff. > > Changes since v20: > * Move UAPI documentation to a dedicated RST file and format it. > > Changes since v19: > * Replace SECBIT_SHOULD_EXEC_CHECK and SECBIT_SHOULD_EXEC_RESTRICT with > SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE: > https://lore.kernel.org/all/20240710.eiKohpa4Phai@digikod.net/ > * Remove the ptrace restrictions, suggested by Andy. > * Improve documentation according to the discussion with Jeff. > > New design since v18: > https://lore.kernel.org/r/20220104155024.48023-3-mic@digikod.net > --- > Documentation/userspace-api/check_exec.rst | 107 +++++++++++++++++++++ > include/uapi/linux/securebits.h | 24 ++++- > security/commoncap.c | 29 ++++-- > 3 files changed, 153 insertions(+), 7 deletions(-) > > diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst > index 393dd7ca19c4..05dfe3b56f71 100644 > --- a/Documentation/userspace-api/check_exec.rst > +++ b/Documentation/userspace-api/check_exec.rst > @@ -5,6 +5,31 @@ > Executability check > =================== > > +The ``AT_EXECVE_CHECK`` :manpage:`execveat(2)` flag, and the > +``SECBIT_EXEC_RESTRICT_FILE`` and ``SECBIT_EXEC_DENY_INTERACTIVE`` securebits > +are intended for script interpreters and dynamic linkers to enforce a > +consistent execution security policy handled by the kernel. See the > +`samples/check-exec/inc.c`_ example. > + > +Whether an interpreter should check these securebits or not depends on the > +security risk of running malicious scripts with respect to the execution > +environment, and whether the kernel can check if a script is trustworthy or > +not. For instance, Python scripts running on a server can use arbitrary > +syscalls and access arbitrary files. Such interpreters should then be > +enlighten to use these securebits and let users define their security policy. > +However, a JavaScript engine running in a web browser should already be > +sandboxed and then should not be able to harm the user's environment. > + > +Script interpreters or dynamic linkers built for tailored execution environments > +(e.g. hardened Linux distributions or hermetic container images) could use > +``AT_EXECVE_CHECK`` without checking the related securebits if backward > +compatibility is handled by something else (e.g. atomic update ensuring that > +all legitimate libraries are allowed to be executed). It is then recommended > +for script interpreters and dynamic linkers to check the securebits at run time > +by default, but also to provide the ability for custom builds to behave like if > +``SECBIT_EXEC_RESTRICT_FILE`` or ``SECBIT_EXEC_DENY_INTERACTIVE`` were always > +set to 1 (i.e. always enforce restrictions). Jeff, does this work for you? I'll update the IMA patch with a last version but otherwise it should be good: https://lore.kernel.org/all/20241210.Wie6ion7Aich@digikod.net/ > + > AT_EXECVE_CHECK > =============== > > @@ -35,3 +60,85 @@ be executable, which also requires integrity guarantees. > To avoid race conditions leading to time-of-check to time-of-use issues, > ``AT_EXECVE_CHECK`` should be used with ``AT_EMPTY_PATH`` to check against a > file descriptor instead of a path. > + > +SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE > +========================================================== > + > +When ``SECBIT_EXEC_RESTRICT_FILE`` is set, a process should only interpret or > +execute a file if a call to :manpage:`execveat(2)` with the related file > +descriptor and the ``AT_EXECVE_CHECK`` flag succeed. > + > +This secure bit may be set by user session managers, service managers, > +container runtimes, sandboxer tools... Except for test environments, the > +related ``SECBIT_EXEC_RESTRICT_FILE_LOCKED`` bit should also be set. > + > +Programs should only enforce consistent restrictions according to the > +securebits but without relying on any other user-controlled configuration. > +Indeed, the use case for these securebits is to only trust executable code > +vetted by the system configuration (through the kernel), so we should be > +careful to not let untrusted users control this configuration. > + > +However, script interpreters may still use user configuration such as > +environment variables as long as it is not a way to disable the securebits > +checks. For instance, the ``PATH`` and ``LD_PRELOAD`` variables can be set by > +a script's caller. Changing these variables may lead to unintended code > +executions, but only from vetted executable programs, which is OK. For this to > +make sense, the system should provide a consistent security policy to avoid > +arbitrary code execution e.g., by enforcing a write xor execute policy. > + > +When ``SECBIT_EXEC_DENY_INTERACTIVE`` is set, a process should never interpret > +interactive user commands (e.g. scripts). However, if such commands are passed > +through a file descriptor (e.g. stdin), its content should be interpreted if a > +call to :manpage:`execveat(2)` with the related file descriptor and the > +``AT_EXECVE_CHECK`` flag succeed. > + > +For instance, script interpreters called with a script snippet as argument > +should always deny such execution if ``SECBIT_EXEC_DENY_INTERACTIVE`` is set. > + > +This secure bit may be set by user session managers, service managers, > +container runtimes, sandboxer tools... Except for test environments, the > +related ``SECBIT_EXEC_DENY_INTERACTIVE_LOCKED`` bit should also be set. > + > +Here is the expected behavior for a script interpreter according to combination > +of any exec securebits: > + > +1. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` > + > + Always interpret scripts, and allow arbitrary user commands (default). > + > + No threat, everyone and everything is trusted, but we can get ahead of > + potential issues thanks to the call to :manpage:`execveat(2)` with > + ``AT_EXECVE_CHECK`` which should always be performed but ignored by the > + script interpreter. Indeed, this check is still important to enable systems > + administrators to verify requests (e.g. with audit) and prepare for > + migration to a secure mode. > + > +2. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` > + > + Deny script interpretation if they are not executable, but allow > + arbitrary user commands. > + > + The threat is (potential) malicious scripts run by trusted (and not fooled) > + users. That can protect against unintended script executions (e.g. ``sh > + /tmp/*.sh``). This makes sense for (semi-restricted) user sessions. > + > +3. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` > + > + Always interpret scripts, but deny arbitrary user commands. > + > + This use case may be useful for secure services (i.e. without interactive > + user session) where scripts' integrity is verified (e.g. with IMA/EVM or > + dm-verity/IPE) but where access rights might not be ready yet. Indeed, > + arbitrary interactive commands would be much more difficult to check. > + > +4. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` > + > + Deny script interpretation if they are not executable, and also deny > + any arbitrary user commands. > + > + The threat is malicious scripts run by untrusted users (but trusted code). > + This makes sense for system services that may only execute trusted scripts. > + > +.. Links > +.. _samples/check-exec/inc.c: > + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/check-exec/inc.c
On Tue, Dec 10, 2024 at 8:48 AM Mickaël Salaün <mic@digikod.net> wrote: > > On Thu, Dec 05, 2024 at 05:09:19PM +0100, Mickaël Salaün wrote: > > The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and > > their *_LOCKED counterparts are designed to be set by processes setting > > up an execution environment, such as a user session, a container, or a > > security sandbox. Unlike other securebits, these ones can be set by > > unprivileged processes. Like seccomp filters or Landlock domains, the > > securebits are inherited across processes. > > > > When SECBIT_EXEC_RESTRICT_FILE is set, programs interpreting code should > > control executable resources according to execveat(2) + AT_EXECVE_CHECK > > (see previous commit). > > > > When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should deny > > execution of user interactive commands (which excludes executable > > regular files). > > > > Being able to configure each of these securebits enables system > > administrators or owner of image containers to gradually validate the > > related changes and to identify potential issues (e.g. with interpreter > > or audit logs). > > > > It should be noted that unlike other security bits, the > > SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE bits are > > dedicated to user space willing to restrict itself. Because of that, > > they only make sense in the context of a trusted environment (e.g. > > sandbox, container, user session, full system) where the process > > changing its behavior (according to these bits) and all its parent > > processes are trusted. Otherwise, any parent process could just execute > > its own malicious code (interpreting a script or not), or even enforce a > > seccomp filter to mask these bits. > > > > Such a secure environment can be achieved with an appropriate access > > control (e.g. mount's noexec option, file access rights, LSM policy) and > > an enlighten ld.so checking that libraries are allowed for execution > > e.g., to protect against illegitimate use of LD_PRELOAD. > > > > Ptrace restrictions according to these securebits would not make sense > > because of the processes' trust assumption. > > > > Scripts may need some changes to deal with untrusted data (e.g. stdin, > > environment variables), but that is outside the scope of the kernel. > > > > See chromeOS's documentation about script execution control and the > > related threat model: > > https://www.chromium.org/chromium-os/developer-library/guides/security/noexec-shell-scripts/ > > > > Cc: Al Viro <viro@zeniv.linux.org.uk> > > Cc: Andy Lutomirski <luto@amacapital.net> > > Cc: Christian Brauner <brauner@kernel.org> > > Cc: Kees Cook <keescook@chromium.org> > > Cc: Paul Moore <paul@paul-moore.com> > > Reviewed-by: Serge Hallyn <serge@hallyn.com> > > Signed-off-by: Mickaël Salaün <mic@digikod.net> > > Link: https://lore.kernel.org/r/20241205160925.230119-3-mic@digikod.net > > --- > > > > Changes since v21: > > * Extend user documentation with exception regarding tailored execution > > environments (e.g. chromeOS's libc) as discussed with Jeff. > > > > Changes since v20: > > * Move UAPI documentation to a dedicated RST file and format it. > > > > Changes since v19: > > * Replace SECBIT_SHOULD_EXEC_CHECK and SECBIT_SHOULD_EXEC_RESTRICT with > > SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE: > > https://lore.kernel.org/all/20240710.eiKohpa4Phai@digikod.net/ > > * Remove the ptrace restrictions, suggested by Andy. > > * Improve documentation according to the discussion with Jeff. > > > > New design since v18: > > https://lore.kernel.org/r/20220104155024.48023-3-mic@digikod.net > > --- > > Documentation/userspace-api/check_exec.rst | 107 +++++++++++++++++++++ > > include/uapi/linux/securebits.h | 24 ++++- > > security/commoncap.c | 29 ++++-- > > 3 files changed, 153 insertions(+), 7 deletions(-) > > > > diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst > > index 393dd7ca19c4..05dfe3b56f71 100644 > > --- a/Documentation/userspace-api/check_exec.rst > > +++ b/Documentation/userspace-api/check_exec.rst > > @@ -5,6 +5,31 @@ > > Executability check > > =================== > > > > +The ``AT_EXECVE_CHECK`` :manpage:`execveat(2)` flag, and the > > +``SECBIT_EXEC_RESTRICT_FILE`` and ``SECBIT_EXEC_DENY_INTERACTIVE`` securebits > > +are intended for script interpreters and dynamic linkers to enforce a > > +consistent execution security policy handled by the kernel. See the > > +`samples/check-exec/inc.c`_ example. > > + > > +Whether an interpreter should check these securebits or not depends on the > > +security risk of running malicious scripts with respect to the execution > > +environment, and whether the kernel can check if a script is trustworthy or > > +not. For instance, Python scripts running on a server can use arbitrary > > +syscalls and access arbitrary files. Such interpreters should then be > > +enlighten to use these securebits and let users define their security policy. > > +However, a JavaScript engine running in a web browser should already be > > +sandboxed and then should not be able to harm the user's environment. > > + > > +Script interpreters or dynamic linkers built for tailored execution environments > > +(e.g. hardened Linux distributions or hermetic container images) could use > > +``AT_EXECVE_CHECK`` without checking the related securebits if backward > > +compatibility is handled by something else (e.g. atomic update ensuring that > > +all legitimate libraries are allowed to be executed). It is then recommended > > +for script interpreters and dynamic linkers to check the securebits at run time > > +by default, but also to provide the ability for custom builds to behave like if > > +``SECBIT_EXEC_RESTRICT_FILE`` or ``SECBIT_EXEC_DENY_INTERACTIVE`` were always > > +set to 1 (i.e. always enforce restrictions). > > Jeff, does this work for you? > Yes. Thanks for updating this section. > I'll update the IMA patch with a last version but otherwise it should be > good: https://lore.kernel.org/all/20241210.Wie6ion7Aich@digikod.net/ > > > + > > AT_EXECVE_CHECK > > =============== > > > > @@ -35,3 +60,85 @@ be executable, which also requires integrity guarantees. > > To avoid race conditions leading to time-of-check to time-of-use issues, > > ``AT_EXECVE_CHECK`` should be used with ``AT_EMPTY_PATH`` to check against a > > file descriptor instead of a path. > > + > > +SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE > > +========================================================== > > + > > +When ``SECBIT_EXEC_RESTRICT_FILE`` is set, a process should only interpret or > > +execute a file if a call to :manpage:`execveat(2)` with the related file > > +descriptor and the ``AT_EXECVE_CHECK`` flag succeed. > > + > > +This secure bit may be set by user session managers, service managers, > > +container runtimes, sandboxer tools... Except for test environments, the > > +related ``SECBIT_EXEC_RESTRICT_FILE_LOCKED`` bit should also be set. > > + > > +Programs should only enforce consistent restrictions according to the > > +securebits but without relying on any other user-controlled configuration. > > +Indeed, the use case for these securebits is to only trust executable code > > +vetted by the system configuration (through the kernel), so we should be > > +careful to not let untrusted users control this configuration. > > + > > +However, script interpreters may still use user configuration such as > > +environment variables as long as it is not a way to disable the securebits > > +checks. For instance, the ``PATH`` and ``LD_PRELOAD`` variables can be set by > > +a script's caller. Changing these variables may lead to unintended code > > +executions, but only from vetted executable programs, which is OK. For this to > > +make sense, the system should provide a consistent security policy to avoid > > +arbitrary code execution e.g., by enforcing a write xor execute policy. > > + > > +When ``SECBIT_EXEC_DENY_INTERACTIVE`` is set, a process should never interpret > > +interactive user commands (e.g. scripts). However, if such commands are passed > > +through a file descriptor (e.g. stdin), its content should be interpreted if a > > +call to :manpage:`execveat(2)` with the related file descriptor and the > > +``AT_EXECVE_CHECK`` flag succeed. > > + > > +For instance, script interpreters called with a script snippet as argument > > +should always deny such execution if ``SECBIT_EXEC_DENY_INTERACTIVE`` is set. > > + > > +This secure bit may be set by user session managers, service managers, > > +container runtimes, sandboxer tools... Except for test environments, the > > +related ``SECBIT_EXEC_DENY_INTERACTIVE_LOCKED`` bit should also be set. > > + > > +Here is the expected behavior for a script interpreter according to combination > > +of any exec securebits: > > + > > +1. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` > > + > > + Always interpret scripts, and allow arbitrary user commands (default). > > + > > + No threat, everyone and everything is trusted, but we can get ahead of > > + potential issues thanks to the call to :manpage:`execveat(2)` with > > + ``AT_EXECVE_CHECK`` which should always be performed but ignored by the > > + script interpreter. Indeed, this check is still important to enable systems > > + administrators to verify requests (e.g. with audit) and prepare for > > + migration to a secure mode. > > + > > +2. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` > > + > > + Deny script interpretation if they are not executable, but allow > > + arbitrary user commands. > > + > > + The threat is (potential) malicious scripts run by trusted (and not fooled) > > + users. That can protect against unintended script executions (e.g. ``sh > > + /tmp/*.sh``). This makes sense for (semi-restricted) user sessions. > > + > > +3. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` > > + > > + Always interpret scripts, but deny arbitrary user commands. > > + > > + This use case may be useful for secure services (i.e. without interactive > > + user session) where scripts' integrity is verified (e.g. with IMA/EVM or > > + dm-verity/IPE) but where access rights might not be ready yet. Indeed, > > + arbitrary interactive commands would be much more difficult to check. > > + > > +4. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` > > + > > + Deny script interpretation if they are not executable, and also deny > > + any arbitrary user commands. > > + > > + The threat is malicious scripts run by untrusted users (but trusted code). > > + This makes sense for system services that may only execute trusted scripts. > > + > > +.. Links > > +.. _samples/check-exec/inc.c: > > + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/check-exec/inc.c > Reviewed-by: Jeff Xu < jeffxu@chromium.org> Tested-by: Jeff Xu <jeffxu@chromium.org>
diff --git a/Documentation/userspace-api/check_exec.rst b/Documentation/userspace-api/check_exec.rst index 393dd7ca19c4..05dfe3b56f71 100644 --- a/Documentation/userspace-api/check_exec.rst +++ b/Documentation/userspace-api/check_exec.rst @@ -5,6 +5,31 @@ Executability check =================== +The ``AT_EXECVE_CHECK`` :manpage:`execveat(2)` flag, and the +``SECBIT_EXEC_RESTRICT_FILE`` and ``SECBIT_EXEC_DENY_INTERACTIVE`` securebits +are intended for script interpreters and dynamic linkers to enforce a +consistent execution security policy handled by the kernel. See the +`samples/check-exec/inc.c`_ example. + +Whether an interpreter should check these securebits or not depends on the +security risk of running malicious scripts with respect to the execution +environment, and whether the kernel can check if a script is trustworthy or +not. For instance, Python scripts running on a server can use arbitrary +syscalls and access arbitrary files. Such interpreters should then be +enlighten to use these securebits and let users define their security policy. +However, a JavaScript engine running in a web browser should already be +sandboxed and then should not be able to harm the user's environment. + +Script interpreters or dynamic linkers built for tailored execution environments +(e.g. hardened Linux distributions or hermetic container images) could use +``AT_EXECVE_CHECK`` without checking the related securebits if backward +compatibility is handled by something else (e.g. atomic update ensuring that +all legitimate libraries are allowed to be executed). It is then recommended +for script interpreters and dynamic linkers to check the securebits at run time +by default, but also to provide the ability for custom builds to behave like if +``SECBIT_EXEC_RESTRICT_FILE`` or ``SECBIT_EXEC_DENY_INTERACTIVE`` were always +set to 1 (i.e. always enforce restrictions). + AT_EXECVE_CHECK =============== @@ -35,3 +60,85 @@ be executable, which also requires integrity guarantees. To avoid race conditions leading to time-of-check to time-of-use issues, ``AT_EXECVE_CHECK`` should be used with ``AT_EMPTY_PATH`` to check against a file descriptor instead of a path. + +SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE +========================================================== + +When ``SECBIT_EXEC_RESTRICT_FILE`` is set, a process should only interpret or +execute a file if a call to :manpage:`execveat(2)` with the related file +descriptor and the ``AT_EXECVE_CHECK`` flag succeed. + +This secure bit may be set by user session managers, service managers, +container runtimes, sandboxer tools... Except for test environments, the +related ``SECBIT_EXEC_RESTRICT_FILE_LOCKED`` bit should also be set. + +Programs should only enforce consistent restrictions according to the +securebits but without relying on any other user-controlled configuration. +Indeed, the use case for these securebits is to only trust executable code +vetted by the system configuration (through the kernel), so we should be +careful to not let untrusted users control this configuration. + +However, script interpreters may still use user configuration such as +environment variables as long as it is not a way to disable the securebits +checks. For instance, the ``PATH`` and ``LD_PRELOAD`` variables can be set by +a script's caller. Changing these variables may lead to unintended code +executions, but only from vetted executable programs, which is OK. For this to +make sense, the system should provide a consistent security policy to avoid +arbitrary code execution e.g., by enforcing a write xor execute policy. + +When ``SECBIT_EXEC_DENY_INTERACTIVE`` is set, a process should never interpret +interactive user commands (e.g. scripts). However, if such commands are passed +through a file descriptor (e.g. stdin), its content should be interpreted if a +call to :manpage:`execveat(2)` with the related file descriptor and the +``AT_EXECVE_CHECK`` flag succeed. + +For instance, script interpreters called with a script snippet as argument +should always deny such execution if ``SECBIT_EXEC_DENY_INTERACTIVE`` is set. + +This secure bit may be set by user session managers, service managers, +container runtimes, sandboxer tools... Except for test environments, the +related ``SECBIT_EXEC_DENY_INTERACTIVE_LOCKED`` bit should also be set. + +Here is the expected behavior for a script interpreter according to combination +of any exec securebits: + +1. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` + + Always interpret scripts, and allow arbitrary user commands (default). + + No threat, everyone and everything is trusted, but we can get ahead of + potential issues thanks to the call to :manpage:`execveat(2)` with + ``AT_EXECVE_CHECK`` which should always be performed but ignored by the + script interpreter. Indeed, this check is still important to enable systems + administrators to verify requests (e.g. with audit) and prepare for + migration to a secure mode. + +2. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=0`` + + Deny script interpretation if they are not executable, but allow + arbitrary user commands. + + The threat is (potential) malicious scripts run by trusted (and not fooled) + users. That can protect against unintended script executions (e.g. ``sh + /tmp/*.sh``). This makes sense for (semi-restricted) user sessions. + +3. ``SECBIT_EXEC_RESTRICT_FILE=0`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` + + Always interpret scripts, but deny arbitrary user commands. + + This use case may be useful for secure services (i.e. without interactive + user session) where scripts' integrity is verified (e.g. with IMA/EVM or + dm-verity/IPE) but where access rights might not be ready yet. Indeed, + arbitrary interactive commands would be much more difficult to check. + +4. ``SECBIT_EXEC_RESTRICT_FILE=1`` and ``SECBIT_EXEC_DENY_INTERACTIVE=1`` + + Deny script interpretation if they are not executable, and also deny + any arbitrary user commands. + + The threat is malicious scripts run by untrusted users (but trusted code). + This makes sense for system services that may only execute trusted scripts. + +.. Links +.. _samples/check-exec/inc.c: + https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/check-exec/inc.c diff --git a/include/uapi/linux/securebits.h b/include/uapi/linux/securebits.h index d6d98877ff1a..3fba30dbd68b 100644 --- a/include/uapi/linux/securebits.h +++ b/include/uapi/linux/securebits.h @@ -52,10 +52,32 @@ #define SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED \ (issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE_LOCKED)) +/* See Documentation/userspace-api/check_exec.rst */ +#define SECURE_EXEC_RESTRICT_FILE 8 +#define SECURE_EXEC_RESTRICT_FILE_LOCKED 9 /* make bit-8 immutable */ + +#define SECBIT_EXEC_RESTRICT_FILE (issecure_mask(SECURE_EXEC_RESTRICT_FILE)) +#define SECBIT_EXEC_RESTRICT_FILE_LOCKED \ + (issecure_mask(SECURE_EXEC_RESTRICT_FILE_LOCKED)) + +/* See Documentation/userspace-api/check_exec.rst */ +#define SECURE_EXEC_DENY_INTERACTIVE 10 +#define SECURE_EXEC_DENY_INTERACTIVE_LOCKED 11 /* make bit-10 immutable */ + +#define SECBIT_EXEC_DENY_INTERACTIVE \ + (issecure_mask(SECURE_EXEC_DENY_INTERACTIVE)) +#define SECBIT_EXEC_DENY_INTERACTIVE_LOCKED \ + (issecure_mask(SECURE_EXEC_DENY_INTERACTIVE_LOCKED)) + #define SECURE_ALL_BITS (issecure_mask(SECURE_NOROOT) | \ issecure_mask(SECURE_NO_SETUID_FIXUP) | \ issecure_mask(SECURE_KEEP_CAPS) | \ - issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE)) + issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE) | \ + issecure_mask(SECURE_EXEC_RESTRICT_FILE) | \ + issecure_mask(SECURE_EXEC_DENY_INTERACTIVE)) #define SECURE_ALL_LOCKS (SECURE_ALL_BITS << 1) +#define SECURE_ALL_UNPRIVILEGED (issecure_mask(SECURE_EXEC_RESTRICT_FILE) | \ + issecure_mask(SECURE_EXEC_DENY_INTERACTIVE)) + #endif /* _UAPI_LINUX_SECUREBITS_H */ diff --git a/security/commoncap.c b/security/commoncap.c index cefad323a0b1..52ea01acb453 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -1302,21 +1302,38 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3, & (old->securebits ^ arg2)) /*[1]*/ || ((old->securebits & SECURE_ALL_LOCKS & ~arg2)) /*[2]*/ || (arg2 & ~(SECURE_ALL_LOCKS | SECURE_ALL_BITS)) /*[3]*/ - || (cap_capable(current_cred(), - current_cred()->user_ns, - CAP_SETPCAP, - CAP_OPT_NONE) != 0) /*[4]*/ /* * [1] no changing of bits that are locked * [2] no unlocking of locks * [3] no setting of unsupported bits - * [4] doing anything requires privilege (go read about - * the "sendmail capabilities bug") */ ) /* cannot change a locked bit */ return -EPERM; + /* + * Doing anything requires privilege (go read about the + * "sendmail capabilities bug"), except for unprivileged bits. + * Indeed, the SECURE_ALL_UNPRIVILEGED bits are not + * restrictions enforced by the kernel but by user space on + * itself. + */ + if (cap_capable(current_cred(), current_cred()->user_ns, + CAP_SETPCAP, CAP_OPT_NONE) != 0) { + const unsigned long unpriv_and_locks = + SECURE_ALL_UNPRIVILEGED | + SECURE_ALL_UNPRIVILEGED << 1; + const unsigned long changed = old->securebits ^ arg2; + + /* For legacy reason, denies non-change. */ + if (!changed) + return -EPERM; + + /* Denies privileged changes. */ + if (changed & ~unpriv_and_locks) + return -EPERM; + } + new = prepare_creds(); if (!new) return -ENOMEM;