| Message ID | 20220814192603.7387-5-gnoack3000@gmail.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere |
| Headers | show |
| Series | landlock: truncate support | expand |
On 14/08/2022 21:26, Günther Noack wrote: > Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial. > > Adapt the backwards compatibility example and discussion to remove the > truncation flag where needed. > > Point out potential surprising behaviour related to truncate. > > Signed-off-by: Günther Noack <gnoack3000@gmail.com> > --- > Documentation/userspace-api/landlock.rst | 48 +++++++++++++++++++----- > 1 file changed, 38 insertions(+), 10 deletions(-) > > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst > index 6648e59fabe7..3ceb97cbe9d1 100644 > --- a/Documentation/userspace-api/landlock.rst > +++ b/Documentation/userspace-api/landlock.rst > @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights. > LANDLOCK_ACCESS_FS_MAKE_FIFO | > LANDLOCK_ACCESS_FS_MAKE_BLOCK | > LANDLOCK_ACCESS_FS_MAKE_SYM | > - LANDLOCK_ACCESS_FS_REFER, > + LANDLOCK_ACCESS_FS_REFER | > + LANDLOCK_ACCESS_FS_TRUNCATE, > }; > > Because we may not know on which kernel version an application will be > @@ -69,16 +70,26 @@ should try to protect users as much as possible whatever the kernel they are > using. To avoid binary enforcement (i.e. either all security features or > none), we can leverage a dedicated Landlock command to get the current version > of the Landlock ABI and adapt the handled accesses. Let's check if we should > -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported > -starting with the second version of the ABI. > +remove the `LANDLOCK_ACCESS_FS_REFER` and `LANDLOCK_ACCESS_FS_TRUNCATE` access s/and/or/ > +rights, which are only supported starting with the second and third version of > +the ABI. > > .. code-block:: c > > int abi; > > abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); > - if (abi < 2) { > - ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; > + switch (abi) { > + case -1: > + perror("The running kernel does not enable to use Landlock"); > + return 1; > + case 1: > + /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ > + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; > + __attribute__((fallthrough)); > + case 2: > + /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ > + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; > } > > This enables to create an inclusive ruleset that will contain our rules. > @@ -127,8 +138,8 @@ descriptor. > > It may also be required to create rules following the same logic as explained > for the ruleset creation, by filtering access rights according to the Landlock > -ABI version. In this example, this is not required because > -`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule. > +ABI version. In this example, this is not required because all of the requested > +``allowed_access`` rights are already available in ABI 1. > > We now have a ruleset with one rule allowing read access to ``/usr`` while > denying all other handled accesses for the filesystem. The next step is to > @@ -251,6 +262,24 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target > process, a sandboxed process should have a subset of the target process rules, > which means the tracee must be in a sub-domain of the tracer. > > +Truncating files > +---------------- > + > +The operations covered by `LANDLOCK_ACCESS_FS_WRITE_FILE` and > +`LANDLOCK_ACCESS_FS_TRUNCATE` both change the contents of a file and sometimes > +overlap in non-intuitive ways. It is recommended to always specify both of > +these together. > + > +A particularly surprising example is :manpage:`creat(2)`. The name suggests > +that this system call requires the rights to create and write files. However, > +it also requires the truncate right if an existing file under the same name is > +already present. > + > +It should also be noted that truncating files does not necessarily require the I think "necessarily" is superfluous here. > +`LANDLOCK_ACCESS_FS_WRITE_FILE` right. Apart from the obvious > +:manpage:`truncate(2)` system call, this can also be done through > +:manpage:`open(2)` with the flags `O_RDONLY` and `O_TRUNC`. `O_RDONLY | O_TRUNC`. > + > Compatibility > ============= > > @@ -386,9 +415,8 @@ File truncation (ABI < 3) > File truncation could not be denied before the third Landlock ABI, so it is > always allowed when using a kernel that only supports the first or second ABI. > > -Starting with the Landlock ABI version 3, it is now possible to securely > -control truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access > -right. > +Starting with the Landlock ABI version 3, it is now possible to securely control > +truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access right. This is an inconsistent hunk, patching the first patch. Please also move this "File truncation" section below the "File renaming and linking". > > File renaming and linking (ABI 1) > ---------------------------------
On Tue, Aug 16, 2022 at 09:18:33PM +0200, Mickaël Salaün wrote: > On 14/08/2022 21:26, Günther Noack wrote: > > diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst > > index 6648e59fabe7..3ceb97cbe9d1 100644 > > --- a/Documentation/userspace-api/landlock.rst > > +++ b/Documentation/userspace-api/landlock.rst > > Because we may not know on which kernel version an application will be > > @@ -69,16 +70,26 @@ should try to protect users as much as possible whatever the kernel they are > > using. To avoid binary enforcement (i.e. either all security features or > > none), we can leverage a dedicated Landlock command to get the current version > > of the Landlock ABI and adapt the handled accesses. Let's check if we should > > -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported > > -starting with the second version of the ABI. > > +remove the `LANDLOCK_ACCESS_FS_REFER` and `LANDLOCK_ACCESS_FS_TRUNCATE` access > > s/and/or/ Done. > > +Truncating files > > +---------------- > > + > > +The operations covered by `LANDLOCK_ACCESS_FS_WRITE_FILE` and > > +`LANDLOCK_ACCESS_FS_TRUNCATE` both change the contents of a file and sometimes > > +overlap in non-intuitive ways. It is recommended to always specify both of > > +these together. > > + > > +A particularly surprising example is :manpage:`creat(2)`. The name suggests > > +that this system call requires the rights to create and write files. However, > > +it also requires the truncate right if an existing file under the same name is > > +already present. > > + > > +It should also be noted that truncating files does not necessarily require the > > I think "necessarily" is superfluous here. Done. I dropped the "obvious" too. > > > > +`LANDLOCK_ACCESS_FS_WRITE_FILE` right. Apart from the obvious > > +:manpage:`truncate(2)` system call, this can also be done through > > +:manpage:`open(2)` with the flags `O_RDONLY` and `O_TRUNC`. > > `O_RDONLY | O_TRUNC`. Done. > > Compatibility > > ============= > > @@ -386,9 +415,8 @@ File truncation (ABI < 3) > > File truncation could not be denied before the third Landlock ABI, so it is > > always allowed when using a kernel that only supports the first or second ABI. > > -Starting with the Landlock ABI version 3, it is now possible to securely > > -control truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access > > -right. > > +Starting with the Landlock ABI version 3, it is now possible to securely control > > +truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access right. > > This is an inconsistent hunk, patching the first patch. > > Please also move this "File truncation" section below the "File renaming and > linking". Thanks, fixed the ordering of commits and moved the truncation section below "File Renaming and Linking". —Günther --
diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst index 6648e59fabe7..3ceb97cbe9d1 100644 --- a/Documentation/userspace-api/landlock.rst +++ b/Documentation/userspace-api/landlock.rst @@ -60,7 +60,8 @@ the need to be explicit about the denied-by-default access rights. LANDLOCK_ACCESS_FS_MAKE_FIFO | LANDLOCK_ACCESS_FS_MAKE_BLOCK | LANDLOCK_ACCESS_FS_MAKE_SYM | - LANDLOCK_ACCESS_FS_REFER, + LANDLOCK_ACCESS_FS_REFER | + LANDLOCK_ACCESS_FS_TRUNCATE, }; Because we may not know on which kernel version an application will be @@ -69,16 +70,26 @@ should try to protect users as much as possible whatever the kernel they are using. To avoid binary enforcement (i.e. either all security features or none), we can leverage a dedicated Landlock command to get the current version of the Landlock ABI and adapt the handled accesses. Let's check if we should -remove the `LANDLOCK_ACCESS_FS_REFER` access right which is only supported -starting with the second version of the ABI. +remove the `LANDLOCK_ACCESS_FS_REFER` and `LANDLOCK_ACCESS_FS_TRUNCATE` access +rights, which are only supported starting with the second and third version of +the ABI. .. code-block:: c int abi; abi = landlock_create_ruleset(NULL, 0, LANDLOCK_CREATE_RULESET_VERSION); - if (abi < 2) { - ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; + switch (abi) { + case -1: + perror("The running kernel does not enable to use Landlock"); + return 1; + case 1: + /* Removes LANDLOCK_ACCESS_FS_REFER for ABI < 2 */ + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_REFER; + __attribute__((fallthrough)); + case 2: + /* Removes LANDLOCK_ACCESS_FS_TRUNCATE for ABI < 3 */ + ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_TRUNCATE; } This enables to create an inclusive ruleset that will contain our rules. @@ -127,8 +138,8 @@ descriptor. It may also be required to create rules following the same logic as explained for the ruleset creation, by filtering access rights according to the Landlock -ABI version. In this example, this is not required because -`LANDLOCK_ACCESS_FS_REFER` is not allowed by any rule. +ABI version. In this example, this is not required because all of the requested +``allowed_access`` rights are already available in ABI 1. We now have a ruleset with one rule allowing read access to ``/usr`` while denying all other handled accesses for the filesystem. The next step is to @@ -251,6 +262,24 @@ To be allowed to use :manpage:`ptrace(2)` and related syscalls on a target process, a sandboxed process should have a subset of the target process rules, which means the tracee must be in a sub-domain of the tracer. +Truncating files +---------------- + +The operations covered by `LANDLOCK_ACCESS_FS_WRITE_FILE` and +`LANDLOCK_ACCESS_FS_TRUNCATE` both change the contents of a file and sometimes +overlap in non-intuitive ways. It is recommended to always specify both of +these together. + +A particularly surprising example is :manpage:`creat(2)`. The name suggests +that this system call requires the rights to create and write files. However, +it also requires the truncate right if an existing file under the same name is +already present. + +It should also be noted that truncating files does not necessarily require the +`LANDLOCK_ACCESS_FS_WRITE_FILE` right. Apart from the obvious +:manpage:`truncate(2)` system call, this can also be done through +:manpage:`open(2)` with the flags `O_RDONLY` and `O_TRUNC`. + Compatibility ============= @@ -386,9 +415,8 @@ File truncation (ABI < 3) File truncation could not be denied before the third Landlock ABI, so it is always allowed when using a kernel that only supports the first or second ABI. -Starting with the Landlock ABI version 3, it is now possible to securely -control truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access -right. +Starting with the Landlock ABI version 3, it is now possible to securely control +truncation thanks to the new `LANDLOCK_ACCESS_FS_TRUNCATE` access right. File renaming and linking (ABI 1) ---------------------------------
Use the LANDLOCK_ACCESS_FS_TRUNCATE flag in the tutorial. Adapt the backwards compatibility example and discussion to remove the truncation flag where needed. Point out potential surprising behaviour related to truncate. Signed-off-by: Günther Noack <gnoack3000@gmail.com> --- Documentation/userspace-api/landlock.rst | 48 +++++++++++++++++++----- 1 file changed, 38 insertions(+), 10 deletions(-)