@@ -3301,7 +3301,9 @@ S: France
N: Aleksa Sarai
E: cyphar@cyphar.com
W: https://www.cyphar.com/
-D: `pids` cgroup subsystem
+D: /sys/fs/cgroup/pids
+D: openat2(2)
+S: Sydney, Australia
N: Dipankar Sarma
E: dipankar@in.ibm.com
@@ -475,3 +475,4 @@
543 common fspick sys_fspick
544 common pidfd_open sys_pidfd_open
# 545 reserved for clone3
+547 common openat2 sys_openat2
@@ -449,3 +449,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
435 common clone3 sys_clone3
+437 common openat2 sys_openat2
@@ -38,7 +38,7 @@
#define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5)
#define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800)
-#define __NR_compat_syscalls 436
+#define __NR_compat_syscalls 438
#endif
#define __ARCH_WANT_SYS_CLONE
@@ -879,6 +879,8 @@ __SYSCALL(__NR_fspick, sys_fspick)
__SYSCALL(__NR_pidfd_open, sys_pidfd_open)
#define __NR_clone3 435
__SYSCALL(__NR_clone3, sys_clone3)
+#define __NR_openat2 437
+__SYSCALL(__NR_openat2, sys_openat2)
/*
* Please add new compat syscalls above this comment and update
@@ -356,3 +356,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
# 435 reserved for clone3
+437 common openat2 sys_openat2
@@ -435,3 +435,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
# 435 reserved for clone3
+437 common openat2 sys_openat2
@@ -441,3 +441,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
435 common clone3 sys_clone3
+437 common openat2 sys_openat2
@@ -374,3 +374,4 @@
433 n32 fspick sys_fspick
434 n32 pidfd_open sys_pidfd_open
435 n32 clone3 __sys_clone3
+437 n32 openat2 sys_openat2
@@ -350,3 +350,4 @@
433 n64 fspick sys_fspick
434 n64 pidfd_open sys_pidfd_open
435 n64 clone3 __sys_clone3
+437 n64 openat2 sys_openat2
@@ -423,3 +423,4 @@
433 o32 fspick sys_fspick
434 o32 pidfd_open sys_pidfd_open
435 o32 clone3 __sys_clone3
+437 o32 openat2 sys_openat2
@@ -433,3 +433,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
435 common clone3 sys_clone3_wrapper
+437 common openat2 sys_openat2
@@ -517,3 +517,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
435 nospu clone3 ppc_clone3
+437 common openat2 sys_openat2
@@ -438,3 +438,4 @@
433 common fspick sys_fspick sys_fspick
434 common pidfd_open sys_pidfd_open sys_pidfd_open
435 common clone3 sys_clone3 sys_clone3
+437 common openat2 sys_openat2 sys_openat2
@@ -438,3 +438,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
# 435 reserved for clone3
+437 common openat2 sys_openat2
@@ -481,3 +481,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
# 435 reserved for clone3
+437 common openat2 sys_openat2
@@ -440,3 +440,4 @@
433 i386 fspick sys_fspick __ia32_sys_fspick
434 i386 pidfd_open sys_pidfd_open __ia32_sys_pidfd_open
435 i386 clone3 sys_clone3 __ia32_sys_clone3
+437 i386 openat2 sys_openat2 __ia32_sys_openat2
@@ -357,6 +357,7 @@
433 common fspick __x64_sys_fspick
434 common pidfd_open __x64_sys_pidfd_open
435 common clone3 __x64_sys_clone3/ptregs
+437 common openat2 __x64_sys_openat2
#
# x32-specific system call numbers start at 512 to avoid cache impact
@@ -406,3 +406,4 @@
433 common fspick sys_fspick
434 common pidfd_open sys_pidfd_open
435 common clone3 sys_clone3
+437 common openat2 sys_openat2
@@ -955,25 +955,82 @@ struct file *open_with_fake_path(const struct path *path, int flags,
}
EXPORT_SYMBOL(open_with_fake_path);
-static inline int build_open_flags(int flags, umode_t mode, struct open_flags *op)
+#define WILL_CREATE(flags) (flags & (O_CREAT | __O_TMPFILE))
+#define O_PATH_FLAGS (O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC)
+
+static inline struct open_how build_open_how(int flags, umode_t mode)
+{
+ struct open_how how = {
+ .flags = flags & VALID_OPEN_FLAGS,
+ .mode = mode & S_IALLUGO,
+ };
+
+ /* O_TMPFILE beats O_PATH. */
+ if (how.flags & __O_TMPFILE)
+ how.flags &= ~O_PATH;
+ /* O_PATH beats everything else. */
+ if (how.flags & O_PATH)
+ how.flags &= O_PATH_FLAGS;
+ /* Modes should only be set for create-like flags. */
+ if (!WILL_CREATE(how.flags))
+ how.mode = 0;
+ return how;
+}
+
+static inline int build_open_flags(const struct open_how *how,
+ struct open_flags *op)
{
+ int flags = how->flags;
int lookup_flags = 0;
int acc_mode = ACC_MODE(flags);
+ /* Must never be set by userspace */
+ flags &= ~(FMODE_NONOTIFY | O_CLOEXEC);
+
/*
- * Clear out all open flags we don't know about so that we don't report
- * them in fcntl(F_GETFD) or similar interfaces.
+ * Older syscalls implicitly clear all of the invalid flags or argument
+ * values before calling build_open_flags(), but openat2(2) checks all
+ * of its arguments.
*/
- flags &= VALID_OPEN_FLAGS;
+ if (flags & ~VALID_OPEN_FLAGS)
+ return -EINVAL;
+ if (how->resolve & ~VALID_RESOLVE_FLAGS)
+ return -EINVAL;
+ if (memchr_inv(how->__padding, 0, sizeof(how->__padding)))
+ return -EINVAL;
+
+ /* Deal with the mode. */
+ if (WILL_CREATE(how->flags)) {
+ if (how->mode & ~S_IALLUGO)
+ return -EINVAL;
+ } else if (how->mode != 0) {
+ return -EINVAL;
+ }
- if (flags & (O_CREAT | __O_TMPFILE))
- op->mode = (mode & S_IALLUGO) | S_IFREG;
+ /*
+ * In order to ensure programs get explicit errors when trying to use
+ * O_TMPFILE on old kernels, O_TMPFILE is implemented such that it
+ * looks like (O_DIRECTORY|O_RDWR & ~O_CREAT) to old kernels. But we
+ * have to require userspace to explicitly set it.
+ */
+ if (flags & __O_TMPFILE) {
+ if ((flags & O_TMPFILE_MASK) != O_TMPFILE)
+ return -EINVAL;
+ if (!(acc_mode & MAY_WRITE))
+ return -EINVAL;
+ }
+ if (flags & O_PATH) {
+ /* O_PATH only permits certain other flags to be set. */
+ if (flags & ~O_PATH_FLAGS)
+ return -EINVAL;
+ acc_mode = 0;
+ }
+
+ if (WILL_CREATE(flags))
+ op->mode = how->mode | S_IFREG;
else
op->mode = 0;
- /* Must never be set by userspace */
- flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC;
-
/*
* O_SYNC is implemented as __O_SYNC|O_DSYNC. As many places only
* check for O_DSYNC if the need any syncing at all we enforce it's
@@ -983,20 +1040,6 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
if (flags & __O_SYNC)
flags |= O_DSYNC;
- if (flags & __O_TMPFILE) {
- if ((flags & O_TMPFILE_MASK) != O_TMPFILE)
- return -EINVAL;
- if (!(acc_mode & MAY_WRITE))
- return -EINVAL;
- } else if (flags & O_PATH) {
- /*
- * If we have O_PATH in the open flag. Then we
- * cannot have anything other than the below set of flags
- */
- flags &= O_DIRECTORY | O_NOFOLLOW | O_PATH;
- acc_mode = 0;
- }
-
op->open_flag = flags;
/* O_TRUNC implies we need access checks for write permissions */
@@ -1022,6 +1065,18 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
lookup_flags |= LOOKUP_DIRECTORY;
if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
+
+ if (how->resolve & RESOLVE_NO_XDEV)
+ lookup_flags |= LOOKUP_NO_XDEV;
+ if (how->resolve & RESOLVE_NO_MAGICLINKS)
+ lookup_flags |= LOOKUP_NO_MAGICLINKS;
+ if (how->resolve & RESOLVE_NO_SYMLINKS)
+ lookup_flags |= LOOKUP_NO_SYMLINKS;
+ if (how->resolve & RESOLVE_BENEATH)
+ lookup_flags |= LOOKUP_BENEATH;
+ if (how->resolve & RESOLVE_IN_ROOT)
+ lookup_flags |= LOOKUP_IN_ROOT;
+
op->lookup_flags = lookup_flags;
return 0;
}
@@ -1040,8 +1095,11 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
struct file *file_open_name(struct filename *name, int flags, umode_t mode)
{
struct open_flags op;
- int err = build_open_flags(flags, mode, &op);
- return err ? ERR_PTR(err) : do_filp_open(AT_FDCWD, name, &op);
+ struct open_how how = build_open_how(flags, mode);
+ int err = build_open_flags(&how, &op);
+ if (err)
+ return ERR_PTR(err);
+ return do_filp_open(AT_FDCWD, name, &op);
}
/**
@@ -1072,17 +1130,19 @@ struct file *file_open_root(struct dentry *dentry, struct vfsmount *mnt,
const char *filename, int flags, umode_t mode)
{
struct open_flags op;
- int err = build_open_flags(flags, mode, &op);
+ struct open_how how = build_open_how(flags, mode);
+ int err = build_open_flags(&how, &op);
if (err)
return ERR_PTR(err);
return do_file_open_root(dentry, mnt, filename, &op);
}
EXPORT_SYMBOL(file_open_root);
-long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
+static long do_sys_openat2(int dfd, const char __user *filename,
+ struct open_how *how)
{
struct open_flags op;
- int fd = build_open_flags(flags, mode, &op);
+ int fd = build_open_flags(how, &op);
struct filename *tmp;
if (fd)
@@ -1092,7 +1152,7 @@ long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
if (IS_ERR(tmp))
return PTR_ERR(tmp);
- fd = get_unused_fd_flags(flags);
+ fd = get_unused_fd_flags(how->flags);
if (fd >= 0) {
struct file *f = do_filp_open(dfd, tmp, &op);
if (IS_ERR(f)) {
@@ -1107,12 +1167,16 @@ long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
return fd;
}
-SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
+long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
{
- if (force_o_largefile())
- flags |= O_LARGEFILE;
+ struct open_how how = build_open_how(flags, mode);
+ return do_sys_openat2(dfd, filename, &how);
+}
- return do_sys_open(AT_FDCWD, filename, flags, mode);
+
+SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
+{
+ return ksys_open(filename, flags, mode);
}
SYSCALL_DEFINE4(openat, int, dfd, const char __user *, filename, int, flags,
@@ -1120,10 +1184,32 @@ SYSCALL_DEFINE4(openat, int, dfd, const char __user *, filename, int, flags,
{
if (force_o_largefile())
flags |= O_LARGEFILE;
-
return do_sys_open(dfd, filename, flags, mode);
}
+SYSCALL_DEFINE4(openat2, int, dfd, const char __user *, filename,
+ struct open_how __user *, how, size_t, usize)
+{
+ int err;
+ struct open_how tmp;
+
+ BUILD_BUG_ON(sizeof(struct open_how) < OPEN_HOW_SIZE_VER0);
+ BUILD_BUG_ON(sizeof(struct open_how) != OPEN_HOW_SIZE_LATEST);
+
+ if (unlikely(usize < OPEN_HOW_SIZE_VER0))
+ return -EINVAL;
+
+ err = copy_struct_from_user(&tmp, sizeof(tmp), how, usize);
+ if (err)
+ return err;
+
+ /* O_LARGEFILE is only allowed for non-O_PATH. */
+ if (!(tmp.flags & O_PATH) && force_o_largefile())
+ tmp.flags |= O_LARGEFILE;
+
+ return do_sys_openat2(dfd, filename, &tmp);
+}
+
#ifdef CONFIG_COMPAT
/*
* Exactly like sys_open(), except that it doesn't set the
@@ -2,15 +2,25 @@
#ifndef _LINUX_FCNTL_H
#define _LINUX_FCNTL_H
+#include <linux/stat.h>
#include <uapi/linux/fcntl.h>
-/* list of all valid flags for the open/openat flags argument: */
+/* List of all valid flags for the open/openat flags argument: */
#define VALID_OPEN_FLAGS \
(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __O_SYNC | O_DSYNC | \
FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
+/* List of all valid flags for the how->upgrade_mask argument: */
+#define VALID_UPGRADE_FLAGS \
+ (UPGRADE_NOWRITE | UPGRADE_NOREAD)
+
+/* List of all valid flags for the how->resolve argument: */
+#define VALID_RESOLVE_FLAGS \
+ (RESOLVE_NO_XDEV | RESOLVE_NO_MAGICLINKS | RESOLVE_NO_SYMLINKS | \
+ RESOLVE_BENEATH | RESOLVE_IN_ROOT)
+
#ifndef force_o_largefile
#define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
#endif
@@ -69,6 +69,7 @@ struct rseq;
union bpf_attr;
struct io_uring_params;
struct clone_args;
+struct open_how;
#include <linux/types.h>
#include <linux/aio_abi.h>
@@ -439,6 +440,8 @@ asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
asmlinkage long sys_fchown(unsigned int fd, uid_t user, gid_t group);
asmlinkage long sys_openat(int dfd, const char __user *filename, int flags,
umode_t mode);
+asmlinkage long sys_openat2(int dfd, const char __user *filename,
+ struct open_how *how, size_t size);
asmlinkage long sys_close(unsigned int fd);
asmlinkage long sys_vhangup(void);
@@ -851,8 +851,11 @@ __SYSCALL(__NR_pidfd_open, sys_pidfd_open)
__SYSCALL(__NR_clone3, sys_clone3)
#endif
+#define __NR_openat2 437
+__SYSCALL(__NR_openat2, sys_openat2)
+
#undef __NR_syscalls
-#define __NR_syscalls 436
+#define __NR_syscalls 438
/*
* 32 bit systems traditionally used different
@@ -93,5 +93,46 @@
#define AT_RECURSIVE 0x8000 /* Apply to the entire subtree */
+/*
+ * Arguments for how openat2(2) should open the target path. If @resolve is
+ * zero, then openat2(2) operates very similarly to openat(2).
+ *
+ * However, unlike openat(2), unknown bits in @flags result in -EINVAL rather
+ * than being silently ignored. @mode must be zero unless one of {O_CREAT,
+ * O_TMPFILE} are set, and @upgrade_mask must be zero unless O_PATH is set.
+ *
+ * @flags: O_* flags.
+ * @mode: O_CREAT/O_TMPFILE file mode.
+ * @upgrade_mask: UPGRADE_* flags (to restrict O_PATH re-opening).
+ * @resolve: RESOLVE_* flags.
+ */
+struct open_how {
+ __aligned_u64 flags;
+ __u16 mode;
+ __u16 __padding[3]; /* must be zeroed */
+ __aligned_u64 resolve;
+};
+
+#define OPEN_HOW_SIZE_VER0 24 /* sizeof first published struct */
+#define OPEN_HOW_SIZE_LATEST OPEN_HOW_SIZE_VER0
+
+/* how->resolve flags for openat2(2). */
+#define RESOLVE_NO_XDEV 0x01 /* Block mount-point crossings
+ (includes bind-mounts). */
+#define RESOLVE_NO_MAGICLINKS 0x02 /* Block traversal through procfs-style
+ "magic-links". */
+#define RESOLVE_NO_SYMLINKS 0x04 /* Block traversal through all symlinks
+ (implies OEXT_NO_MAGICLINKS) */
+#define RESOLVE_BENEATH 0x08 /* Block "lexical" trickery like
+ "..", symlinks, and absolute
+ paths which escape the dirfd. */
+#define RESOLVE_IN_ROOT 0x10 /* Make all jumps to "/" and ".."
+ be scoped inside the dirfd
+ (similar to chroot(2)). */
+
+/* how->upgrade flags for openat2(2). */
+/* First bit is reserved for a future UPGRADE_NOEXEC flag. */
+#define UPGRADE_NOREAD 0x02 /* Block re-opening with MAY_READ. */
+#define UPGRADE_NOWRITE 0x04 /* Block re-opening with MAY_WRITE. */
#endif /* _UAPI_LINUX_FCNTL_H */
/* Background. */ For a very long time, extending openat(2) with new features has been incredibly frustrating. This stems from the fact that openat(2) is possibly the most famous counter-example to the mantra "don't silently accept garbage from userspace" -- it doesn't check whether unknown flags are present[1]. This means that (generally) the addition of new flags to openat(2) has been fraught with backwards-compatibility issues (O_TMPFILE has to be defined as __O_TMPFILE|O_DIRECTORY|[O_RDWR or O_WRONLY] to ensure old kernels gave errors, since it's insecure to silently ignore the flag[2]). All new security-related flags therefore have a tough road to being added to openat(2). In addition, the newly-added path resolution restriction LOOKUP flags (which we would like to expose to user-space) don't feel related to the pre-existing O_* flag set -- they affect all components of path lookup. Thus it's necessary to (at the very least) add an additional flag. Adding a new syscall allows us to finally fix the flag-ignoring problem, and we can make it extensible enough so that we will hopefully never need an openat3(2). /* Syscall Prototype. */ /* * open_how is an extensible structure (similar in interface to * clone3(2) or sched_setattr(2)). The size parameter must be set to * sizeof(struct open_how), to allow for future extensions. All future * extensions will be appended to open_how, with their zero value * acting as a no-op default. */ struct open_how { /* ... */ }; int openat2(int dfd, const char *pathname, struct open_how *how, size_t size); /* Description. */ The initial version of 'struct open_how' contains the following fields: flags Used to specify openat(2)-style flags. However, any unknown flag bits or otherwise incorrect flag combinations (like O_PATH|O_RDWR) will result in -EINVAL. In addition, this field is 64-bits wide to allow for more O_ flags than currently permitted with openat(2). mode The file mode for O_CREAT or O_TMPFILE. Must be set to zero if flags does not contain O_CREAT or O_TMPFILE. __padding Must be set to all zeroes. resolve Restrict path resolution (in contrast to O_* flags they affect all path components). The current set of flags are as follows (at the moment, all of the RESOLVE_ flags are implemented as just passing the corresponding LOOKUP_ flag). RESOLVE_NO_XDEV => LOOKUP_NO_XDEV RESOLVE_NO_SYMLINKS => LOOKUP_NO_SYMLINKS RESOLVE_NO_MAGICLINKS => LOOKUP_NO_MAGICLINKS RESOLVE_BENEATH => LOOKUP_BENEATH RESOLVE_IN_ROOT => LOOKUP_IN_ROOT open_how does not contain an embedded size field, because it is of little benefit (userspace can figure out the kernel open_how size at runtime fairly easily without it). Note that as a result of the new how->flags handling, O_PATH|O_TMPFILE is no longer permitted for openat(2). As far as I can tell, this has always been a bug and appears to not be used by userspace (and I've not seen any problems on my machines by disallowing it). If it turns out this breaks something, we can special-case it and only permit it for openat(2) but not openat2(2). /* Testing. */ In a follow-up patch there are over 200 selftests which ensure that this syscall has the correct semantics and will correctly handle several attack scenarios. In addition, I've written a userspace library[4] which provides convenient wrappers around openat2(RESOLVE_IN_ROOT) (this is necessary because no other syscalls support RESOLVE_IN_ROOT, and thus lots of care must be taken when using RESOLVE_IN_ROOT'd file descriptors with other syscalls). During the development of this patch, I've run numerous verification tests using libpathrs (showing that the API is reasonably usable by userspace). /* Future Work. */ Additional RESOLVE_ flags have been suggested during the review period. These can be easily implemented separately (such as blocking automount during resolution). Furthermore, there are some other proposed changes to the openat(2) interface (the most obvious example is magic-link hardening[4]) which would be a good opportunity to add a way for userspace to restrict how O_PATH file descriptors can be re-opened. [1]: https://lwn.net/Articles/588444/ [2]: https://lore.kernel.org/lkml/CA+55aFyyxJL1LyXZeBsf2ypriraj5ut1XkNDsunRBqgVjZU_6Q@mail.gmail.com [3]: https://sourceware.org/bugzilla/show_bug.cgi?id=17523 [4]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/ Suggested-by: Christian Brauner <christian@brauner.io> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> --- CREDITS | 4 +- arch/alpha/kernel/syscalls/syscall.tbl | 1 + arch/arm/tools/syscall.tbl | 1 + arch/arm64/include/asm/unistd.h | 2 +- arch/arm64/include/asm/unistd32.h | 2 + arch/ia64/kernel/syscalls/syscall.tbl | 1 + arch/m68k/kernel/syscalls/syscall.tbl | 1 + arch/microblaze/kernel/syscalls/syscall.tbl | 1 + arch/mips/kernel/syscalls/syscall_n32.tbl | 1 + arch/mips/kernel/syscalls/syscall_n64.tbl | 1 + arch/mips/kernel/syscalls/syscall_o32.tbl | 1 + arch/parisc/kernel/syscalls/syscall.tbl | 1 + arch/powerpc/kernel/syscalls/syscall.tbl | 1 + arch/s390/kernel/syscalls/syscall.tbl | 1 + arch/sh/kernel/syscalls/syscall.tbl | 1 + arch/sparc/kernel/syscalls/syscall.tbl | 1 + arch/x86/entry/syscalls/syscall_32.tbl | 1 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + arch/xtensa/kernel/syscalls/syscall.tbl | 1 + fs/open.c | 154 +++++++++++++++----- include/linux/fcntl.h | 12 +- include/linux/syscalls.h | 3 + include/uapi/asm-generic/unistd.h | 5 +- include/uapi/linux/fcntl.h | 41 ++++++ 24 files changed, 201 insertions(+), 38 deletions(-)