[RFC PATCH] f*xattr: allow O_PATH descriptors

Christian Göttsche
2022-06-07T15:32:10Z

From: Miklos Szeredi <mszeredi@redhat.com>

Support file descriptors obtained via O_PATH for extended attribute
operations.

Extended attributes are for example used by SELinux for the security
context of file objects. To avoid time-of-check-time-of-use issues while
setting those contexts it is advisable to pin the file in question and
operate on a file descriptor instead of the path name. This can be
emulated in userspace via /proc/self/fd/NN [1] but requires a procfs,
which might not be mounted e.g. inside of chroots, see[2].

[1]: https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7233d2ac2d12ca50
[2]: https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e070793889c9466845

Original patch by Miklos Szeredi <mszeredi@redhat.com>
https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@redhat.com/

> While this carries a minute risk of someone relying on the property of
> xattr syscalls rejecting O_PATH descriptors, it saves the trouble of
> introducing another set of syscalls.
>
> Only file->f_path and file->f_inode are accessed in these functions.
>
> Current versions return EBADF, hence easy to detect the presense of
> this feature and fall back in case it's missing.

CC: linux-api@vger.kernel.org
CC: linux-man@vger.kernel.org
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
fs/xattr.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/xattr.c b/fs/xattr.c
index e8dd03e4561e..16360ac4eb1b 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -656,7 +656,7 @@ SYSCALL_DEFINE5(lsetxattr, const char __user *, pathname,
SYSCALL_DEFINE5(fsetxattr, int, fd, const char __user *, name,
const void __user *,value, size_t, size, int, flags)
{
- struct fd f = fdget(fd);
+ struct fd f = fdget_raw(fd);
int error = -EBADF;

if (!f.file)
@@ -768,7 +768,7 @@ SYSCALL_DEFINE4(lgetxattr, const char __user *, pathname,
SYSCALL_DEFINE4(fgetxattr, int, fd, const char __user *, name,
void __user *, value, size_t, size)
{
- struct fd f = fdget(fd);
+ struct fd f = fdget_raw(fd);
ssize_t error = -EBADF;

if (!f.file)
@@ -844,7 +844,7 @@ SYSCALL_DEFINE3(llistxattr, const char __user *, pathname, char __user *, list,

SYSCALL_DEFINE3(flistxattr, int, fd, char __user *, list, size_t, size)
{
- struct fd f = fdget(fd);
+ struct fd f = fdget_raw(fd);
ssize_t error = -EBADF;

if (!f.file)
@@ -910,7 +910,7 @@ SYSCALL_DEFINE2(lremovexattr, const char __user *, pathname,

SYSCALL_DEFINE2(fremovexattr, int, fd, const char __user *, name)
{
- struct fd f = fdget(fd);
+ struct fd f = fdget_raw(fd);
int error = -EBADF;

if (!f.file)
--
2.36.1