Messages in this thread | | | Date | Tue, 1 Mar 2022 11:15:09 +0100 | From | Mickaël Salaün <> | Subject | Re: [PATCH v1] fs: Fix inconsistent f_mode |
| |
On 01/03/2022 10:22, Christian Brauner wrote: > On Mon, Feb 28, 2022 at 10:59:35PM +0100, Mickaël Salaün wrote: >> From: Mickaël Salaün <mic@linux.microsoft.com> >> >> While transitionning to ACC_MODE() with commit 5300990c0370 ("Sanitize >> f_flags helpers") and then fixing it with commit 6d125529c6cb ("Fix >> ACC_MODE() for real"), we lost an open flags consistency check. Opening >> a file with O_WRONLY | O_RDWR leads to an f_flags containing MAY_READ | >> MAY_WRITE (thanks to the ACC_MODE() helper) and an empty f_mode. >> Indeed, the OPEN_FMODE() helper transforms 3 (an incorrect value) to 0. >> >> Fortunately, vfs_read() and vfs_write() both check for FMODE_READ, or >> respectively FMODE_WRITE, and return an EBADF error if it is absent. >> Before commit 5300990c0370 ("Sanitize f_flags helpers"), opening a file >> with O_WRONLY | O_RDWR returned an EINVAL error. Let's restore this safe >> behavior. > > That specific part seems a bit risky at first glance. Given that the > patch referenced is from 2009 this means we've been allowing O_WRONLY | > O_RDWR to succeed for almost 13 years now.
Yeah, it's an old bug, but we should keep in mind that a file descriptor created with such flags cannot be used to read nor write. However, unfortunately, it can be used for things like ioctl, fstat, chdir… I don't know if there is any user of this trick.
Either way, there is an inconsistency between those using ACC_MODE() and those using OPEN_FMODE(). If we decide to take a side for the behavior of one or the other, without denying to create such FD, it could also break security policies. We have to choose what to potentially break…
> >> >> To make it consistent with ACC_MODE(), this patch also changes >> OPEN_FMODE() to return FMODE_READ | FMODE_WRITE for O_WRONLY | O_RDWR. >> This may help protect from potential spurious issues. >> >> This issue could result in inconsistencies with AppArmor, Landlock and >> SELinux, but the VFS checks would still forbid read and write accesses. >> Tomoyo uses the ACC_MODE() transformation which is correct, and Smack >> doesn't check the file mode. Filesystems using OPEN_FMODE() should also >> be protected by the VFS checks. >> >> Fixes: 5300990c0370 ("Sanitize f_flags helpers") >> Cc: Al Viro <viro@zeniv.linux.org.uk> >> Cc: Casey Schaufler <casey@schaufler-ca.com> >> Cc: Darrick J. Wong <djwong@kernel.org> >> Cc: Eric Paris <eparis@parisplace.org> >> Cc: John Johansen <john.johansen@canonical.com> >> Cc: Kentaro Takeda <takedakn@nttdata.co.jp> >> Cc: Miklos Szeredi <miklos@szeredi.hu> >> Cc: Paul Moore <paul@paul-moore.com> >> Cc: Stephen Smalley <stephen.smalley.work@gmail.com> >> Cc: Steve French <sfrench@samba.org> >> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> >> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com> >> Link: https://lore.kernel.org/r/20220228215935.748017-1-mic@digikod.net >> --- >> fs/file_table.c | 3 +++ >> include/linux/fs.h | 5 +++-- >> 2 files changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/fs/file_table.c b/fs/file_table.c >> index 7d2e692b66a9..b936f69525d0 100644 >> --- a/fs/file_table.c >> +++ b/fs/file_table.c >> @@ -135,6 +135,9 @@ static struct file *__alloc_file(int flags, const struct cred *cred) >> struct file *f; >> int error; >> >> + if ((flags & O_ACCMODE) == O_ACCMODE) >> + return ERR_PTR(-EINVAL); >> + >> f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL); >> if (unlikely(!f)) >> return ERR_PTR(-ENOMEM); >> diff --git a/include/linux/fs.h b/include/linux/fs.h >> index e2d892b201b0..83bc5aaf1c41 100644 >> --- a/include/linux/fs.h >> +++ b/include/linux/fs.h >> @@ -3527,8 +3527,9 @@ int __init list_bdev_fs_names(char *buf, size_t size); >> #define __FMODE_NONOTIFY ((__force int) FMODE_NONOTIFY) >> >> #define ACC_MODE(x) ("\004\002\006\006"[(x)&O_ACCMODE]) >> -#define OPEN_FMODE(flag) ((__force fmode_t)(((flag + 1) & O_ACCMODE) | \ >> - (flag & __FMODE_NONOTIFY))) >> +#define OPEN_FMODE(flag) ((__force fmode_t)( \ >> + (((flag + 1) & O_ACCMODE) ?: O_ACCMODE) | \ >> + (flag & __FMODE_NONOTIFY))) >> >> static inline bool is_sxid(umode_t mode) >> { >> >> base-commit: 7e57714cd0ad2d5bb90e50b5096a0e671dec1ef3 >> -- >> 2.35.1 >>
| |