Re: [PATCH v1] fs: Fix inconsistent f_mode

[Christian Brauner <brauner@kernel.org>]

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.

> 
> 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
>