Re: [PATCH v2] fs/{posix_acl,ext2,jfs,ceph}: apply umask if ACL support is disabled

[Dave Kleikamp]

On 10/9/23 9:43AM, Max Kellermann wrote:
> One important implementation detail of the posix_acl_create() function
> is that it applies the umask to the "mode" parameter.  If
> CONFIG_FS_POSIX_ACL is disabled, this detail is missing and the umask
> may not get applied.
>
> This patch adds the missing code to posix_acl_create() and to three
> filesystems that omit the posix_acl_create() call if their individual
> ACL support is disabled (CONFIG_EXT2_FS_POSIX_ACL,
> CONFIG_JFS_POSIX_ACL, CONFIG_CEPH_FS_POSIX_ACL).  If
> posix_acl_create() never gets called, the umask needs to be applied
> anyway.
>
> This bug used to be exploitable easily with O_TMPFILE (see
> https://bugzilla.kernel.org/show_bug.cgi?id=203625) but that part was
> fixed by commit ac6800e279a2 ("fs: Add missing umask strip in
> vfs_tmpfile") last year.  The bug may not be reachable by userspace
> anymore, but since it is apparently still necessary to apply the umask
> again in posix_acl_create(), there is no reason to assume it's not
> necessary with ACL support is disabled.
>
> Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>

Acked-by: Dave Kleikamp <dave.kleikamp@xxxxxxxxxx>

> ---
>   fs/ceph/super.h           | 6 ++++++
>   fs/ext2/acl.h             | 6 ++++++
>   fs/jfs/jfs_acl.h          | 6 ++++++
>   include/linux/posix_acl.h | 1 +
>   4 files changed, 19 insertions(+)
>
> diff --git a/fs/ceph/super.h b/fs/ceph/super.h
> index 51c7f2b14f6f..58349639bd57 100644
> --- a/fs/ceph/super.h
> +++ b/fs/ceph/super.h
> @@ -1194,6 +1194,12 @@ static inline void ceph_forget_all_cached_acls(struct inode *inode)
>   static inline int ceph_pre_init_acls(struct inode *dir, umode_t *mode,
>   				     struct ceph_acl_sec_ctx *as_ctx)
>   {
> +	/* usually, the umask is applied by posix_acl_create(), but if
> +	 * ACL support is disabled at compile time, we need to do it
> +	 * here, because posix_acl_create() will never be called
> +	 */
> +	*mode &= ~current_umask();
> +
>   	return 0;
>   }
>   static inline void ceph_init_inode_acls(struct inode *inode,
> diff --git a/fs/ext2/acl.h b/fs/ext2/acl.h
> index 4a8443a2b8ec..0ecaa9c20c0c 100644
> --- a/fs/ext2/acl.h
> +++ b/fs/ext2/acl.h
> @@ -67,6 +67,12 @@ extern int ext2_init_acl (struct inode *, struct inode *);
>   
>   static inline int ext2_init_acl (struct inode *inode, struct inode *dir)
>   {
> +	/* usually, the umask is applied by posix_acl_create(), but if
> +	 * ACL support is disabled at compile time, we need to do it
> +	 * here, because posix_acl_create() will never be called
> +	 */
> +	inode->i_mode &= ~current_umask();
> +
>   	return 0;
>   }
>   #endif
> diff --git a/fs/jfs/jfs_acl.h b/fs/jfs/jfs_acl.h
> index f892e54d0fcd..64a05e663a45 100644
> --- a/fs/jfs/jfs_acl.h
> +++ b/fs/jfs/jfs_acl.h
> @@ -17,6 +17,12 @@ int jfs_init_acl(tid_t, struct inode *, struct inode *);
>   static inline int jfs_init_acl(tid_t tid, struct inode *inode,
>   			       struct inode *dir)
>   {
> +	/* usually, the umask is applied by posix_acl_create(), but if
> +	 * ACL support is disabled at compile time, we need to do it
> +	 * here, because posix_acl_create() will never be called
> +	 */
> +	inode->i_mode &= ~current_umask();
> +
>   	return 0;
>   }
>   
> diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
> index 0e65b3d634d9..54bc9b1061ca 100644
> --- a/include/linux/posix_acl.h
> +++ b/include/linux/posix_acl.h
> @@ -128,6 +128,7 @@ static inline void cache_no_acl(struct inode *inode)
>   static inline int posix_acl_create(struct inode *inode, umode_t *mode,
>   		struct posix_acl **default_acl, struct posix_acl **acl)
>   {
> +	*mode &= ~current_umask();
>   	*default_acl = *acl = NULL;
>   	return 0;
>   }