linux-ext4 - Re: [PATCH v2] fs/{posix_acl,ext2,jfs,ceph}: apply umask if ACL support is disabled

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Date:   Tue, 10 Oct 2023 15:11:25 +0200
From:   Jan Kara <jack@...e.cz>
To:     Max Kellermann <max.kellermann@...os.com>
Cc:     Xiubo Li <xiubli@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>, Jan Kara <jack@...e.com>,
        Dave Kleikamp <shaggy@...nel.org>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
        jfs-discussion@...ts.sourceforge.net
Subject: Re: [PATCH v2] fs/{posix_acl,ext2,jfs,ceph}: apply umask if ACL
 support is disabled

On Mon 09-10-23 16:43:39, 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@...os.com>

Thanks for the updated changelog! But as I'm looking into VFS code isn't
this already handled by mode_strip_umask() / vfs_prepare_mode() in
fs/namei.c? Because posix_acl_create() doesn't do anything to 'mode' for
!IS_POSIXACL() filesystems either. So at least ext2 (where I've checked
the mount option handling) does seem to have umask properly applied in all
the cases. But I might be missing something...

								Honza

> ---
>  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;
>  }
> -- 
> 2.39.2
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR