Re: [PATCH 6.1] f2fs: Require FMODE_WRITE for atomic write ioctls

[Sasha Levin <sashal@kernel.org>]

On Fri, Oct 04, 2024 at 07:34:41PM +0000, Eric Biggers wrote:
>From: Jann Horn <jannh@google.com>
>
>commit 4f5a100f87f32cb65d4bb1ad282a08c92f6f591e upstream.
>
>The F2FS ioctls for starting and committing atomic writes check for
>inode_owner_or_capable(), but this does not give LSMs like SELinux or
>Landlock an opportunity to deny the write access - if the caller's FSUID
>matches the inode's UID, inode_owner_or_capable() immediately returns true.
>
>There are scenarios where LSMs want to deny a process the ability to write
>particular files, even files that the FSUID of the process owns; but this
>can currently partially be bypassed using atomic write ioctls in two ways:
>
> - F2FS_IOC_START_ATOMIC_REPLACE + F2FS_IOC_COMMIT_ATOMIC_WRITE can
>   truncate an inode to size 0
> - F2FS_IOC_START_ATOMIC_WRITE + F2FS_IOC_ABORT_ATOMIC_WRITE can revert
>   changes another process concurrently made to a file
>
>Fix it by requiring FMODE_WRITE for these operations, just like for
>F2FS_IOC_MOVE_RANGE. Since any legitimate caller should only be using these
>ioctls when intending to write into the file, that seems unlikely to break
>anything.
>
>Fixes: 88b88a667971 ("f2fs: support atomic writes")
>Cc: stable@vger.kernel.org
>Signed-off-by: Jann Horn <jannh@google.com>
>Reviewed-by: Chao Yu <chao@kernel.org>
>Reviewed-by: Eric Biggers <ebiggers@google.com>
>Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>Signed-off-by: Eric Biggers <ebiggers@google.com>

I've queued these backports, thanks!

-- 
Thanks,
Sasha