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Date: Wed, 12 Jun 2024 09:50:27 +0200
From: Jan Kara <jack@suse.cz>
To: Hugh Dickins <hughd@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>,
	syzbot <syzbot+9c1fe13fcb51574b249b@syzkaller.appspotmail.com>,
	akpm@linux-foundation.org, chengming.zhou@linux.dev,
	hannes@cmpxchg.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, nphamcs@gmail.com,
	syzkaller-bugs@googlegroups.com, Jan Kara <jack@suse.cz>,
	linux-ext4@vger.kernel.org
Subject: Re: [syzbot] [mm?] KMSAN: uninit-value in zswap_store
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On Tue 11-06-24 02:13:59, Hugh Dickins wrote:
> On Mon, 10 Jun 2024, Yosry Ahmed wrote:
> > Hugh, do you mind taking a quick look at whether this is a real bug?
> > 
> > If this is expected behavior, perhaps there is some annotation we can
> > use in zswap that it is fine to encounter uninitialized data when
> > reading the folio.
> 
> I've not been faced with a KMSAN report before, so I might well be
> misunderstanding its language: but this looks like an ext4 "bug" to me.
> 
> I think the story that the three KMSAN stacks are telling is this:
> syzbot has an ext4 filesystem on a loop device on a tmpfs file (I do
> exactly that too); ext4 is doing some ext4_xattr_inode_write() business,
> in the course of which it writes back a not-fully-initialized block to
> the loop device; shmem faithfully copies that data into its pagecache,
> then later when under memory pressure that page gets "written" out to
> zswap: where zswap_is_page_same_filled() takes an interest in the data,
> and KMSAN objects because some of it was not originally initialized.
> 
> If that's a correct interpretation, then it's probably not a big deal:
> it's probably the uninitialized end of a buffer that's written out,
> not any part of the "disk" which ext4 would ever show to a user; but
> I do agree with KMSAN that ext4 would do better to clear that area,
> rather than accidentally storing someone else's super-secret info.

Yes, that seems to be accurate.  ext4_xattr_inode_write() stores large
extended attribute in the inode and we don't bother to zero out the tail of
the block we use since we never access bytes beyond xattr size. Frankly I
don't consider this a bug since the uninitialized bytes are never exposed
to (unpriviledged) userspace. But I agree that out of pure precaution and
because it doesn't cost much in terms of performance we could zero out the
block tail.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR