linux-ext4 - Re: KASAN: use-after-free Read in iov_iter

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Date:   Mon, 16 Dec 2019 11:58:07 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dave Chinner <david@...morbit.com>
Cc:     syzbot <syzbot+bea68382bae9490e7dd6@...kaller.appspotmail.com>,
        darrick.wong@...cle.com, hch@...radead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        linux-ext4@...r.kernel.org, David Howells <dhowells@...hat.com>
Subject: Re: KASAN: use-after-free Read in iov_iter_alignment

On Mon 16-12-19 11:48:36, Jan Kara wrote:
> On Tue 03-12-19 08:10:37, Dave Chinner wrote:
> > [cc linux-ext4@...r.kernel.org - this is reported from the new ext4
> > dio->iomap code]
> > 
> > On Mon, Dec 02, 2019 at 09:15:08AM -0800, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following crash on:
> > > 
> > > HEAD commit:    b94ae8ad Merge tag 'seccomp-v5.5-rc1' of git://git.kernel...
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=135a8d7ae00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=c2e464ae414aee8c
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=bea68382bae9490e7dd6
> > > compiler:       clang version 9.0.0 (/home/glider/llvm/clang
> > > 80fee25776c2fb61e74c1ecb1a523375c2500b69)
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1135cb36e00000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14e90abce00000
> > > 
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+bea68382bae9490e7dd6@...kaller.appspotmail.com
> > > 
...

> > Looks like buffered read IO on a loopback device on an ext4 image
> > file, and something is being tripped over in the new ext4 direct IO
> > path.  Might be an iomap issue, might be an ext4 issue, but it looks
> > like the buffered read bio completion is running while the iov is
> > still being submitted...
> 
> Looking a bit more into this, I'm pretty sure this is caused by commit
> 8cefc107ca54c "pipe: Use head and tail pointers for the ring, not cursor
> and length". The pipe dereference it has added to iov_iter_alignment() is
> just bogus for all iter types except for pipes. I'll send a fix.

For reference the fix I've sent is attached.

								Honza

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

View attachment "0001-pipe-Fix-bogus-dereference-in-iov_iter_alignment.patch" of type "text/x-patch" (1350 bytes)