Re: [syzbot] [nilfs?] KASAN: slab-out-of-bounds Read in wb_writeback

[Christian Brauner]

On Fri, Apr 05, 2024 at 03:23:46PM +0200, Jan Kara wrote:
> On Fri 05-04-24 13:05:59, Christian Brauner wrote:
> > On Wed, Apr 03, 2024 at 11:47:17AM +0200, Jan Kara wrote:
> > > On Tue 02-04-24 07:38:25, syzbot wrote:
> > > > syzbot has found a reproducer for the following issue on:
> > > > 
> > > > HEAD commit:    c0b832517f62 Add linux-next specific files for 20240402
> > > > git tree:       linux-next
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=14af7dd9180000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=afcaf46d374cec8c
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=7b219b86935220db6dd8
> > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1729f003180000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17fa4341180000
> > > > 
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/0d36ec76edc7/disk-c0b83251.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/6f9bb4e37dd0/vmlinux-c0b83251.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/2349287b14b7/bzImage-c0b83251.xz
> > > > mounted in repro: https://storage.googleapis.com/syzbot-assets/9760c52a227c/mount_0.gz
> > > > 
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+7b219b86935220db6dd8@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > 
> > > > ==================================================================
> > > > BUG: KASAN: slab-out-of-bounds in __lock_acquire+0x78/0x1fd0 kernel/locking/lockdep.c:5005
> > > > Read of size 8 at addr ffff888020485fa8 by task kworker/u8:2/35
> > > 
> > > Looks like the writeback cleanups are causing some use-after-free issues.
> > > The code KASAN is complaining about is:
> > > 
> > > 		/*
> > > 		 * Nothing written. Wait for some inode to
> > > 		 * become available for writeback. Otherwise
> > > 		 * we'll just busyloop.
> > > 		 */
> > > 		trace_writeback_wait(wb, work);
> > > 		inode = wb_inode(wb->b_more_io.prev);
> > > >>>>>		spin_lock(&inode->i_lock); <<<<<<
> > > 		spin_unlock(&wb->list_lock);
> > > 		/* This function drops i_lock... */
> > > 		inode_sleep_on_writeback(inode);
> > > 
> > > in wb_writeback(). Now looking at the changes indeed the commit
> > > 167d6693deb ("fs/writeback: bail out if there is no more inodes for IO and
> > > queued once") is buggy because it will result in trying to fetch 'inode'
> > > from empty b_more_io list and thus we'll corrupt memory. I think instead of
> > > modifying the condition:
> > > 
> > > 		if (list_empty(&wb->b_more_io)) {
> > > 
> > > we should do:
> > > 
> > > -		if (progress) {
> > > +		if (progress || !queued) {
> > >                         spin_unlock(&wb->list_lock);
> > >                         continue;
> > >                 }
> > > 
> > > Kemeng?
> > 
> > Fwiw, I observed this on xfstest too the last few days and tracked it
> > down to this series. Here's the splat I got in case it helps:
> 
> OK, since this is apparently causing more issues and Kemeng didn't reply
> yet, here's a fix in the form of the patch. It has passed some basic
> testing. Feel free to fold it into Kemeng's patch so that we don't keep
> linux-next broken longer than necessary. Thanks!

Thanks! Folded and I mentioned that I folded a fix from you into the
commit with a link to this patch.