Re: kernel BUG at fs/pipe.c:LINE!

[Eric Biggers]

David,

On Sun, Dec 01, 2019 at 10:45:08PM -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=1387ab12e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=ff560c3de405258c
> dashboard link: https://syzkaller.appspot.com/bug?extid=d37abaade33a934f16f2
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12945c41e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=161e202ee00000
> 
> The bug was bisected to:
> 
> commit 8cefc107ca54c8b06438b7dc9cc08bc0a11d5b98
> Author: David Howells <dhowells@xxxxxxxxxx>
> Date:   Fri Nov 15 13:30:32 2019 +0000
> 
>     pipe: Use head and tail pointers for the ring, not cursor and length
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=118cce96e00000
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=138cce96e00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=158cce96e00000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+d37abaade33a934f16f2@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 8cefc107ca54 ("pipe: Use head and tail pointers for the ring, not
> cursor and length")
> 
> ------------[ cut here ]------------
> kernel BUG at fs/pipe.c:582!

This same BUG_ON() crashed my system during normal use, no syzkaller involved at
all, on mainline 937d6eefc7.  Can you please take a look?  This syzbot report
has a reproducer so that might be the easiest place to start.

- Eric