linux-kernel - Re: [syzbot] [mm?] kernel BUG in __page_table_check

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Message-Id: <20241105110236.40819b7effad3f44de73dddf@linux-foundation.org>
Date: Tue, 5 Nov 2024 11:02:36 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Alan Stern <stern@...land.harvard.edu>
Cc: syzbot <syzbot+ccc0e1cfdb72b664f0d8@...kaller.appspotmail.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 pasha.tatashin@...een.com, syzkaller-bugs@...glegroups.com,
 linux-usb@...r.kernel.org
Subject: Re: [syzbot] [mm?] kernel BUG in __page_table_check_zero (2)

On Tue, 5 Nov 2024 11:39:59 -0500 Alan Stern <stern@...land.harvard.edu> wrote:

> On Mon, Nov 04, 2024 at 08:00:07PM -0800, Andrew Morton wrote:
> > On Wed, 30 Oct 2024 21:54:22 -0700 syzbot <syzbot+ccc0e1cfdb72b664f0d8@...kaller.appspotmail.com> wrote:
> > 
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > 
> > Thanks.  I'm suspecting some USB issue - fault injection was used to
> > trigger a memory allocation failure and dec_usb_memory_use_count() ended
> > up freeing an in-use page.  Could USB folks please have a look?
> 
> Andrew, I'm not sure what to look for.

Thanks for looking.

> Can you read through 
> usbdev_mmap() in drivers/usb/core/devio.c, along with the four short 
> routines preceding it, and let us know if anything seems obviously 
> wrong?

All I see is lots of USB code which I don't understand ;) It seems odd
that usbdev_mmap() calls dec_usb_memory_use_count() on some error
paths, but goes direct to usbfs_decrease_memory_usage() on others.

Did you try running the "C reproducer"?