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

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Message-ID: <c5a71213-ca55-4165-8e50-2686d05614e3@rowland.harvard.edu>
Date: Tue, 5 Nov 2024 15:42:12 -0500
From: Alan Stern <stern@...land.harvard.edu>
To: Andrew Morton <akpm@...ux-foundation.org>
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, Nov 05, 2024 at 11:02:36AM -0800, Andrew Morton wrote:
> 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

Well, I wouldn't expect you to understand the USB-specific stuff.  I was 
really asking about the memory-management calls and error handling.

> that usbdev_mmap() calls dec_usb_memory_use_count() on some error
> paths, but goes direct to usbfs_decrease_memory_usage() on others.

The paths that call dec_usb_memory_use_count() are those on which a 
memory buffer has been allocated and needs to be deallocated.  That 
routine then calls usbfs_decrease_memory_usage() as needed.

> Did you try running the "C reproducer"?

No, I haven't.  I haven't had much time to work on this.  In fact, I 
couldn't even tell exactly which call in dec_usb_memory_use_count() 
caused the fault; the line number listed in the bug report didn't match 
up with any obvious suspects in my copy of the kernel source.  Was it 
the kfree(usbm) call?

Alan Stern