linux-kernel - Re: [fuse-devel] [PATCH] fuse: break infinite loop in fuse_fill_write

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Message-ID: <5612C357.1050000@parallels.com>
Date:	Mon, 5 Oct 2015 11:37:11 -0700
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Roman Gushchin <klamm@...dex-team.ru>,
	<fuse-devel@...ts.sourceforge.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	<fuse-devel@...ts.sourceforge.net>, <linux-kernel@...r.kernel.org>
Subject: Re: [fuse-devel] [PATCH] fuse: break infinite loop in
 fuse_fill_write_pages()

On 10/02/2015 06:58 PM, Konstantin Khlebnikov wrote:
> On Sat, Oct 3, 2015 at 1:04 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
>> On Fri, 2 Oct 2015 12:27:45 -0700 Maxim Patlasov <mpatlasov@...allels.com> wrote:
>>
>>> On 10/02/2015 04:21 AM, Konstantin Khlebnikov wrote:
>>>> Bump. Add more peopple in CC.
>>>>
>>>> On Mon, Sep 21, 2015 at 1:02 PM, Roman Gushchin <klamm@...dex-team.ru> wrote:
>>>>> I got a report about unkillable task eating CPU. Thge further
>>>>> investigation shows, that the problem is in the fuse_fill_write_pages()
>>>>> function. If iov's first segment has zero length, we get an infinite
>>>>> loop, because we never reach iov_iter_advance() call.
>>> iov_iter_copy_from_user_atomic() eventually calls iterate_iovec(). The
>>> latter silently consumes zero-length iov. So I don't think "iov's first
>>> segment has zero length" can cause infinite loop.
>> I'm suspecting it got stuck because local variable `bytes' is zero, so
>> the code does `goto again' repeatedly.
>>
>> Or maybe not.   A more complete description of the bug would help.
> I suspect here is the same scenario like in 124d3b7041f:
> Zero-length segmend is followed by segment with invalid address:
> iov_iter_fault_in_readable() checks only first segment (zero-length)
> iov_iter_copy_from_user_atomic() skips it, fails at second and
> returns zero -> goto again without skipping zero-length segment.
>
> Patch calls iov_iter_advance() before goto again: we'll skip zero-length
> segment at second iteraction and iov_iter_fault_in_readable() will detect
> invalid address.

Makes sense to me. The patch looks fine.

Thanks,
Maxim
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