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

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Date:	Fri, 2 Oct 2015 12:27:45 -0700
From:	Maxim Patlasov <mpatlasov@...allels.com>
To:	Konstantin Khlebnikov <koct9i@...il.com>,
	Roman Gushchin <klamm@...dex-team.ru>
CC:	<fuse-devel@...ts.sourceforge.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [fuse-devel] [PATCH] fuse: break infinite loop in
 fuse_fill_write_pages()

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.

Thanks,
Maxim

>>
>> Fix this by calling iov_iter_advance() before repeating an attempt to
>> copy data from userspace.
>>
>> A similar problem is described in 124d3b7041f ("fix writev regression:
>> pan hanging unkillable and un-straceable").
>>
>> Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
>> Cc: Miklos Szeredi <miklos@...redi.hu>
>> ---
>>   fs/fuse/file.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
>> index f523f2f..195476a 100644
>> --- a/fs/fuse/file.c
>> +++ b/fs/fuse/file.c
>> @@ -1049,6 +1049,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
>>                  tmp = iov_iter_copy_from_user_atomic(page, ii, offset, bytes);
>>                  flush_dcache_page(page);
>>
>> +               iov_iter_advance(ii, tmp);
>>                  if (!tmp) {
>>                          unlock_page(page);
>>                          page_cache_release(page);
>> @@ -1061,7 +1062,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
>>                  req->page_descs[req->num_pages].length = tmp;
>>                  req->num_pages++;
>>
>> -               iov_iter_advance(ii, tmp);
>>                  count += tmp;
>>                  pos += tmp;
>>                  offset += tmp;
>> --
>> 2.4.3
>>
>> --
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