linux-kernel - [PATCH 3.2 01/77] fuse: break infinite loop in fuse_fill_write

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Message-ID: <lsq.1450971462.224162794@decadent.org.uk>
Date:	Thu, 24 Dec 2015 15:37:42 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Maxim Patlasov" <mpatlasov@...allels.com>,
	"Konstantin Khlebnikov" <khlebnikov@...dex-team.ru>,
	"Roman Gushchin" <klamm@...dex-team.ru>,
	"Miklos Szeredi" <miklos@...redi.hu>
Subject: [PATCH 3.2 01/77] fuse: break infinite loop in fuse_fill_write_pages()

3.2.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Roman Gushchin <klamm@...dex-team.ru>

commit 3ca8138f014a913f98e6ef40e939868e1e9ea876 upstream.

I got a report about unkillable task eating CPU. 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.

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"). If 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.

Special thanks to Konstantin Khlebnikov, who helped a lot with the commit
description.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Maxim Patlasov <mpatlasov@...allels.com>
Cc: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
Signed-off-by: Miklos Szeredi <miklos@...redi.hu>
Fixes: ea9b9907b82a ("fuse: implement perform_write")
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/fuse/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -850,6 +850,7 @@ static ssize_t fuse_fill_write_pages(str

 		mark_page_accessed(page);

+		iov_iter_advance(ii, tmp);
 		if (!tmp) {
 			unlock_page(page);
 			page_cache_release(page);
@@ -861,7 +862,6 @@ static ssize_t fuse_fill_write_pages(str
 		req->pages[req->num_pages] = page;
 		req->num_pages++;

-		iov_iter_advance(ii, tmp);
 		count += tmp;
 		pos += tmp;
 		offset += tmp;

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