linux-kernel - [PATCH v2] fuse: break infinite loop in fuse_fill_write

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Date:	Mon, 12 Oct 2015 16:33:44 +0300
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	unlisted-recipients:; (no To-header on input)
Cc:	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	fuse-devel@...ts.sourceforge.net,
	Roman Gushchin <klamm@...dex-team.ru>,
	Miklos Szeredi <miklos@...redi.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Maxim Patlasov <mpatlasov@...allels.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: [PATCH v2] fuse: break infinite loop in fuse_fill_write_pages()

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.

Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
Cc: Miklos Szeredi <miklos@...redi.hu>
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>
---
 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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