linux-kernel - [ 008/117] bio: dont overflow in bio_get_nr

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Message-Id: <20120528031203.986470805@decadent.org.uk>
Date:	Mon, 28 May 2012 04:12:10 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk,
	Kent Overstreet <kent.overstreet@...il.com>,
	Kent Overstreet <koverstreet@...gle.com>,
	Tejun Heo <tj@...nel.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Jens Axboe <axboe@...nel.dk>
Subject: [ 008/117] bio: dont overflow in bio_get_nr_vecs()

3.2-stable review patch.  If anyone has any objections, please let me know.

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

From: Kent Overstreet <kent.overstreet@...il.com>

commit 5abebfdd02450fa1349daacf242e70b3736581e3 upstream.

There were two places bio_get_nr_vecs() could overflow:

First, it did a left shift to convert from sectors to bytes immediately
before dividing by PAGE_SIZE.  If PAGE_SIZE ever was less than 512 a great
many things would break, so dividing by PAGE_SIZE >> 9 is safe and will
generate smaller code too.

The nastier overflow was in the DIV_ROUND_UP() (that's what the code was
effectively doing, anyways).  If n + d overflowed, the whole thing would
return 0 which breaks things rather effectively.

bio_get_nr_vecs() doesn't claim to give an exact value anyways, so the
DIV_ROUND_UP() is silly; we could do a straight divide except if a
device's queue_max_sectors was less than PAGE_SIZE we'd return 0.  So we
just add 1; this should always be safe - things will break badly if
bio_get_nr_vecs() returns > BIO_MAX_PAGES (bio_alloc() will suddenly start
failing) but it's queue_max_segments that must guard against this, if
queue_max_sectors is preventing this from happen things are going to
explode on architectures with different PAGE_SIZE.

Signed-off-by: Kent Overstreet <koverstreet@...gle.com>
Cc: Tejun Heo <tj@...nel.org>
Acked-by: Valdis Kletnieks <Valdis.Kletnieks@...edu>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/bio.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/fs/bio.c b/fs/bio.c
index b1fe82c..b980ecd 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -505,13 +505,9 @@ EXPORT_SYMBOL(bio_clone);
 int bio_get_nr_vecs(struct block_device *bdev)
 {
 	struct request_queue *q = bdev_get_queue(bdev);
-	int nr_pages;
-
-	nr_pages = ((queue_max_sectors(q) << 9) + PAGE_SIZE - 1) >> PAGE_SHIFT;
-	if (nr_pages > queue_max_segments(q))
-		nr_pages = queue_max_segments(q);
-
-	return nr_pages;
+	return min_t(unsigned,
+		     queue_max_segments(q),
+		     queue_max_sectors(q) / (PAGE_SIZE >> 9) + 1);
 }
 EXPORT_SYMBOL(bio_get_nr_vecs);

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