Re: [PATCH 3/3] vhost: fix get_user_pages_fast error handling

On Tue, Feb 23, 2010 at 07:39:08PM +0200, Michael S. Tsirkin wrote:

On Tue, Feb 23, 2010 at 07:39:52PM +0200, Gleb Natapov wrote:

quoted

On Tue, Feb 23, 2010 at 07:32:58PM +0200, Michael S. Tsirkin wrote:

quoted

On Tue, Feb 23, 2010 at 07:34:34PM +0200, Gleb Natapov wrote:

quoted

On Tue, Feb 23, 2010 at 06:57:58PM +0200, Michael S. Tsirkin wrote:

quoted

get_user_pages_fast returns number of pages on success, negative value
on failure, but never 0. Fix vhost code to match this logic.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 drivers/vhost/vhost.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d4f8fdf..d003504 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -646,8 +646,9 @@ static int set_bit_to_user(int nr, void __user *addr)
 	int bit = nr + (log % PAGE_SIZE) * 8;
 	int r;
 	r = get_user_pages_fast(log, 1, 1, &page);
-	if (r)
+	if (r < 0)
 		return r;
+	BUG_ON(r != 1);

Can't this be easily triggered from user space?

I think no. get_user_pages_fast always returns number of pages
pinned (in this case always 1) or an error (< 0).
Anything else is a kernel bug.

But what if page is unmapped from userspace?

Then we get -EFAULT

Ah correct.

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 	base = kmap_atomic(page, KM_USER0);
 	set_bit(bit, base);
 	kunmap_atomic(base, KM_USER0);
-- 
1.7.0.18.g0d53a5

--
			Gleb.

--
			Gleb.

--
			Gleb.