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Date:   Wed, 19 Aug 2020 07:19:45 +0200
From:   Christoph Hellwig <hch@....de>
To:     davem@...emloft.net, kuba@...nel.org
Cc:     colyli@...e.de, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] net: bypass ->sendpage for slab pages

Sending Slab or tail pages into ->sendpage will cause really strange
delayed oops.  Prevent it right in the networking code instead of
requiring drivers to guess the exact conditions where sendpage works.

Based on a patch from Coly Li <colyli@...e.de>.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 net/socket.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/socket.c b/net/socket.c
index dbbe8ea7d395da..b4e65688915fe3 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -3638,7 +3638,11 @@ EXPORT_SYMBOL(kernel_getpeername);
 int kernel_sendpage(struct socket *sock, struct page *page, int offset,
 		    size_t size, int flags)
 {
-	if (sock->ops->sendpage)
+	/* sendpage does manipulates the refcount of the sent in page, which
+	 * does not work for Slab pages, or for tails of non-__GFP_COMP
+	 * high order pages.
+	 */
+	if (sock->ops->sendpage && !PageSlab(page) && page_count(page) > 0)
 		return sock->ops->sendpage(sock, page, offset, size, flags);
 
 	return sock_no_sendpage(sock, page, offset, size, flags);
-- 
2.28.0

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