[PATCH v5 1/3] net: introduce helper sendpage_ok() in include/linux/net.h

The original problem was from nvme-over-tcp code, who mistakenly uses
kernel_sendpage() to send pages allocated by __get_free_pages() without
__GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
tail pages, sending them by kernel_sendpage() may trigger a kernel panic
from a corrupted kernel heap, because these pages are incorrectly freed
in network stack as page_count 0 pages.

This patch introduces a helper sendpage_ok(), it returns true if the
checking page,
- is not slab page: PageSlab(page) is false.
- has page refcount: page_count(page) is not zero

All drivers who want to send page to remote end by kernel_sendpage()
may use this helper to check whether the page is OK. If the helper does
not return true, the driver should try other non sendpage method (e.g.
sock_no_sendpage()) to handle the page.

Signed-off-by: Coly Li <redacted>
Cc: Chaitanya Kulkarni <redacted>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Jan Kara <jack@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mikhail Skorzhinskii <redacted>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Vlastimil Babka <redacted>
Cc: stable@vger.kernel.org
---
v5, include linux/mm.h in include/linux/net.h
v4, change sendpage_ok() as an inline helper, and post it as
    separate patch.
v3, introduce a more common sendpage_ok()
v2, fix typo in patch subject
v1, the initial version.
 include/linux/net.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/net.h b/include/linux/net.h
index d48ff1180879..a807fad31958 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h

@@ -21,6 +21,7 @@
 #include <linux/rcupdate.h>
 #include <linux/once.h>
 #include <linux/fs.h>
+#include <linux/mm.h>
 #include <linux/sockptr.h>
 
 #include <uapi/linux/net.h>

@@ -286,6 +287,21 @@ do {									\
 #define net_get_random_once_wait(buf, nbytes)			\
 	get_random_once_wait((buf), (nbytes))
 
+/*
+ * E.g. XFS meta- & log-data is in slab pages, or bcache meta
+ * data pages, or other high order pages allocated by
+ * __get_free_pages() without __GFP_COMP, which have a page_count
+ * of 0 and/or have PageSlab() set. We cannot use send_page for
+ * those, as that does get_page(); put_page(); and would cause
+ * either a VM_BUG directly, or __page_cache_release a page that
+ * would actually still be referenced by someone, leading to some
+ * obscure delayed Oops somewhere else.
+ */
+static inline bool sendpage_ok(struct page *page)
+{
+	return  (!PageSlab(page) && page_count(page) >= 1);
+}
+
 int kernel_sendmsg(struct socket *sock, struct msghdr *msg, struct kvec *vec,
 		   size_t num, size_t len);
 int kernel_sendmsg_locked(struct sock *sk, struct msghdr *msg,

-- 
2.26.2

Currently nvme_tcp_try_send_data() doesn't use kernel_sendpage() to
send slab pages. But for pages allocated by __get_free_pages() without
__GFP_COMP, which also have refcount as 0, they are still sent by
kernel_sendpage() to remote end, this is problematic.

The new introduced helper sendpage_ok() checks both PageSlab tag and
page_count counter, and returns true if the checking page is OK to be
sent by kernel_sendpage().

This patch fixes the page checking issue of nvme_tcp_try_send_data()
with sendpage_ok(). If sendpage_ok() returns true, send this page by
kernel_sendpage(), otherwise use sock_no_sendpage to handle this page.

Signed-off-by: Coly Li <redacted>
Cc: Chaitanya Kulkarni <redacted>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Jan Kara <jack@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mikhail Skorzhinskii <redacted>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Vlastimil Babka <redacted>
Cc: stable@vger.kernel.org
---
 drivers/nvme/host/tcp.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/tcp.c b/drivers/nvme/host/tcp.c
index 62fbaecdc960..902fe742762b 100644
--- a/drivers/nvme/host/tcp.c
+++ b/drivers/nvme/host/tcp.c

@@ -912,12 +912,11 @@ static int nvme_tcp_try_send_data(struct nvme_tcp_request *req)
 		else
 			flags |= MSG_MORE | MSG_SENDPAGE_NOTLAST;
 
-		/* can't zcopy slab pages */
-		if (unlikely(PageSlab(page))) {
-			ret = sock_no_sendpage(queue->sock, page, offset, len,
+		if (sendpage_ok(page)) {
+			ret = kernel_sendpage(queue->sock, page, offset, len,
 					flags);
 		} else {
-			ret = kernel_sendpage(queue->sock, page, offset, len,
+			ret = sock_no_sendpage(queue->sock, page, offset, len,
 					flags);
 		}
 		if (ret <= 0)

-- 
2.26.2

In _drbd_send_page() a page is checked by following code before sending
it by kernel_sendpage(),
        (page_count(page) < 1) || PageSlab(page)
If the check is true, this page won't be send by kernel_sendpage() and
handled by sock_no_sendpage().

This kind of check is exactly what macro sendpage_ok() does, which is
introduced into include/linux/net.h to solve a similar send page issue
in nvme-tcp code.

This patch uses macro sendpage_ok() to replace the open coded checks to
page type and refcount in _drbd_send_page(), as a code cleanup.

Signed-off-by: Coly Li <redacted>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
---
 drivers/block/drbd/drbd_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
index cb687ccdbd96..55dc0c91781e 100644
--- a/drivers/block/drbd/drbd_main.c
+++ b/drivers/block/drbd/drbd_main.c

@@ -1553,7 +1553,7 @@ static int _drbd_send_page(struct drbd_peer_device *peer_device, struct page *pa
 	 * put_page(); and would cause either a VM_BUG directly, or
 	 * __page_cache_release a page that would actually still be referenced
 	 * by someone, leading to some obscure delayed Oops somewhere else. */
-	if (drbd_disable_sendpage || (page_count(page) < 1) || PageSlab(page))
+	if (drbd_disable_sendpage || !sendpage_ok(page))
 		return _drbd_no_send_page(peer_device, page, offset, size, msg_flags);
 
 	msg_flags |= MSG_NOSIGNAL;

-- 
2.26.2

On 8/16/20 12:08 AM, Coly Li wrote:

return (A);

Can simply be written :

return A;

In this case :

return !PageSlab(page) && page_count(page) >= 1;

BTW, do you have plans to refine code added with commit a10674bf2406afc2554f9c7d31b2dc65d6a27fd9
("tcp: detecting the misuse of .sendpage for Slab objects")

On 2020/8/18 16:08, Eric Dumazet wrote:

On 8/16/20 12:08 AM, Coly Li wrote:

quoted

The original problem was from nvme-over-tcp code, who mistakenly uses
kernel_sendpage() to send pages allocated by __get_free_pages() without
__GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
tail pages, sending them by kernel_sendpage() may trigger a kernel panic
from a corrupted kernel heap, because these pages are incorrectly freed
in network stack as page_count 0 pages.

This patch introduces a helper sendpage_ok(), it returns true if the
checking page,
- is not slab page: PageSlab(page) is false.
- has page refcount: page_count(page) is not zero

All drivers who want to send page to remote end by kernel_sendpage()
may use this helper to check whether the page is OK. If the helper does
not return true, the driver should try other non sendpage method (e.g.
sock_no_sendpage()) to handle the page.

diff --git a/include/linux/net.h b/include/linux/net.h
index d48ff1180879..a807fad31958 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h

@@ -21,6 +21,7 @@
 #include <linux/rcupdate.h>
 #include <linux/once.h>
 #include <linux/fs.h>
+#include <linux/mm.h>
 #include <linux/sockptr.h>
 
 #include <uapi/linux/net.h>

@@ -286,6 +287,21 @@ do {									\
 #define net_get_random_once_wait(buf, nbytes)			\
 	get_random_once_wait((buf), (nbytes))
 
+/*
+ * E.g. XFS meta- & log-data is in slab pages, or bcache meta
+ * data pages, or other high order pages allocated by
+ * __get_free_pages() without __GFP_COMP, which have a page_count
+ * of 0 and/or have PageSlab() set. We cannot use send_page for
+ * those, as that does get_page(); put_page(); and would cause
+ * either a VM_BUG directly, or __page_cache_release a page that
+ * would actually still be referenced by someone, leading to some
+ * obscure delayed Oops somewhere else.
+ */
+static inline bool sendpage_ok(struct page *page)
+{
+	return  (!PageSlab(page) && page_count(page) >= 1);
+}

Hi Eric,

return (A);

Can simply be written :

return A;

In this case :

return !PageSlab(page) && page_count(page) >= 1;

Sure, I update it in v6 series.

BTW, do you have plans to refine code added with commit a10674bf2406afc2554f9c7d31b2dc65d6a27fd9
("tcp: detecting the misuse of .sendpage for Slab objects")

Thanks for the hint, I will remove this piece in v6 series.

Coly Li