On 2020/9/25 23:18, Greg KH wrote:

On Fri, Sep 25, 2020 at 11:01:13PM +0800, 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.

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
---
 include/linux/net.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/net.h b/include/linux/net.h
index d48ff1180879..05db8690f67e 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;

Do you have one extra ' ' after "return" there?

It should be fixed in next version.

And this feels like a mm thing, why put it in net.h and not mm.h?

This check is specific for kernel_sendpage(), so I want to place it
closer to where kernel_sendpage() is declared.

And indeed there was similar discussion about why this helper is not in
mm code in v5 series. Christoph supported to place sendpage_ok() in
net.h, an uncompleted piece of his opinion was "It is not a mm bug, it
is a networking quirk."

Thanks.

Coly Li