FAILED: patch "[PATCH] tcp: fix page frag corruption on page fault" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From dacb5d8875cc6cd3a553363b4d6f06760fcbe70c Mon Sep 17 00:00:00 2001
From: Paolo Abeni <[email protected]>
Date: Fri, 26 Nov 2021 19:34:21 +0100
Subject: [PATCH] tcp: fix page frag corruption on page fault

Steffen reported a TCP stream corruption for HTTP requests served by the apache web-server using a cifs mount-point and memory mapping the relevant file.

The root cause is quite similar to the one addressed by commit 20eb4f29b602 ("net: fix sk_page_frag() recursion from memory reclaim"). Here the nested access to the task page frag is caused by a page fault on the (mmapped) user-space memory buffer coming from the cifs file.

The page fault handler performs an smb transaction on a different socket, inside the same process context. Since sk->sk_allaction for such socket does not prevent the usage for the task_frag, the nested allocation modify "under the hood" the page frag in use by the outer sendmsg call, corrupting the stream.

The overall relevant stack trace looks like the following:

httpd 78268 [001] 3461630.850950: probe:tcp_sendmsg_locked:

        ffffffff91461d91 tcp_sendmsg_locked+0x1
        ffffffff91462b57 tcp_sendmsg+0x27
        ffffffff9139814e sock_sendmsg+0x3e
        ffffffffc06dfe1d smb_send_kvec+0x28
        [...]
        ffffffffc06cfaf8 cifs_readpages+0x213
        ffffffff90e83c4b read_pages+0x6b
        ffffffff90e83f31 __do_page_cache_readahead+0x1c1
        ffffffff90e79e98 filemap_fault+0x788
        ffffffff90eb0458 __do_fault+0x38
        ffffffff90eb5280 do_fault+0x1a0
        ffffffff90eb7c84 __handle_mm_fault+0x4d4
        ffffffff90eb8093 handle_mm_fault+0xc3
        ffffffff90c74f6d __do_page_fault+0x1ed
        ffffffff90c75277 do_page_fault+0x37
        ffffffff9160111e page_fault+0x1e
        ffffffff9109e7b5 copyin+0x25
        ffffffff9109eb40 _copy_from_iter_full+0xe0
        ffffffff91462370 tcp_sendmsg_locked+0x5e0
        ffffffff91462370 tcp_sendmsg_locked+0x5e0
        ffffffff91462b57 tcp_sendmsg+0x27
        ffffffff9139815c sock_sendmsg+0x4c
        ffffffff913981f7 sock_write_iter+0x97
        ffffffff90f2cc56 do_iter_readv_writev+0x156
        ffffffff90f2dff0 do_iter_write+0x80
        ffffffff90f2e1c3 vfs_writev+0xa3
        ffffffff90f2e27c do_writev+0x5c
        ffffffff90c042bb do_syscall_64+0x5b
        ffffffff916000ad entry_SYSCALL_64_after_hwframe+0x65

The cifs filesystem rightfully sets sk_allocations to GFP_NOFS, we can avoid the nesting using the sk page frag for allocation lacking the __GFP_FS flag. Do not define an additional mm-helper for that, as this is strictly tied to the sk page frag usage.

v1 -> v2:
- use a stricted sk_page_frag() check instead of reordering the code (Eric)

index b32906e1ab55..715cdb4b2b79 100644
@@ -2430,19 +2430,22 @@ static inline void sk_stream_moderate_sndbuf(struct sock *sk)
  * @sk: socket
  *
  * Use the per task page_frag instead of the per socket one for
- * optimization when we know that we're in the normal context and owns
+ * optimization when we know that we're in process context and own
  * everything that's associated with %current.
  *
- * gfpflags_allow_blocking() isn't enough here as direct reclaim may nest
- * inside other socket operations and end up recursing into sk_page_frag()
- * while it's already in use.
+ * Both direct reclaim and page faults can nest inside other
+ * socket operations and end up recursing into sk_page_frag()
+ * while it's already in use: explicitly avoid task page_frag
+ * usage if the caller is potentially doing any of them.
+ * This assumes that page fault handlers use the GFP_NOFS flags.
  *
  * Return: a per task page_frag if context allows that,
  * otherwise a per socket one.
  */
 static inline struct page_frag *sk_page_frag(struct sock *sk)
 {
-	if (gfpflags_normal_context(sk->sk_allocation))
+	if ((sk->sk_allocation & (__GFP_DIRECT_RECLAIM | __GFP_MEMALLOC | __GFP_FS)) ==
+	    (__GFP_DIRECT_RECLAIM | __GFP_FS))
 		return &current->task_frag;
 
 	return &sk->sk_frag;