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tcp: fix page frag corruption on page fault

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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 20eb4f29b6 ("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)

Reported-by: Steffen Froemer <sfroemer@redhat.com>
Fixes: 5640f76858 ("net: use a per task frag allocator")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Paolo Abeni 2021-11-26 19:34:21 +01:00 committed by David S. Miller
parent f8e7dfd6fd
commit dacb5d8875
1 changed files with 8 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
include/net/sock.h
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@ -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;