tcp: fix page frag corruption on page fault

In the Linux kernel, the following vulnerability has been resolved:

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)