CVE-2021-47544

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)