In the Linux kernel, the following vulnerability has been resolved:tcp: fix page frag corruption on page faultSteffen reported a TCP stream corruption for HTTP requestsserved by the apache web-server using a cifs mount-pointand memory mapping the relevant file.The root cause is quite similar to the one addressed bycommit 20eb4f29b602 ("net: fix sk_page_frag() recursion frommemory reclaim"). Here the nested access to the task page fragis caused by a page fault on the (mmapped) user-space memorybuffer coming from the cifs file.The page fault handler performs an smb transaction on a differentsocket, inside the same process context. Since sk->sk_allactionfor such socket does not prevent the usage for the task_frag,the nested allocation modify "under the hood" the page fragin 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+0x65The cifs filesystem rightfully sets sk_allocations to GFP_NOFS,we can avoid the nesting using the sk page frag for allocationlacking the __GFP_FS flag. Do not define an additional mm-helperfor 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)
CREATE(Triage):(User=admin) [CVE-2021-47544 (https://nvd.nist.gov/vuln/detail/CVE-2021-47544)
