CVE-2021-47544 in Linux
Summary
by MITRE • 05/24/2024
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)
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Analysis
by VulDB Data Team • 09/19/2025
The vulnerability described in CVE-2021-47544 represents a critical TCP stream corruption issue within the Linux kernel that stems from improper handling of page fragment management during memory fault conditions. This flaw manifests specifically when Apache HTTPD serves content through CIFS mount points while utilizing memory mapping for file access. The root cause lies in the recursive access pattern that occurs when a page fault handler executes SMB transactions on different sockets within the same process context, creating a scenario where nested memory allocation modifies the page fragment currently in use by an outer sendmsg operation.
The technical implementation of this vulnerability involves a complex interaction between multiple kernel subsystems including TCP networking, memory management, and CIFS file system operations. When a page fault occurs during the reading of mapped CIFS files, the fault handler performs SMB transactions on alternative sockets that share the same memory management context. The socket's sk_page_frag() function, which is designed to manage page fragment allocation for TCP operations, becomes corrupted because the page fragment structure is modified during the nested allocation process. This corruption directly impacts the TCP stream integrity as the outer sendmsg call continues with an inconsistent page fragment state, leading to data corruption in HTTP responses.
The operational impact of this vulnerability extends beyond simple data corruption, potentially enabling attackers to manipulate HTTP responses served through Apache when using CIFS mounts, which could lead to information disclosure or service disruption. The vulnerability is particularly concerning in enterprise environments where CIFS mounts are commonly used for shared file access and where Apache HTTPD serves dynamic content. The stack trace demonstrates a clear recursive pattern where tcp_sendmsg_locked calls are interrupted by page fault handling which in turn triggers CIFS read operations that modify the same page fragment structure, creating a race condition that corrupts the TCP stream. This type of vulnerability falls under CWE-367, specifically addressing Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities in kernel memory management.
Mitigation strategies for this vulnerability focus on preventing the recursive access pattern that causes the page fragment corruption. The fix implements a stricter check in sk_page_frag() that prevents the use of page fragments during memory allocation operations that lack the __GFP_FS flag, effectively avoiding the nesting issue. This approach aligns with ATT&CK technique T1059.003 for privilege escalation through kernel exploits and represents a defensive coding pattern that prevents recursive memory management operations. Organizations should prioritize applying the kernel patch that implements this fix, particularly in environments running Apache HTTPD with CIFS mount points. The solution avoids introducing additional memory management helpers and instead modifies the existing page fragment allocation logic to maintain consistency during concurrent memory operations. Security teams should monitor for any potential side effects of this patch on memory allocation performance and ensure that the fix does not inadvertently impact other legitimate network operations.