medium CVSS 5.9 EPSS <0.1%

CVE-2021-47544·Kernel vulnerability

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)

Severity
medium
Software
Kernel
Fixed in
5.15.7
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 17% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

From: 5.11

Until: 5.15.7

Fixed in: 5.15.7

How to fix this CVE

Update your Linux kernel to version 5.15.7 or later to resolve this TCP stream corruption vulnerability affecting memory-mapped file operations. This patch prevents nested page fragment allocation that can corrupt data when HTTP requests are served from CIFS mount-points or other scenarios involving page faults during socket operations. Apply the update immediately to all production systems, particularly those using CIFS filesystems with memory-mapped I/O.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check installed kernel version with 'uname -r' and compare against 5.15.7 (vulnerable if < 5.15.7 on 5.11-5.15 series)
  2. Step 2: Verify CIFS mounts are active with 'mount | grep cifs' and check if applications use memory-mapped I/O on these mounts
  3. Step 3: Search system logs for TCP corruption indicators with 'journalctl -b | grep -i "tcp\|corruption\|cifs"' and check application error logs for data integrity issues
  4. Step 4: After patching, reboot the system with 'sudo reboot' and confirm new kernel version with 'uname -r' to ensure update is active

FAQ

What is CVE-2021-47544?

This vulnerability is a TCP stream corruption bug in the Linux kernel caused by nested page fragment allocations when handling page faults during socket operations. It specifically affects scenarios where HTTP requests are served from CIFS-mounted files with memory-mapped buffers, leading to data corruption in the TCP stream.

Is CVE-2021-47544 being actively exploited?

No, this vulnerability is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available. However, it can cause silent data corruption in production environments.

What versions of Kernel are affected by CVE-2021-47544?

Linux kernel versions 5.11 through 5.15.6 are vulnerable. The fix was released in kernel 5.15.7 and backported to stable branches.

How do I check if my server is vulnerable to CVE-2021-47544?

Run 'uname -r' to check your kernel version. If it shows a version between 5.11 and 5.15.6, your system is vulnerable. Additionally, check if CIFS is mounted with 'mount | grep cifs' to determine exposure risk.

Does Defensia detect CVE-2021-47544?

Yes — Defensia's CVE advisory scanner compares installed kernel versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2021-47544 will appear in your dashboard with remediation steps.

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References

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