medium CVSS 5.5 EPSS <0.1%

CVE-2021-47370·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: mptcp: ensure tx skbs always have the MPTCP ext Due to signed/unsigned comparison, the expression: info->size_goal - skb->len > 0 evaluates to true when the size goal is smaller than the skb size. That results in lack of tx cache refill, so that the skb allocated by the core TCP code lacks the required MPTCP skb extensions. Due to the above, syzbot is able to trigger the following WARN_ON(): WARNING: CPU: 1 PID: 810 at net/mptcp/protocol.c:1366 mptcp_sendmsg_frag+0x1362/0x1bc0 net/mptcp/protocol.c:1366 Modules linked in: CPU: 1 PID: 810 Comm: syz-executor.4 Not tainted 5.14.0-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 RIP: 0010:mptcp_sendmsg_frag+0x1362/0x1bc0 net/mptcp/protocol.c:1366 Code: ff 4c 8b 74 24 50 48 8b 5c 24 58 e9 0f fb ff ff e8 13 44 8b f8 4c 89 e7 45 31 ed e8 98 57 2e fe e9 81 f4 ff ff e8 fe 43 8b f8 <0f> 0b 41 bd ea ff ff ff e9 6f f4 ff ff 4c 89 e7 e8 b9 8e d2 f8 e9 RSP: 0018:ffffc9000531f6a0 EFLAGS: 00010216 RAX: 000000000000697f RBX: 0000000000000000 RCX: ffffc90012107000 RDX: 0000000000040000 RSI: ffffffff88eac9e2 RDI: 0000000000000003 RBP: ffff888078b15780 R08: 0000000000000000 R09: 0000000000000000 R10: ffffffff88eac017 R11: 0000000000000000 R12: ffff88801de0a280 R13: 0000000000006b58 R14: ffff888066278280 R15: ffff88803c2fe9c0 FS: 00007fd9f866e700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007faebcb2f718 CR3: 00000000267cb000 CR4: 00000000001506e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: __mptcp_push_pending+0x1fb/0x6b0 net/mptcp/protocol.c:1547 mptcp_release_cb+0xfe/0x210 net/mptcp/protocol.c:3003 release_sock+0xb4/0x1b0 net/core/sock.c:3206 sk_stream_wait_memory+0x604/0xed0 net/core/stream.c:145 mptcp_sendmsg+0xc39/0x1bc0 net/mptcp/protocol.c:1749 inet6_sendmsg+0x99/0xe0 net/ipv6/af_inet6.c:643 sock_sendmsg_nosec net/socket.c:704 [inline] sock_sendmsg+0xcf/0x120 net/socket.c:724 sock_write_iter+0x2a0/0x3e0 net/socket.c:1057 call_write_iter include/linux/fs.h:2163 [inline] new_sync_write+0x40b/0x640 fs/read_write.c:507 vfs_write+0x7cf/0xae0 fs/read_write.c:594 ksys_write+0x1ee/0x250 fs/read_write.c:647 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x4665f9 Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007fd9f866e188 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 RAX: ffffffffffffffda RBX: 000000000056c038 RCX: 00000000004665f9 RDX: 00000000000e7b78 RSI: 0000000020000000 RDI: 0000000000000003 RBP: 00000000004bfcc4 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 000000000056c038 R13: 0000000000a9fb1f R14: 00007fd9f866e300 R15: 0000000000022000 Fix the issue rewriting the relevant expression to avoid sign-related problems - note: size_goal is always >= 0. Additionally, ensure that the skb in the tx cache always carries the relevant extension.

Severity
medium
Software
Kernel
Fixed in
5.14.9
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 6% 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: 5.14.7

Until: 5.14.9

Fixed in: 5.14.9

How to fix this CVE

Update your Linux kernel to version 5.14.9 or later to address a critical MPTCP transmission buffer handling flaw. This vulnerability affects kernel versions 5.14.7 through 5.14.9 and causes system instability when MPTCP extensions are improperly allocated during packet transmission. Apply the kernel update via your distribution's package manager and reboot your system to fully resolve the issue.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check kernel version with `uname -r` and compare against affected range 5.14.7-5.14.9
  2. Step 2: Verify MPTCP support is enabled in kernel with `grep CONFIG_MPTCP /boot/config-$(uname -r)` (should show CONFIG_MPTCP=y)
  3. Step 3: Search kernel logs for MPTCP warnings with `dmesg | grep -i 'mptcp_sendmsg_frag\|WARN_ON'` or `journalctl -k | grep -i mptcp`
  4. Step 4: After applying patches, verify new kernel version with `uname -r` and confirm it is 5.14.9 or later, then check for absence of MPTCP-related WARN_ON messages in recent logs

FAQ

What is CVE-2021-47370?

This vulnerability is a type confusion bug in the Linux kernel's MPTCP implementation where a signed/unsigned comparison error causes the transmission buffer cache to fail refilling, leaving transmitted socket buffers without required MPTCP extensions. This triggers kernel warnings and potential denial of service on systems using MPTCP.

Is CVE-2021-47370 being actively exploited?

No, this vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog and has no publicly available exploit code, though it can be reliably triggered through fuzzing.

What versions of Kernel are affected by CVE-2021-47370?

Linux kernel versions 5.14.7, 5.14.8, and 5.14.9 are affected. The fix was included in kernel 5.14.9 and later releases.

How do I check if my server is vulnerable to CVE-2021-47370?

Run `uname -r` and verify if your kernel version falls between 5.14.7 and 5.14.9, then check `grep CONFIG_MPTCP /boot/config-$(uname -r)` to confirm MPTCP is enabled on your system.

Does Defensia detect CVE-2021-47370?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2021-47370 will appear in your dashboard with remediation steps.

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References

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