CVE-2023-52881 (GCVE-0-2023-52881)
Vulnerability from cvelistv5
Published
2024-05-29 10:15
Modified
2025-05-04 12:49
Severity ?
Summary
In the Linux kernel, the following vulnerability has been resolved: tcp: do not accept ACK of bytes we never sent This patch is based on a detailed report and ideas from Yepeng Pan and Christian Rossow. ACK seq validation is currently following RFC 5961 5.2 guidelines: The ACK value is considered acceptable only if it is in the range of ((SND.UNA - MAX.SND.WND) <= SEG.ACK <= SND.NXT). All incoming segments whose ACK value doesn't satisfy the above condition MUST be discarded and an ACK sent back. It needs to be noted that RFC 793 on page 72 (fifth check) says: "If the ACK is a duplicate (SEG.ACK < SND.UNA), it can be ignored. If the ACK acknowledges something not yet sent (SEG.ACK > SND.NXT) then send an ACK, drop the segment, and return". The "ignored" above implies that the processing of the incoming data segment continues, which means the ACK value is treated as acceptable. This mitigation makes the ACK check more stringent since any ACK < SND.UNA wouldn't be accepted, instead only ACKs that are in the range ((SND.UNA - MAX.SND.WND) <= SEG.ACK <= SND.NXT) get through. This can be refined for new (and possibly spoofed) flows, by not accepting ACK for bytes that were never sent. This greatly improves TCP security at a little cost. I added a Fixes: tag to make sure this patch will reach stable trees, even if the 'blamed' patch was adhering to the RFC. tp->bytes_acked was added in linux-4.2 Following packetdrill test (courtesy of Yepeng Pan) shows the issue at hand: 0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 +0 bind(3, ..., ...) = 0 +0 listen(3, 1024) = 0 // ---------------- Handshake ------------------- // // when window scale is set to 14 the window size can be extended to // 65535 * (2^14) = 1073725440. Linux would accept an ACK packet // with ack number in (Server_ISN+1-1073725440. Server_ISN+1) // ,though this ack number acknowledges some data never // sent by the server. +0 < S 0:0(0) win 65535 <mss 1400,nop,wscale 14> +0 > S. 0:0(0) ack 1 <...> +0 < . 1:1(0) ack 1 win 65535 +0 accept(3, ..., ...) = 4 // For the established connection, we send an ACK packet, // the ack packet uses ack number 1 - 1073725300 + 2^32, // where 2^32 is used to wrap around. // Note: we used 1073725300 instead of 1073725440 to avoid possible // edge cases. // 1 - 1073725300 + 2^32 = 3221241997 // Oops, old kernels happily accept this packet. +0 < . 1:1001(1000) ack 3221241997 win 65535 // After the kernel fix the following will be replaced by a challenge ACK, // and prior malicious frame would be dropped. +0 > . 1:1(0) ack 1001
References
Impacted products
Vendor Product Version
Linux Linux Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 354e4aa391ed50a4d827ff6fc11e0667d0859b25
Version: 8d15569e14cfcf9151e9e3b4c0cb98369943a2bb
Version: e252bbd8c87b95e9cecdc01350fbb0b46a0f9bf1
Version: 2ee4432e82437a7c051c254b065fbf5d4581e1a3
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\"lessThanOrEqual\": \"4.19.*\"}, {\"status\": \"unaffected\", \"version\": \"5.4.264\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.4.*\"}, {\"status\": \"unaffected\", \"version\": \"5.10.204\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.10.*\"}, {\"status\": \"unaffected\", \"version\": \"5.15.143\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.15.*\"}, {\"status\": \"unaffected\", \"version\": \"6.1.68\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.1.*\"}, {\"status\": \"unaffected\", \"version\": \"6.6.7\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"6.6.*\"}, {\"status\": \"unaffected\", \"version\": \"6.7\", \"versionType\": \"original_commit_for_fix\", \"lessThanOrEqual\": \"*\"}], \"programFiles\": [\"net/ipv4/tcp_input.c\"], \"defaultStatus\": \"affected\"}], \"references\": [{\"url\": \"https://git.kernel.org/stable/c/69eae75ca5255e876628ac5cee9eaab31f644b57\"}, {\"url\": \"https://git.kernel.org/stable/c/458f07ffeccd17f99942311e09ef574ddf4a414a\"}, {\"url\": \"https://git.kernel.org/stable/c/7ffff0cc929fdfc62a74b384c4903d6496c910f0\"}, {\"url\": \"https://git.kernel.org/stable/c/b17a886ed29f3b70b78ccf632dad03e0c69e3c1a\"}, {\"url\": \"https://git.kernel.org/stable/c/0d4e0afdd6658cd21dd5be61880411a2553fd1fc\"}, {\"url\": \"https://git.kernel.org/stable/c/008b807fe487e0b15a3a6c39add4eb477f73e440\"}, {\"url\": \"https://git.kernel.org/stable/c/2087d53a66e97a5eb5d1bf558d5bef9e5f891757\"}, {\"url\": \"https://git.kernel.org/stable/c/3d501dd326fb1c73f1b8206d4c6e1d7b15c07e27\"}], \"x_generator\": {\"engine\": \"bippy-5f407fcff5a0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"In the Linux kernel, the following vulnerability has been resolved:\\n\\ntcp: do not accept ACK of bytes we never sent\\n\\nThis patch is based on a detailed report and ideas from Yepeng Pan\\nand Christian Rossow.\\n\\nACK seq validation is currently following RFC 5961 5.2 guidelines:\\n\\n   The ACK value is considered acceptable only if\\n   it is in the range of ((SND.UNA - MAX.SND.WND) \u003c= SEG.ACK \u003c=\\n   SND.NXT).  All incoming segments whose ACK value doesn\u0027t satisfy the\\n   above condition MUST be discarded and an ACK sent back.  It needs to\\n   be noted that RFC 793 on page 72 (fifth check) says: \\\"If the ACK is a\\n   duplicate (SEG.ACK \u003c SND.UNA), it can be ignored.  If the ACK\\n   acknowledges something not yet sent (SEG.ACK \u003e SND.NXT) then send an\\n   ACK, drop the segment, and return\\\".  The \\\"ignored\\\" above implies that\\n   the processing of the incoming data segment continues, which means\\n   the ACK value is treated as acceptable.  This mitigation makes the\\n   ACK check more stringent since any ACK \u003c SND.UNA wouldn\u0027t be\\n   accepted, instead only ACKs that are in the range ((SND.UNA -\\n   MAX.SND.WND) \u003c= SEG.ACK \u003c= SND.NXT) get through.\\n\\nThis can be refined for new (and possibly spoofed) flows,\\nby not accepting ACK for bytes that were never sent.\\n\\nThis greatly improves TCP security at a little cost.\\n\\nI added a Fixes: tag to make sure this patch will reach stable trees,\\neven if the \u0027blamed\u0027 patch was adhering to the RFC.\\n\\ntp-\u003ebytes_acked was added in linux-4.2\\n\\nFollowing packetdrill test (courtesy of Yepeng Pan) shows\\nthe issue at hand:\\n\\n0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3\\n+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0\\n+0 bind(3, ..., ...) = 0\\n+0 listen(3, 1024) = 0\\n\\n// ---------------- Handshake ------------------- //\\n\\n// when window scale is set to 14 the window size can be extended to\\n// 65535 * (2^14) = 1073725440. Linux would accept an ACK packet\\n// with ack number in (Server_ISN+1-1073725440. Server_ISN+1)\\n// ,though this ack number acknowledges some data never\\n// sent by the server.\\n\\n+0 \u003c S 0:0(0) win 65535 \u003cmss 1400,nop,wscale 14\u003e\\n+0 \u003e S. 0:0(0) ack 1 \u003c...\u003e\\n+0 \u003c . 1:1(0) ack 1 win 65535\\n+0 accept(3, ..., ...) = 4\\n\\n// For the established connection, we send an ACK packet,\\n// the ack packet uses ack number 1 - 1073725300 + 2^32,\\n// where 2^32 is used to wrap around.\\n// Note: we used 1073725300 instead of 1073725440 to avoid possible\\n// edge cases.\\n// 1 - 1073725300 + 2^32 = 3221241997\\n\\n// Oops, old kernels happily accept this packet.\\n+0 \u003c . 1:1001(1000) ack 3221241997 win 65535\\n\\n// After the kernel fix the following will be replaced by a challenge ACK,\\n// and prior malicious frame would be dropped.\\n+0 \u003e . 1:1(0) ack 1001\"}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2024-12-19T08:27:49.339Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2023-52881\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2024-12-19T08:27:49.339Z\", \"dateReserved\": \"2024-05-21T15:35:00.781Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2024-05-29T10:15:14.186Z\", \"assignerShortName\": \"Linux\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.1"
    }
  }
}


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