CVE-2024-36927 in Linux
Summary
by MITRE • 05/30/2024
In the Linux kernel, the following vulnerability has been resolved:
ipv4: Fix uninit-value access in __ip_make_skb()
KMSAN reported uninit-value access in __ip_make_skb() [1]. __ip_make_skb()
tests HDRINCL to know if the skb has icmphdr. However, HDRINCL can cause a race condition. If calling setsockopt(2) with IP_HDRINCL changes HDRINCL while __ip_make_skb() is running, the function will access icmphdr in the skb even if it is not included. This causes the issue reported by KMSAN.
Check FLOWI_FLAG_KNOWN_NH on fl4->flowi4_flags instead of testing HDRINCL on the socket.
Also, fl4->fl4_icmp_type and fl4->fl4_icmp_code are not initialized. These are union in struct flowi4 and are implicitly initialized by flowi4_init_output(), but we should not rely on specific union layout.
Initialize these explicitly in raw_sendmsg().
[1]
BUG: KMSAN: uninit-value in __ip_make_skb+0x2b74/0x2d20 net/ipv4/ip_output.c:1481 __ip_make_skb+0x2b74/0x2d20 net/ipv4/ip_output.c:1481 ip_finish_skb include/net/ip.h:243 [inline]
ip_push_pending_frames+0x4c/0x5c0 net/ipv4/ip_output.c:1508 raw_sendmsg+0x2381/0x2690 net/ipv4/raw.c:654 inet_sendmsg+0x27b/0x2a0 net/ipv4/af_inet.c:851 sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg+0x274/0x3c0 net/socket.c:745 __sys_sendto+0x62c/0x7b0 net/socket.c:2191 __do_sys_sendto net/socket.c:2203 [inline]
__se_sys_sendto net/socket.c:2199 [inline]
__x64_sys_sendto+0x130/0x200 net/socket.c:2199 do_syscall_64+0xd8/0x1f0 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x6d/0x75
Uninit was created at: slab_post_alloc_hook mm/slub.c:3804 [inline]
slab_alloc_node mm/slub.c:3845 [inline]
kmem_cache_alloc_node+0x5f6/0xc50 mm/slub.c:3888 kmalloc_reserve+0x13c/0x4a0 net/core/skbuff.c:577 __alloc_skb+0x35a/0x7c0 net/core/skbuff.c:668 alloc_skb include/linux/skbuff.h:1318 [inline]
__ip_append_data+0x49ab/0x68c0 net/ipv4/ip_output.c:1128 ip_append_data+0x1e7/0x260 net/ipv4/ip_output.c:1365 raw_sendmsg+0x22b1/0x2690 net/ipv4/raw.c:648 inet_sendmsg+0x27b/0x2a0 net/ipv4/af_inet.c:851 sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg+0x274/0x3c0 net/socket.c:745 __sys_sendto+0x62c/0x7b0 net/socket.c:2191 __do_sys_sendto net/socket.c:2203 [inline]
__se_sys_sendto net/socket.c:2199 [inline]
__x64_sys_sendto+0x130/0x200 net/socket.c:2199 do_syscall_64+0xd8/0x1f0 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x6d/0x75
CPU: 1 PID: 15709 Comm: syz-executor.7 Not tainted 6.8.0-11567-gb3603fcb79b1 #25 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-1.fc39 04/01/2014
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Analysis
by VulDB Data Team • 12/04/2025
The vulnerability described in CVE-2024-36927 resides within the Linux kernel's IPv4 networking stack, specifically in the `__ip_make_skb()` function located in `net/ipv4/ip_output.c`. This flaw manifests as an uninitialized value access, a category of issue commonly classified under CWE-457: Use of Uninitialized Variable. The vulnerability was identified through KMSAN (Kernel Memory Sanitizer), a tool designed to detect memory safety issues in kernel code, and is particularly concerning due to its potential for causing unpredictable behavior or system instability. The root cause lies in how the function handles the `HDRINCL` socket option, which allows applications to include IP headers in raw socket packets, creating a race condition scenario where concurrent access to socket flags during packet processing leads to invalid memory access.
The problematic code path involves the `__ip_make_skb()` function checking the `HDRINCL` flag to determine whether an ICMP header is present in the socket buffer (skb). However, this check is not atomic and can be modified by a concurrent `setsockopt()` call with `IP_HDRINCL`, leading to a situation where the function accesses `icmphdr` in the skb even when it is not properly initialized or included. This race condition results in accessing uninitialized memory locations, which KMSAN flags as a critical error. The vulnerability is further exacerbated by the fact that `fl4->fl4_icmp_type` and `fl4->fl4_icmp_code` fields within the `flowi4` structure are not explicitly initialized, despite being part of a union that gets implicitly initialized by `flowi4_init_output()`. This implicit initialization is unreliable and violates best practices for memory safety, as highlighted in CWE-665: Improper Initialization.
The operational impact of this vulnerability is significant, particularly in environments where raw sockets and concurrent network operations are common. An attacker could potentially exploit this race condition to cause system crashes, data corruption, or even privilege escalation depending on the context of use. The vulnerability is triggered during packet transmission through the `raw_sendmsg()` function, which is part of the raw socket interface and commonly used in network utilities and security tools. The stack trace indicates the vulnerability flows through `raw_sendmsg` → `inet_sendmsg` → `sock_sendmsg_nosec` → `__sock_sendmsg` → `__sys_sendto`, emphasizing the deep integration into core network I/O operations. This path suggests that any application using raw sockets with concurrent modifications to `IP_HDRINCL` could be at risk, making it a critical issue for network security and system stability.
The fix implemented addresses the core issue by replacing the `HDRINCL` flag check with a check against `FLOWI_FLAG_KNOWN_NH` in `fl4->flowi4_flags`, which provides a more reliable and atomic method for determining packet header inclusion. Additionally, the solution ensures explicit initialization of `fl4->fl4_icmp_type` and `fl4->fl4_icmp_code` fields in `raw_sendmsg()` to eliminate reliance on implicit union initialization. This approach aligns with security best practices and mitigates the race condition by ensuring that packet header state is properly synchronized and initialized before access. The fix also reflects principles from the ATT&CK framework under T1059.001: Command and Scripting Interpreter - PowerShell, as it addresses a fundamental system-level vulnerability that could be exploited to gain unauthorized access or disrupt services. The mitigation strategy involves ensuring proper synchronization of socket flags and explicit memory initialization, reducing the attack surface for potential exploitation through concurrent network operations.