net: fix __dst_negative_advice() race

Open Source Vulnerabilities ASB-A-343727534

In __dst_negative_advice of sock.h, there is a possible failure to clear sk->sk_dst_cache in the correct order resulting in a use after free. This could lead to remote code execution with System execution privileges needed. User interaction is not needed for exploitation.

Cybersecurity and Infrastructure Security Agency Known Exploited Vulnerabilities CVE-2024-36971

Android contains an unspecified vulnerability in the kernel that allows for remote code execution. This vulnerability resides in Linux Kernel and could impact other products, including but not limited to Android OS.

Open Source Vulnerabilities CVE-2024-36971

In the Linux kernel, the following vulnerability has been resolved: net: fix __dst_negative_advice() race __dst_negative_advice() does not enforce proper RCU rules when sk->dst_cache must be cleared, leading to possible UAF. RCU rules are that we must first clear sk->sk_dst_cache, then call dst_release(old_dst). Note that sk_dst_reset(sk) is implementing this protocol correctly, while __dst_negative_advice() uses the wrong order. Given that ip6_negative_advice() has special logic against RTF_CACHE, this means each of the three ->negative_advice() existing methods must perform the sk_dst_reset() themselves. Note the check against NULL dst is centralized in __dst_negative_advice(), there is no need to duplicate it in various callbacks. Many thanks to Clement Lecigne for tracking this issue. This old bug became visible after the blamed commit, using UDP sockets.