bpf: check_cond_jmp_op(): properly infer if register is null

Summary

In the Linux kernel, the following vulnerability has been resolved: bpf: check_cond_jmp_op(): properly infer if register is null Nicholas Carlini reported a bug when verifier can incorrectly infer that a pointer is non-null. The bug occurs when two pointers are compared and one of them has a type w/o PTR_MAYBE_NULL flag, but which allows a value to be NULL at runtime. Here is an example: // `a` is PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED // `a` is 0 at runtime. // `b` is PTR_TO_MAP_VALUE | PTR_MAYBE_NULL void *a = bpf_rdonly_cast(0, 0); int *b = bpf_map_lookup_elem(...); if (a == b) *b = 42; // verifier does not catch null pointer dereference This happens because of a special case in check_cond_jmp_op(), which attempts to strip PTR_MAYBE_NULL flags from pointer types, when processing comparisons like `rA == rB`, if either rA or rB can't be null. The non-null property is derived based on the absence of PTR_MAYBE_NULL flag on rA's or rB's type. But that is not sufficient for types like PTR_TO_MEM, as in the example. This patch replaces type_may_be_null() call with reg_not_null(), which contains an allowlist of types for which absence of PTR_MAYBE_NULL actually means that the value can't be NULL at runtime. At the moment, the list in the reg_not_null() omits two types for which PTR_MAYBE_NULL is applicable: PTR_TO_XDP_SOCK and PTR_TO_BUF. In order to remain backward compatible, and assuming that only comparison between pointers of the same type makes sense, this commit extends reg_not_null(). W/o such an extension e.g. verifier_jeq_infer_not_null/null_ptr_to_map_value fails. reg_not_null() can be extended further, but I deem that out of scope for the fix at hand. Explicit base_type(...) != PTR_TO_BTF_ID checks in the check_cond_jmp_op() can be removed with migration to reg_not_null(), but that is a behavioural change, as the special case would start matching for PTR_TO_BTF_ID that is also is_trusted_reg(). I omit the behavioural change from this commit.

Timeline

1. Sep 25, 2026, 10:36 AM UTCCVE published
2. Oct 21, 2026, 12:54 PM UTCRecord last updated

Affected versions

• Linux Linux

  From befae75856ab406a3f3fab2aa2118cf3b2dfe3e6 · fixed in d51cd486930f5bcfd4b55d55ab8e47b03f93d2ed; From befae75856ab406a3f3fab2aa2118cf3b2dfe3e6 · fixed in dfd8f8adda565e08c370c45cf9d6b2511ba9b5c6; From befae75856ab406a3f3fab2aa2118cf3b2dfe3e6 · fixed in 9bf96b4347f541604b3caadeabda0773d9ceaf2a; From befae75856ab406a3f3fab2aa2118cf3b2dfe3e6 · fixed in b55c9f019e169ed0d01394f96e172286a8b21b99; From befae75856ab406a3f3fab2aa2118cf3b2dfe3e6 · fixed in d3ef6c097ba078e1f8c7239d76a0ce8b61e75095

• Linux Linux

  6.2

• KernelLinux

  From 6.2.0 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.114; From 6.13.0 · fixed in 6.18.57; From 6.19.0 · fixed in 7.2.7

References 5