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.
References
Configurations
Configuration 1 (hide)
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History
21 Oct 2026, 13:21
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| References |
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15 Oct 2026, 11:18
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| References |
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08 Oct 2026, 18:12
| Type | Values Removed | Values Added |
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| References | () https://git.kernel.org/stable/c/b55c9f019e169ed0d01394f96e172286a8b21b99 - Patch | |
| References | () https://git.kernel.org/stable/c/d3ef6c097ba078e1f8c7239d76a0ce8b61e75095 - Patch | |
| CVSS |
v2 : v3 : |
v2 : unknown
v3 : 5.5 |
| CWE | CWE-476 | |
| First Time |
Linux
Linux linux Kernel |
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| CPE | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* cpe:2.3:o:linux:linux_kernel:7.3:rc1:*:*:*:*:*:* |
25 Sep 2026, 11:17
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| New CVE |
Information
Published : 2026-09-25 11:17
Updated : 2026-10-21 13:21
NVD link : CVE-2026-98134
Mitre link : CVE-2026-98134
CVE.ORG link : CVE-2026-98134
JSON object : View
Products Affected
linux
- linux_kernel
CWE
CWE-476
NULL Pointer Dereference