Wind River Support Network

Description

In the Linux kernel, the following vulnerability has been resolved:  bpf: Fix incorrect pruning due to atomic fetch precision tracking  When backtrack_insn encounters a BPF_STX instruction with BPF_ATOMIC and BPF_FETCH, the src register (or r0 for BPF_CMPXCHG) also acts as a destination, thus receiving the old value from the memory location.  The current backtracking logic does not account for this. It treats atomic fetch operations the same as regular stores where the src register is only an input. This leads the backtrack_insn to fail to propagate precision to the stack location, which is then not marked as precise!  Later, the verifier's path pruning can incorrectly consider two states equivalent when they differ in terms of stack state. Meaning, two branches can be treated as equivalent and thus get pruned when they should not be seen as such.  Fix it as follows: Extend the BPF_LDX handling in backtrack_insn to also cover atomic fetch operations via is_atomic_fetch_insn() helper. When the fetch dst register is being tracked for precision, clear it, and propagate precision over to the stack slot. For non-stack memory, the precision walk stops at the atomic instruction, same as regular BPF_LDX. This covers all fetch variants.  Before:    0: (b7) r1 = 8                        ; R1=8   1: (7b) *(u64 *)(r10 -8) = r1         ; R1=8 R10=fp0 fp-8=8   2: (b7) r2 = 0                        ; R2=0   3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)          ; R2=8 R10=fp0 fp-8=mmmmmmmm   4: (bf) r3 = r10                      ; R3=fp0 R10=fp0   5: (0f) r3 += r2   mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1   mark_precise: frame0: regs=r2 stack= before 4: (bf) r3 = r10   mark_precise: frame0: regs=r2 stack= before 3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)   mark_precise: frame0: regs=r2 stack= before 2: (b7) r2 = 0   6: R2=8 R3=fp8   6: (b7) r0 = 0                        ; R0=0   7: (95) exit  After:    0: (b7) r1 = 8                        ; R1=8   1: (7b) *(u64 *)(r10 -8) = r1         ; R1=8 R10=fp0 fp-8=8   2: (b7) r2 = 0                        ; R2=0   3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)          ; R2=8 R10=fp0 fp-8=mmmmmmmm   4: (bf) r3 = r10                      ; R3=fp0 R10=fp0   5: (0f) r3 += r2   mark_precise: frame0: last_idx 5 first_idx 0 subseq_idx -1   mark_precise: frame0: regs=r2 stack= before 4: (bf) r3 = r10   mark_precise: frame0: regs=r2 stack= before 3: (db) r2 = atomic64_fetch_add((u64 *)(r10 -8), r2)   mark_precise: frame0: regs= stack=-8 before 2: (b7) r2 = 0   mark_precise: frame0: regs= stack=-8 before 1: (7b) *(u64 *)(r10 -8) = r1   mark_precise: frame0: regs=r1 stack= before 0: (b7) r1 = 8   6: R2=8 R3=fp8   6: (b7) r0 = 0                        ; R0=0   7: (95) exit

========Wind River Notice========
*Mitigation:*
Customers can use kernel.unprivileged_bpf_disabled sysctl to prevent unprivileged users from being able to use eBPF. This would require a privileged user with CAP_SYS_ADMIN or root to be able to abuse this flaw reducing its attack space.

Inspect kernel.unprivileged_bpf_disabled sysctl with the command:

cat /proc/sys/kernel/unprivileged_bpf_disabled

The setting of 1 would mean that unprivileged users can not use eBPF, mitigating the flaw.

echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled


For more details, please refer to Linux kernel official document:
https://docs.kernel.org/admin-guide/sysctl/kernel.html#unprivileged-bpf-disabled

CVEs

• CVE-2026-43009