CVE-2026-43009

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