CVE-2026-98132

In the Linux kernel, the following vulnerability has been resolved:

bpf: don't downgrade half-dead scalar zero spills to STACK_ZERO

states.c:__clean_func_state() can downgrade scalar zero spill to

STACK_ZERO in the following case:

*(u64 *)(r10 - 8) = 0;

... checkpoint ...

r1 = *(u32 *)(r10 - 4);

... no reads from r10-8 ...

Here 4 bytes at r10-8 are dead and verifier changes scalar spill to a

combination: 0000pppp (p stands for poison). Such a change breaks

precision propagation chains. All places that produce STACK_ZERO

should call bpf_mark_chain_precision() for the zero source.

This patch fixes the bug in a simplest way possible:

avoids converting stack spills of zero to STACK_ZERO.

Two smarter approaches are possible:

- do bpf_mark_chain_precision() from __clean_func_state()

- check slot liveness information in check_stack_write_fixed_off()

I investigated both and the changes required are a bit tricky,

hence go with a simple fix for the time being.