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

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.