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.
