Re: [PATCH bpf-next v2 2/4] bpf: Fix narrow scalar spill onto 64-bit spilled scalar slots

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-11-27 at 13:20 -0800, Kumar Kartikeya Dwivedi wrote:
> From: Tao Lyu <tao.lyu@epfl.ch>
> 
> When CAP_PERFMON and CAP_SYS_ADMIN (allow_ptr_leaks) are disabled, the
> verifier aims to reject partial overwrite on an 8-byte stack slot that
> contains a spilled pointer.
> 
> However, in such a scenario, it rejects all partial stack overwrites as
> long as the targeted stack slot is a spilled register, because it does
> not check if the stack slot is a spilled pointer.
> 
> Incomplete checks will result in the rejection of valid programs, which
> spill narrower scalar values onto scalar slots, as shown below.
> 
> 0: R1=ctx() R10=fp0
> ; asm volatile ( @ repro.bpf.c:679
> 0: (7a) *(u64 *)(r10 -8) = 1          ; R10=fp0 fp-8_w=1
> 1: (62) *(u32 *)(r10 -8) = 1
> attempt to corrupt spilled pointer on stack
> processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0.
> 
> Fix this by expanding the check to not consider spilled scalar registers
> when rejecting the write into the stack.
> 
> Previous discussion on this patch is at link [0].
> 
>   [0]: https://lore.kernel.org/bpf/20240403202409.2615469-1-tao.lyu@epfl.ch
> 
> Fixes: ab125ed3ec1c ("bpf: fix check for attempt to corrupt spilled pointer")
> Signed-off-by: Tao Lyu <tao.lyu@epfl.ch>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

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