linux-kernel - Re: [PATCH bpf-next v3 1/2] bpf: fix nullness propagation for reg to reg comparisons

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Date:   Thu, 22 Dec 2022 17:31:02 -0800
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
        andrii@...nel.org, song@...nel.org, yhs@...com, kpsingh@...nel.org,
        sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/2] bpf: fix nullness propagation for reg to
 reg comparisons

On 12/21/22 6:44 PM, Hao Sun wrote:
> After befae75856ab, the verifier would propagate null information after
> JEQ/JNE, e.g., if two pointers, one is maybe_null and the other is not,
> the former would be marked as non-null in eq path. However, as comment
> "PTR_TO_BTF_ID points to a kernel struct that does not need to be null
> checked by the BPF program ... The verifier must keep this in mind and
> can make no assumptions about null or non-null when doing branch ...".
> If one pointer is maybe_null and the other is PTR_TO_BTF, the former is
> incorrectly marked non-null. The following BPF prog can trigger a
> null-ptr-deref, also see this report for more details[1]:
> 
> 	0: (18) r1 = map_fd	        ; R1_w=map_ptr(ks=4, vs=4)
> 	2: (79) r6 = *(u64 *)(r1 +8)    ; R6_w=bpf_map->inner_map_data
> 					; R6 is PTR_TO_BTF_ID
> 					; equals to null at runtime
> 	3: (bf) r2 = r10
> 	4: (07) r2 += -4
> 	5: (62) *(u32 *)(r2 +0) = 0
> 	6: (85) call bpf_map_lookup_elem#1    ; R0_w=map_value_or_null
> 	7: (1d) if r6 == r0 goto pc+1
> 	8: (95) exit
> 	; from 7 to 9: R0=map_value R6=ptr_bpf_map
> 	9: (61) r0 = *(u32 *)(r0 +0)          ; null-ptr-deref
> 	10: (95) exit
> 
> So, make the verifier propagate nullness information for reg to reg
> comparisons only if neither reg is PTR_TO_BTF_ID.
> 
> [1] https://lore.kernel.org/bpf/CACkBjsaFJwjC5oiw-1KXvcazywodwXo4zGYsRHwbr2gSG9WcSw@mail.gmail.com/T/#u
> 
> Fixes: befae75856ab4 ("bpf: propagate nullness information for reg to reg comparisons")
The "Fixes" tag has one more hex digit. I have corrected it and applied to the 
bpf tree.  Thanks.

Please run checkpatch.pl in the future:

WARNING: Please use correct Fixes: style 'Fixes: <12 chars of sha1> ("<title 
line>")' - ie: 'Fixes: befae75856ab ("bpf: propagate nullness information for 
reg to reg comparisons")'
#35:
Fixes: befae75856ab4 ("bpf: propagate nullness information for reg to reg 
comparisons")