Re: [PATCH bpf-next v3 1/2] bpf, verifier: fold reg->var_off into PTR_TO_FLOW_KEYS bounds check

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2026-06-06 at 18:50 +0800, Nuoqi Gui wrote:
> Constant pointer arithmetic on a PTR_TO_FLOW_KEYS register lands the
> constant in reg->var_off (e.g. flow_keys(imm=4096)), but the
> PTR_TO_FLOW_KEYS path in check_mem_access() passes only insn->off to
> check_flow_keys_access() and never folds reg->var_off.value.  The
> verifier therefore accepts an access that, at runtime, dereferences past
> struct bpf_flow_keys -- a verifier/runtime divergence that yields an
> out-of-bounds read and write of kernel stack memory.
> 
> Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for
> pointers") removed the generic "off += reg->off" that check_mem_access()
> applied before the per-type dispatch and replaced it with per-path
> folding of reg->var_off.value (for example the ctx path now folds the
> register offset via check_ctx_access()).  The PTR_TO_FLOW_KEYS path was
> not given the equivalent fold, so a constant offset that used to be
> folded and rejected is now silently accepted:
> 
>   before 022ac0750883: the offset stays in reg->off and is folded
>     generically, so the access is checked with off=4096 and rejected.
>   after  022ac0750883: the offset lands in reg->var_off, the flow_keys
>     path checks off=0 and accepts; at runtime the access dereferences
>     base + 0x1000.
> 
> For a BPF_PROG_TYPE_FLOW_DISSECTOR program the following is accepted:
> 
>   r2 = *(u64 *)(r1 + 144)   ; R2=flow_keys (PTR_TO_FLOW_KEYS)
>   r2 += 0x1000              ; R2=flow_keys(imm=4096), accepted
>   r0 = *(u64 *)(r2 + 0)     ; accepted, var_off.value=0x1000 ignored
> 
> while the equivalent insn->off form
> 
>   r0 = *(u64 *)(r2 + 0x1000)
> 
> has the same effective offset but is correctly rejected with
> "invalid access to flow keys off=4096 size=8", which isolates the defect
> to the missing var_off fold.  Once attached as a flow dissector, the
> accepted program reads kernel stack past struct bpf_flow_keys (a
> kernel-stack / KASLR information leak) and can likewise write past it,
> corrupting kernel memory.
> 
> Fix it by folding reg->var_off.value into the offset before the bounds
> check and rejecting non-constant offsets, mirroring the other pointer
> types (e.g. check_ctx_access()).
> 
> No released kernel is affected; the regression is confined to the 7.1
> development cycle (reproduced on v7.1-rc1..rc5), and v7.0.x rejects the
> program above.
> 
> Fixes: 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers")
> Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
> ---

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

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