Re: [PATCH bpf-next 1/2] bpf: Fix check_stack_write_fixed_off() to correctly spill imm

[Shung-Hsi Yu]

On Fri, Oct 27, 2023 at 03:14:10PM +0800, Shung-Hsi Yu wrote:
> On Thu, Oct 26, 2023 at 05:13:10PM +0200, Hao Sun wrote:
> > In check_stack_write_fixed_off(), imm value is cast to u32 before being
> > spilled to the stack. Therefore, the sign information is lost, and the
> > range information is incorrect when load from the stack again.
> > 
> > For the following prog:
> > 0: r2 = r10
> > 1: *(u64*)(r2 -40) = -44
> > 2: r0 = *(u64*)(r2 - 40)
> > 3: if r0 s<= 0xa goto +2
> > 4: r0 = 1
> > 5: exit
> > 6: r0  = 0
> > 7: exit
> > 
> > The verifier gives:
> > func#0 @0
> > 0: R1=ctx(off=0,imm=0) R10=fp0
> > 0: (bf) r2 = r10                      ; R2_w=fp0 R10=fp0
> > 1: (7a) *(u64 *)(r2 -40) = -44        ; R2_w=fp0 fp-40_w=4294967252
> > 2: (79) r0 = *(u64 *)(r2 -40)         ; R0_w=4294967252 R2_w=fp0
> > fp-40_w=4294967252
> > 3: (c5) if r0 s< 0xa goto pc+2
> > mark_precise: frame0: last_idx 3 first_idx 0 subseq_idx -1
> > mark_precise: frame0: regs=r0 stack= before 2: (79) r0 = *(u64 *)(r2 -40)
> > 3: R0_w=4294967252
> > 4: (b7) r0 = 1                        ; R0_w=1
> > 5: (95) exit
> > verification time 7971 usec
> > stack depth 40
> > processed 6 insns (limit 1000000) max_states_per_insn 0 total_states 0
> > peak_states 0 mark_read 0
> > 
> > So remove the incorrect cast, since imm field is declared as s32, and
> > __mark_reg_known() takes u64, so imm would be correctly sign extended
> > by compiler.
> > 
> > Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
> 
> Acked-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>
> 
> The acked-by applies to future version of the patchset as well.

Oh and since this is a fix it would be great to have the fixes tag[1] to
specify when the bug was introduced

Fixes: ecdf985d7615 ("bpf: track immediate values written to stack by BPF_ST instruction")

Add Cc tag for stable[2] so stable kernels pick up the fix as well

Cc: stable@xxxxxxxxxxxxxxx

And ideally specify that the patch should be applied to the bpf tree rather
than bpf-next[3] (though the BPF maintainers has the final say on which tree
this patch should be applied).

I'd owe you a big thank as well since this helps with our internal process
at my company. So thank you in advance!

1: https://docs.kernel.org/process/submitting-patches.html#describe-your-changes
2: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#option-1
3: https://docs.kernel.org/bpf/bpf_devel_QA.html#q-how-do-the-changes-make-their-way-into-linux

> FWIW I think we'd also need the same treatment for the (BPF_ALU | BPF_MOV |
> BPF_K) case in check_alu_op().
> 
> > ---
> >  kernel/bpf/verifier.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 857d76694517..44af69ce1301 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -4674,7 +4674,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> >  		   insn->imm != 0 && env->bpf_capable) {
> >  		struct bpf_reg_state fake_reg = {};
> >  
> > -		__mark_reg_known(&fake_reg, (u32)insn->imm);
> > +		__mark_reg_known(&fake_reg, insn->imm);
> >  		fake_reg.type = SCALAR_VALUE;
> >  		save_register_state(state, spi, &fake_reg, size);
> >  	} else if (reg && is_spillable_regtype(reg->type)) {
> > 
> > -- 
> > 2.34.1
> >