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

[Hao Sun]

On Fri, Oct 27, 2023 at 9:44 AM Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx> wrote:
>
> 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.
>

(BPF_ALU | BPF_MOV | BPF_K) is handled correctly in the current
code, i.e., no cast in BPF_ALU64 so that the sign is extended, and
the cast in BPF_ALU correctly zero extend the reg.

> 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")
>

Noted, thanks.

> 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
> > >