On Fri, 2023-09-01 at 16:19 +0200, Puranjay Mohan wrote:
> Hi Ilya
>
> On Wed, Aug 30, 2023 at 3:12 AM Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> wrote:
> >
> > On the architectures that use bpf_jit_needs_zext(), e.g., s390x,
> > the
> > verifier incorrectly inserts a zero-extension after BPF_MEMSX,
> > leading
> > to miscompilations like the one below:
> >
> > 24: 89 1a ff fe 00 00 00 00 "r1 = *(s16 *)(r10 -
> > 2);" # zext_dst set
> > 0x3ff7fdb910e: lgh %r2,-
> > 2(%r13,%r0) # load halfword
> > 0x3ff7fdb9114: llgfr
> > %r2,%r2 # wrong!
> > 25: 65 10 00 03 00 00 7f ff if r1 s> 32767 goto +3
> > <l0_1> # check_cond_jmp_op()
> >
> > Disable such zero-extensions. The JITs need to insert sign-
> > extension
> > themselves, if necessary.
> >
> > Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
> > ---
> > kernel/bpf/verifier.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index bb78212fa5b2..097985a46edc 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3110,7 +3110,9 @@ static void mark_insn_zext(struct
> > bpf_verifier_env *env,
> > {
> > s32 def_idx = reg->subreg_def;
> >
> > - if (def_idx == DEF_NOT_SUBREG)
>
> The problem here is that reg->subreg_def should be set as
> DEF_NOT_SUBREG for
> registers that are used as destination registers of BPF_LDX |
> BPF_MEMSX. I am seeing
> the same problem on ARM32 and was going to send a patch today.
>
> The problem is that is_reg64() returns false for destination
> registers
> of BPF_LDX | BPF_MEMSX.
> But BPF_LDX | BPF_MEMSX always loads a 64 bit value because of the
> sign extension so
> is_reg64() should return true.
>
> I have written a patch that I will be sending as a reply to this.
> Please let me know if that makes sense.
>
> > + if (def_idx == DEF_NOT_SUBREG ||
> > + (BPF_CLASS(env->prog->insnsi[def_idx - 1].code) ==
> > BPF_LDX &&
> > + BPF_MODE(env->prog->insnsi[def_idx - 1].code) ==
> > BPF_MEMSX))
> > return;
> >
> > env->insn_aux_data[def_idx - 1].zext_dst = true;
> > --
> > 2.41.0
> >
> >
>
> Thanks,
> Puranjay
Hi,
I also considered doing this, and I think both approaches are
functionally equivalent and work in practice.
However, I can envision that, just like we have the zext_dst
optimization today, we might want a sext_dst optimization in the
future. Therefore I think it's better to fix this by not setting
zext_dst instead of not setting subreg_def.
Best regards,
Ilya
