Re: [PATCH bpf v2] bpf: verifier: prevent userspace memory access

From: Puranjay Mohan
Date: Thu Mar 21 2024 - 07:49:09 EST


Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:

> On Thu, Mar 21, 2024 at 4:05 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
>>
>> On Thu, Mar 21, 2024 at 3:11 AM Puranjay Mohan <puranjay12@gmailcom> wrote:
>> >
>> > diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_compc
>> > index e613eebfd349..e61a51a5b4be 100644
>> > --- a/arch/s390/net/bpf_jit_comp.c
>> > +++ b/arch/s390/net/bpf_jit_comp.c
>> > @@ -2691,3 +2691,8 @@ bool bpf_jit_supports_subprog_tailcalls(void)
>> > {
>> > return true;
>> > }
>> > +
>> > +u64 bpf_arch_uaddress_limit(void)
>> > +{
>> > + return -ENOTSUPP;
>> > +}
>>
>> Looks good and should work, but s390 CI is still not happy.
>> Ideas?
>> sock tests were not failing before. So something is going on.
>
> I think I have an explanation.
> -ENOTSUPP and u64... and later:
> u64 uaddress_limit = bpf_arch_uaddress_limit()
> if (uaddress_limit < 0)
>
> I bet the compiler simply removes this check since unsigned cannot
> be negative.
> Odd that there is no compiler warning.
>
> pw-bot: cr
>

Yes, I verified that the compiler is removing this:


if (BPF_CLASS(insn->code) == BPF_LDX &&
a944: 7100047f cmp w3, #0x1
a948: 540013e1 b.ne abc4 <do_misc_fixups+0x66c> // b.any
a94c: 721a041f tst w0, #0xc0
a950: 54fff4e1 b.ne a7ec <do_misc_fixups+0x294> // b.any
u64 uaddress_limit = bpf_arch_uaddress_limit();
a954: b90003e6 str w6, [sp]
a958: 94000000 bl 0 <bpf_arch_uaddress_limit>
*patch++ = BPF_MOV64_REG(BPF_REG_AX, insn->src_reg);


We should do:
if (!uaddress_limit)
goto next_insn;

and in the disabled case return 0 in place of -ENOSUPP.

Doing this adds the check:


if (BPF_CLASS(insn->code) == BPF_LDX &&
a944: 7100047f cmp w3, #0x1
a948: 54001401 b.ne abc8 <do_misc_fixups+0x670> // b.any
a94c: 721a041f tst w0, #0xc0
a950: 54fff4e1 b.ne a7ec <do_misc_fixups+0x294> // b.any
u64 uaddress_limit = bpf_arch_uaddress_limit();
a954: b90003e6 str w6, [sp]
a958: 94000000 bl 0 <bpf_arch_uaddress_limit>
if (!uaddress_limit)
a95c: b4fff020 cbz x0, a760 <do_misc_fixups+0x208>
*patch++ = BPF_MOV64_REG(BPF_REG_AX, insn->src_reg);



I will send v3 with this approach.


Thanks,
Puranjay