Re: [PATCH bpf v4] bpf: verifier: prevent userspace memory access

[Daniel Borkmann]

On 3/22/24 4:05 PM, Puranjay Mohan wrote:
[...]
> > > +		/* Make it impossible to de-reference a userspace address */
> > > +		if (BPF_CLASS(insn->code) == BPF_LDX &&
> > > +		    (BPF_MODE(insn->code) == BPF_PROBE_MEM ||
> > > +		     BPF_MODE(insn->code) == BPF_PROBE_MEMSX)) {
> > > +			struct bpf_insn *patch = &insn_buf[0];
> > > +			u64 uaddress_limit = bpf_arch_uaddress_limit();
> > > +
> > > +			if (!uaddress_limit)
> > > +				goto next_insn;
> > > +
> > > +			*patch++ = BPF_MOV64_REG(BPF_REG_AX, insn->src_reg);
> > > +			if (insn->off)
> > > +				*patch++ = BPF_ALU64_IMM(BPF_ADD, BPF_REG_AX, insn->off);
> > > +			*patch++ = BPF_ALU64_IMM(BPF_RSH, BPF_REG_AX, 32);
> > > +			*patch++ = BPF_JMP_IMM(BPF_JLE, BPF_REG_AX, uaddress_limit >> 32, 2);
> > > +			*patch++ = *insn;
> > > +			*patch++ = BPF_JMP_IMM(BPF_JA, 0, 0, 1);
> > > +			*patch++ = BPF_MOV64_IMM(insn->dst_reg, 0);
> >
> > But how does this address other cases where we could fault e.g. non-canonical,
> > vsyscall page, etc? Technically, we would have to call to copy_from_kernel_nofault_allowed()
> > to really address all the cases aside from the overflow (good catch btw!) where kernel
> > turns into user address.
>
> So, we are trying to ~simulate a call to
> copy_from_kernel_nofault_allowed() here. If the address under
> consideration is below TASK_SIZE (TASK_SIZE + 4GB to be precise) then we
> skip that load because that address could be mapped by the user.
>
> If the address is above TASK_SIZE + 4GB, we allow the load and it could
> cause a fault if the address is invalid, non-canonical etc. Taking the
> fault is fine because JIT will add an exception table entry for
> for that load with BPF_PBOBE_MEM.

Are you sure? I don't think the kernel handles non-canonical fixup.

> The vsyscall page is special, this approach skips all loads from this
> page. I am not sure if that is acceptable.

The bpf_probe_read_kernel() does handle it fine via copy_from_kernel_nofault().

So there is tail risk that BPF_PROBE_* could trigger a crash. Other archs might
have other quirks, e.g. in case of loongarch it says highest bit set means kernel
space.