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

[Daniel Borkmann <daniel@iogearbox.net>]

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.