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

From: Puranjay Mohan <[email-masked]>
Date: Fri, 22 Mar 2024 16:53:47 +0000

Daniel Borkmann <[email-masked]> writes:

> 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.

Atleast for ARM64 for I don't see a differentiation between the handling
of canonical and non-canonical addresses.
do_translation_fault() checks if addr < TASK_SIZE and calls
do_page_fault() or if the address is greater than TASK_SIZE (it is a
kernel address), do_bad_area() is called.

Both of these call __do_kernel_fault() if fault is from kernel mode and it
does fixup_exception().

>
>> 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().
bpf_probe_read_kernel() is skipping reading from the vsyscall page, that
is what this patch does as well.

ARM64, RISCV, and some other archs don't implement
copy_from_kernel_nofault_allowed() so I think the we should fix the
common case where the BPF program should not be allowed to access memory
below TASK_SIZE. This would be true for all architectures. 

>
> So there is tail risk that BPF_PROBE_* could trigger a crash. Other archs might

Can you explain this a bit more, how will BPF_PROBE_* trigger a crash?

> have other quirks, e.g. in case of loongarch it says highest bit set means kernel
> space.

Thanks,
Puranjay