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Subject: Re: [PATCH bpf v4] bpf: verifier: prevent userspace memory access
To: Puranjay Mohan <puranjay12@gmail.com>,
 "David S. Miller" <davem@davemloft.net>, David Ahern <dsahern@kernel.org>,
 Alexei Starovoitov <ast@kernel.org>, Andrii Nakryiko <andrii@kernel.org>,
 Martin KaFai Lau <martin.lau@linux.dev>, Eduard Zingerman
 <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
 Yonghong Song <yonghong.song@linux.dev>,
 John Fastabend <john.fastabend@gmail.com>, KP Singh <kpsingh@kernel.org>,
 Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
 Jiri Olsa <jolsa@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
 Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
 "H. Peter Anvin" <hpa@zytor.com>,
 Jean-Philippe Brucker <jean-philippe@linaro.org>, netdev@vger.kernel.org,
 bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
 Ilya Leoshkevich <iii@linux.ibm.com>
References: <20240321124640.8870-1-puranjay12@gmail.com>
 <9f2b63b5-569c-1e00-a635-93d9cd695517@iogearbox.net>
 <mb61p4jcyxq5m.fsf@gmail.com>
From: Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <15ba79e3-14b2-d92e-3f94-e4f5f963e15d@iogearbox.net>
Date: Fri, 22 Mar 2024 17:28:27 +0100
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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.