Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

[David Miller]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Fri, 22 Feb 2019 13:20:58 -0800

> On Fri, Feb 22, 2019 at 11:27 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
>>
>> On bpf side the bpf_probe_read() helper just calls probe_kernel_read()
>> and users pass both user and kernel addresses into it and expect
>> that the helper will actually try to read from that address.
> 
> As mentioned earlier in the thread, that's actually fundamentally broken.
> 
> There are architectures that have physically separate address spaces,
> with the same pointer value in both kernel and user space.
> 
> They are rare, but they exist. At least sparc32 and the old 4G:4G split x86.

And sparc64.

> So a pointer really should always unambiguously always be explicitly
> _either_ a kernel pointer, or a user pointer. You can't have "this is
> a pointer", and then try to figure it out by looking at the value.
> That may happen to work on x86-64, but it's literally a "happen to
> work on the most common architectures", not a design thing.

Don't be surprised if we see more separation like this in the future too.

So it's not a smart thing to code against even if you can discount all of
the examples Linus gives above.