[PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR

[Nicholas Piggin]

Nicholas Piggin's on April 3, 2020 9:05 pm:
> Christophe Leroy's on April 3, 2020 8:31 pm:
>> 
>> 
>> Le 03/04/2020 à 11:35, Nicholas Piggin a écrit :
>>> There is no need to allow user accesses when probing kernel addresses.
>> 
>> I just discovered the following commit 
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4
>> 
>> This commit adds probe_kernel_read_strict() and probe_kernel_write_strict().
>> 
>> When reading the commit log, I understand that probe_kernel_read() may 
>> be used to access some user memory. Which will not work anymore with 
>> your patch.
> 
> Hmm, I looked at _strict but obviously not hard enough. Good catch.
> 
> I don't think probe_kernel_read() should ever access user memory,
> the comment certainly says it doesn't, but that patch sort of implies
> that they do.
> 
> I think it's wrong. The non-_strict maybe could return userspace data to 
> you if you did pass a user address? I don't see why that shouldn't just 
> be disallowed always though.
> 
> And if the _strict version is required to be safe, then it seems like a
> bug or security issue to just allow everyone that doesn't explicitly
> override it to use the default implementation.
> 
> Also, the way the weak linkage is done in that patch, means parisc and
> um archs that were previously overriding probe_kernel_read() now get
> the default probe_kernel_read_strict(), which would be wrong for them.

The changelog in commit 75a1a607bb7 makes it a bit clearer. If the
non-_strict variant is used on non-kernel addresses, then it might not 
return -EFAULT or it might cause a kernel warning. The _strict variant 
is supposed to be usable with any address and it will return -EFAULT if 
it was not a valid and mapped kernel address.

The non-_strict variant can not portably access user memory because it
uses KERNEL_DS, and its documentation says its only for kernel pointers.
So powerpc should be fine to run that under KUAP AFAIKS.

I don't know why the _strict behaviour is not just made default, but
the implementation of it does seem to be broken on the archs that
override the non-_strict variant.

Thanks,
Nick