linux-kernel - Re: objtool clac/stac handling change..

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Message-ID: <87lfk26nx4.fsf@mpe.ellerman.id.au>
Date:   Thu, 02 Jul 2020 23:34:31 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: objtool clac/stac handling change..

Linus Torvalds <torvalds@...ux-foundation.org> writes:
> On Wed, Jul 1, 2020 at 12:59 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>> On Wed, Jul 01, 2020 at 12:04:36PM -0700, Linus Torvalds wrote:
>> >
>> > That's actually for the access granting. Shutting the access down ends
>> > up always doing the same thing anyway..
>>
>> #define user_read_access_end            prevent_current_read_from_user
>> #define user_write_access_end           prevent_current_write_to_user
>> static inline void prevent_current_read_from_user(void)
>> {
>>         prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT_READ);
>> }
>>
>> static inline void prevent_current_write_to_user(void)
>> {
>>         prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT_WRITE);
>> }
>>
>> and prevent_user_access() has instances that do care about the direction...
>
> Go and look closer.
>
> There are three cases:
>
>  (a) the 32-bit book3s case. It looks like it cares, but when you look
> closer, it ends up not caring about the read side, and saving the
> "which address to I allow user writes to" in current->thread.kuap
>
>  (b) the nohash 32-bit case - doesn't care
>
>  (c) the 64-bit books case - doesn't care
>
> So yes, in the (a) case it does make a difference between reads and
> writes, but at least as far as I can tell, it ignores the read case,
> and has code to avoid the unnecessary "disable user writes" case when
> there was only a read enable done.

Yeah that's my understanding too.

Christophe is the expert on that code so I'll defer to him if I'm wrong.

> Now, it's possible that I'm wrong, but the upshot of that is that even
> on powerpc, I think that if we just made the rule be that "taking a
> user exception should automatically do the 'user_access_end()' for us"
> is trivial.

I think we can do something to make it work.

We don't have an equivalent of x86's ex_handler_uaccess(), so it's not
quite as easy as whacking a user_access_end() in there.

Probably the simplest option for us is to just handle it in our
unsafe_op_wrap(). I'll try and come up with something tomorrow.

cheers