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

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Message-ID: <8be7cf19-9fc9-ce9c-091f-c8824a01a3c8@csgroup.eu>
Date:   Thu, 2 Jul 2020 17:13:28 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        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..



Le 02/07/2020 à 15:34, Michael Ellerman a écrit :
> 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.

Isn't it something easy to do in bad_page_fault() ?

Not exactly a call to user_access_end() but altering regs->kuap so that 
user access is not restored on exception exit.

> 
> 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.

unsafe_op_wrap() is not used anymore for unsafe_put_user() as we are now 
using asm goto.

Christophe