From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Michael Ellerman <mpe@ellerman.id.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
the arch/x86 maintainers <x86@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: objtool clac/stac handling change..
Date: Thu, 2 Jul 2020 17:13:28 +0200 [thread overview]
Message-ID: <8be7cf19-9fc9-ce9c-091f-c8824a01a3c8@csgroup.eu> (raw)
In-Reply-To: <87lfk26nx4.fsf@mpe.ellerman.id.au>
Le 02/07/2020 à 15:34, Michael Ellerman a écrit :
> Linus Torvalds <torvalds@linux-foundation.org> writes:
>> On Wed, Jul 1, 2020 at 12:59 PM Al Viro <viro@zeniv.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
next prev parent reply other threads:[~2020-07-02 15:13 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-01 18:22 objtool clac/stac handling change Linus Torvalds
2020-07-01 18:29 ` Andy Lutomirski
2020-07-01 19:35 ` Linus Torvalds
2020-07-01 20:36 ` Andy Lutomirski
2020-07-01 20:51 ` Josh Poimboeuf
2020-07-01 21:02 ` Linus Torvalds
2020-07-02 0:00 ` Josh Poimboeuf
2020-07-02 8:05 ` Peter Zijlstra
2020-07-01 20:51 ` Linus Torvalds
2020-07-02 0:47 ` Andy Lutomirski
2020-07-02 2:30 ` Linus Torvalds
2020-07-02 2:35 ` Linus Torvalds
2020-07-02 3:08 ` Andy Lutomirski
2020-07-01 18:41 ` Al Viro
2020-07-01 19:04 ` Linus Torvalds
2020-07-01 19:59 ` Al Viro
2020-07-01 20:25 ` Linus Torvalds
2020-07-02 13:34 ` Michael Ellerman
2020-07-02 14:01 ` Al Viro
2020-07-02 14:04 ` Al Viro
2020-07-02 15:13 ` Christophe Leroy [this message]
2020-07-02 20:13 ` Linus Torvalds
2020-07-03 3:59 ` Michael Ellerman
2020-07-03 3:17 ` Michael Ellerman
2020-07-03 5:27 ` Christophe Leroy
2020-07-02 19:52 ` Linus Torvalds
2020-07-02 20:17 ` Al Viro
2020-07-02 20:32 ` Linus Torvalds
2020-07-02 20:59 ` Al Viro
2020-07-02 21:55 ` Linus Torvalds
2020-07-03 1:33 ` Al Viro
2020-07-03 3:32 ` Linus Torvalds
2020-07-03 21:02 ` Al Viro
2020-07-03 21:10 ` Linus Torvalds
2020-07-03 21:41 ` Andy Lutomirski
2020-07-03 22:25 ` Al Viro
2020-07-03 21:59 ` Al Viro
2020-07-03 22:04 ` Al Viro
2020-07-03 22:12 ` Al Viro
2020-07-04 0:49 ` Al Viro
2020-07-04 1:54 ` Linus Torvalds
2020-07-04 2:30 ` Al Viro
2020-07-04 3:06 ` Linus Torvalds
2020-07-04 2:11 ` Al Viro
2020-07-07 12:35 ` David Laight
2020-07-10 22:37 ` Linus Torvalds
2020-07-13 9:32 ` David Laight
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