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Subject: Re: [PATCH v3 2/7] uaccess: Tell user_access_begin() if it's for a
 write or not
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
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        dri-devel <dri-devel@lists.freedesktop.org>,
        the arch/x86 maintainers <x86@kernel.org>
References: <fed4f49349913cb6739dac647ba6a61d56b989d2.1579783936.git.christophe.leroy@c-s.fr>
 <e11a8f0670251267f87e3114e0bdbacb1eb72980.1579783936.git.christophe.leroy@c-s.fr>
 <CAHk-=wg4HEABOZdjxMzbembNmxs1zYfrNAEc2L+JS9FBSnM8JA@mail.gmail.com>
From:   christophe leroy <christophe.leroy@c-s.fr>
Message-ID: <fc5c94a2-5a25-0715-5240-5ba3cbe0f2b2@c-s.fr>
Date:   Thu, 23 Jan 2020 20:47:06 +0100
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Le 23/01/2020 à 19:02, Linus Torvalds a écrit :
> On Thu, Jan 23, 2020 at 4:59 AM Christophe Leroy
> <christophe.leroy@c-s.fr> wrote:
>>
>> On 32 bits powerPC (book3s/32), only write accesses to user are
>> protected and there is no point spending time on unlocking for reads.
> 
> Honestly, I'm starting to think that 32-bit ppc just needs to look
> more like everybody else, than make these changes.

Well, beside ppc32, I was also seen it as an opportunity for the modern 
ppc64. On it, you can unlock either read or write or both. And this is 
what is done for get_user() / put_user() and friends: unlock only reads 
for get_user() and only writes for put_user().

Could also be a compromise between performance and security: keeping 
reads allowed at all time and only protect against writes on modern 
architectures which support it like ppc64.

> 
> We used to have a read/write argument to the old "verify_area()" and
> "access_ok()" model, and it was a mistake. It was due to odd i386 user
> access issues. We got rid of it. I'm not convinced this is any better
> - it looks very similar and for odd ppc access issues.

I'm going to leave it aside, at least for the time being, and do it as a 
second step later after evaluating the real performance impact. I'll 
respin tomorrow in that way.

> 
> But if we really do want to do this, then:

Indeed I took the idea from a discussion in last Octobre (Subject: 
"book3s/32 KUAP (was Re: [PATCH] Convert filldir[64]() from __put_user() 
to unsafe_put_user())" )

https://lore.kernel.org/lkml/87h84avffi.fsf@mpe.ellerman.id.au/


> 
>> Add an argument to user_access_begin() to tell when it's for write and
>> return an opaque key that will be used by user_access_end() to know
>> what was done by user_access_begin().
> 
> You should make it more opaque than "unsigned long".
> 
> Also, it shouldn't be a "is this a write". What if it's a read _and_ a
> write? Only a write? Only a read?

Indeed that was more: does it includes a write. It's either RO or RW

Christophe