linux-kernel - Re: [PATCH PTI v2 3/6] x86/vsyscall/64: Explicitly set _PAGE

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Date:   Mon, 11 Dec 2017 08:01:09 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH PTI v2 3/6] x86/vsyscall/64: Explicitly set _PAGE_USER in
 the pagetable hierarchy

On Mon, Dec 11, 2017 at 5:39 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...nel.org> wrote:
>
>> The kernel is very erratic as to which pagetables have _PAGE_USER
>> set.  The vsyscall page gets lucky: it seems that all of the
>> relevant pagetables are among the apparently arbitrary ones that set
>> _PAGE_USER.  Rather than relying on chance, just explicitly set
>> _PAGE_USER.
>>
>> This will let us clean up pagetable setup to stop setting
>> _PAGE_USER.  The added code can also be reused by pagetable
>> isolation to manage the _PAGE_USER bit in the usermode tables.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/entry/vsyscall/vsyscall_64.c | 33 ++++++++++++++++++++++++++++++++-
>>  1 file changed, 32 insertions(+), 1 deletion(-)
>
> Btw., would it make sense to clean up all this confusion?
>
> In particular a 'KERNEL' pre of post fix is ambiguous in this context I think, and
> the PAGE_KERNEL_ prefix is actively harmful I think and is at the root of the
> confusion.
>
> So if renamed it and used this nomenclature consistently instead:
>
>   PAGE_USER_
>   PAGE_SYSTEM_

Like _PAGE_USER_VSYSCALL?

Anyway, that's not the confusion I'm talking about.  I'm talking about
_KERNPG_TABLE vs _PAGE_TABLE.  The latter should be called
_USERPG_TABLE, and a whole bunch of its users should be switched to
_KERNPG_TABLE.

But, since PTI is intended for backporting, I think these types of big
cleanups should wait.

>
> ... and got rid of PAGE_KERNEL uses in arch/x86/, then it would be obvious at
> first glance what kind of mapping is established in a particular place - and it
> would stay so in the future as well.
>
> ( There's some interaction with generic MM code which needs the original defines
>   like PAGE_KERNEL[_EXEC], but those generic masks could be defined as aliases, to
>   keep this cleanup within x86 for now. )
>
> Thanks,
>
>         Ingo