linux-kernel - Re: [RFC][PATCH 08/24] x86/entry: Add C version of SWAPGS and SWAPGS_UNSAFE

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Date:   Mon, 9 Nov 2020 20:55:02 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, jroedel@...e.de
Cc:     konrad.wilk@...cle.com, jan.setjeeilers@...cle.com,
        junaids@...gle.com, oweisse@...gle.com, rppt@...ux.vnet.ibm.com,
        graf@...zon.de, mgross@...ux.intel.com, kuzuno@...il.com
Subject: Re: [RFC][PATCH 08/24] x86/entry: Add C version of SWAPGS and
 SWAPGS_UNSAFE_STACK

[Copying the reply to Andy in the thread with the right email addresses]

On 11/9/20 6:38 PM, Andy Lutomirski wrote:
> On Mon, Nov 9, 2020 at 3:22 AM Alexandre Chartre
> <alexandre.chartre@...cle.com> wrote:
>>
>> SWAPGS and SWAPGS_UNSAFE_STACK are assembly macros. Add C versions
>> of these macros (swapgs() and swapgs_unsafe_stack()).
>
> This needs a very good justification.  It also needs some kind of
> static verification that these helpers are only used by noinstr code,
> and they need to be __always_inline.  And I cannot fathom how C code
> could possibly use SWAPGS_UNSAFE_STACK in a meaningful way.
>

You're right, I probably need to revisit the usage of SWAPGS_UNSAFE_STACK
in C code, that doesn't make sense. Looks like only SWAPGS is then needed.

Or maybe we can just use native_swapgs() instead?

I have added a C version of SWAPGS for moving paranoid_entry() to C because,
in this function, we need to switch CR3 before doing the updating GS. But I
really wonder if we need a paravirt swapgs here, and we can probably just use
native_swapgs().

Also, if we map the per cpu offsets (__per_cpu_offset) in the user page-table
then we will be able to update GS before switching CR3. That way we can keep the
GS update in assembly code, and just do the CR3 switch in C code. This would also
avoid having to disable stack-protector (patch 21).

alex.