Re: [RFC][PATCH 08/24] x86/entry: Add C version of SWAPGS and SWAPGS_UNSAFE_STACK

From: Alexandre Chartre
Date: Mon Nov 09 2020 - 14:55:09 EST



[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@xxxxxxxxxx> 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.