Re: [PATCH] x86/smap: Fix the smap_save() asm

From: Borislav Petkov
Date: Thu Sep 17 2020 - 02:04:51 EST

On Wed, Sep 16, 2020 at 11:48:42PM +0100, Andrew Cooper wrote:
> Every day is a school day.

Tell me about it...

> This is very definitely one to be filed in obscure x86 corner cases.
> The code snippet above is actually wrong for the kernel, as it uses one
> slot of the red-zone.  Recompiling with -mno-red-zone makes something
> which looks safe stack-wise, give or take this behaviour.

Right, we recently disabled red zone in the early decompression stage,
for SEV-ES:

I probably should go audit that for similar funnies:

$ objdump -d arch/x86/boot/compressed/vmlinux | grep -E "pop.*\(%[er]?sp"

Nope, nothing. Because building your snippet with:

$ gcc -Wall -O2 -mno-red-zone -o flags{,.c}

still does use that one slot:

0000000000001050 <main>:
1050: 48 83 ec 18 sub $0x18,%rsp
1054: 48 8d 3d a9 0f 00 00 lea 0xfa9(%rip),%rdi # 2004 <_IO_stdin_used+0x4>
105b: 31 c0 xor %eax,%eax
105d: 9c pushfq
105e: 8f 44 24 08 popq 0x8(%rsp)
1062: 48 8b 74 24 08 mov 0x8(%rsp),%rsi

Wonder if that flag -mno-red-zone even does anything...