Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE

Borislav Petkov
2020-02-20T12:17:35Z

On Wed, Feb 19, 2020 at 04:57:15PM +0100, Peter Zijlstra wrote:
> - memmove(&gpregs->ip, (void *)regs->sp, 5*8);
> + for (i = 0; i < count; i++) {
> + int idx = (dst <= src) ? i : count - i;
> + dst[idx] = src[idx];
> + }

Or, you can actually unroll it. This way it even documents clearly what
it does:

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index fe38015ed50a..2b790a574ba5 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -298,6 +298,7 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
regs->ip == (unsigned long)native_irq_return_iret)
{
struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
+ unsigned long *p = (unsigned long *)regs->sp;

/*
* regs->sp points to the failing IRET frame on the
@@ -305,7 +306,11 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code, unsign
* in gpregs->ss through gpregs->ip.
*
*/
- memmove(&gpregs->ip, (void *)regs->sp, 5*8);
+ gpregs->ip = *p;
+ gpregs->cs = *(p + 1);
+ gpregs->flags = *(p + 2);
+ gpregs->sp = *(p + 3);
+ gpregs->ss = *(p + 4);
gpregs->orig_ax = 0; /* Missing (lost) #GP error code */

/*

--
Regards/Gruss,
Boris.