[patch V4 part 1 08/36] x86/doublefault: Remove memmove() call

From: Thomas Gleixner
Date: Tue May 05 2020 - 10:13:51 EST


Use of memmove() in #DF is problematic considered tracing and other
instrumentation.

Remove the memmove() call and simply write out what needs doing; this
even clarifies the code, win-win! The code copies from the espfix64
stack to the normal task stack, there is no possible way for that to
overlap.

Survives selftests/x86, specifically sigreturn_64.

Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200220121727.GB507@xxxxxxx
---
arch/x86/kernel/traps.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -278,6 +278,7 @@ dotraplinkage void do_double_fault(struc
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
@@ -285,7 +286,11 @@ dotraplinkage void do_double_fault(struc
* in gpregs->ss through gpregs->ip.
*
*/
- memmove(&gpregs->ip, (void *)regs->sp, 5*8);
+ gpregs->ip = p[0];
+ 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 */

/*