[tip: x86/entry] x86/doublefault: Remove memmove() call

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     f44e70325748c7998cf089d2ae67b4b5bc8d8ad9
Gitweb:        https://git.kernel.org/tip/f44e70325748c7998cf089d2ae67b4b5bc8d8ad9
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 20 Feb 2020 13:17:27 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Fri, 15 May 2020 20:03:04 +02:00

x86/doublefault: Remove memmove() call

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>
Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200505134058.863038566@xxxxxxxxxxxxx

---
 arch/x86/kernel/traps.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index e85561f..75fa765 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -369,6 +369,7 @@ 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
@@ -376,7 +377,11 @@ 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[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 */
 
 		/*