[PATCH 1/2] x86/asm/entry/64: Enable interrupts *after* we fetch PER_CPU_VAR(old_rsp)

[Denys Vlasenko <dvlasenk@redhat.com>]

Without this change, it is still not possible to get rid of
PER_CPU_VAR(old_rsp) usage in switch_to: if preemption happens
while we did not fetch PER_CPU_VAR(old_rsp) and stored it in pt_regs->sp,
PER_CPU_VAR(old_rsp) gets corrupted by other task's user sp.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---

Run-tested, including with PARAVIRT on.

 arch/x86/kernel/entry_64.S | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index d86788c..3054a9d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -241,17 +241,17 @@ GLOBAL(system_call_after_swapgs)
 	movq	%rsp,PER_CPU_VAR(old_rsp)
 	/* kernel_stack is set so that 5 slots (iret frame) are preallocated */
 	movq	PER_CPU_VAR(kernel_stack),%rsp
-	/*
-	 * No need to follow this irqs off/on section - it's straight
-	 * and short:
-	 */
-	ENABLE_INTERRUPTS(CLBR_NONE)
 	ALLOC_PT_GPREGS_ON_STACK 8		/* +8: space for orig_ax */
 	movq	%rcx,RIP(%rsp)
 	movq	PER_CPU_VAR(old_rsp),%rcx
 	movq	%r11,EFLAGS(%rsp)
 	movq	%rcx,RSP(%rsp)
 	movq_cfi rax,ORIG_RAX
+	/*
+	 * No need to follow this irqs off/on section - it's straight
+	 * and short:
+	 */
+	ENABLE_INTERRUPTS(CLBR_RAX)
 	SAVE_C_REGS_EXCEPT_RAX_RCX_R11
 	movq	$-ENOSYS,RAX(%rsp)
 	CFI_REL_OFFSET rip,RIP
-- 
1.8.1.4