[tip:x86/asm] x86/entry/64: Fix fast-path syscall return register state

[tip-bot for Andy Lutomirski]

Commit-ID:  eb2a54c3271cb6443ae93ec44a91687b60c559a3
Gitweb:     http://git.kernel.org/tip/eb2a54c3271cb6443ae93ec44a91687b60c559a3
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Sun, 31 Jan 2016 09:33:27 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 1 Feb 2016 08:53:25 +0100

x86/entry/64: Fix fast-path syscall return register state

I was fishing RIP (i.e. RCX) out of pt_regs->cx and RFLAGS (i.e.
R11) out of pt_regs->r11.  While it usually worked (pt_regs
started out with CX == IP and R11 == FLAGS), it was very
fragile.  In particular, it broke sys_iopl() because sys_iopl()
forgot to mark itself as using ptregs.

Undo that part of the syscall rework.  There was no compelling
reason to do it this way.  While I'm at it, load RCX and R11
before the other regs to be a little friendlier to the CPU, as
they will be the first of the reloaded registers to be used.

Reported-and-tested-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: 1e423bff959e x86/entry/64: ("Migrate the 64-bit syscall slow path to C")
Link: http://lkml.kernel.org/r/a85f8360c397e48186a9bc3e565ad74307a7b011.1454261517.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9f7bb80..70eadb0 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -212,7 +212,9 @@ entry_SYSCALL_64_fastpath:
 
 	LOCKDEP_SYS_EXIT
 	TRACE_IRQS_ON		/* user mode is traced as IRQs on */
-	RESTORE_C_REGS
+	movq	RIP(%rsp), %rcx
+	movq	EFLAGS(%rsp), %r11
+	RESTORE_C_REGS_EXCEPT_RCX_R11
 	movq	RSP(%rsp), %rsp
 	USERGS_SYSRET64