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@suse.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1e423bff959e x86/entry/64: ("Migrate the 64-bit syscall slow path to C")
Link: http://lkml.kernel.org/r/a85f8360c397e48186a9bc3e565ad74307a7b011.1454261517.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9f7bb808035efd1ffc0bbdefa264bf4f83279f94..70eadb0ea5fa46407ba16e7bafd6f30386ec0a46 100644 (file)
--- 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