x86/entry/64: Fix and clean up paranoid_exit

paranoid_exit needs to restore CR3 before GSBASE.  Doing it in the opposite
order crashes if the exception came from a context with user GSBASE and
user CR3 -- RESTORE_CR3 cannot resture user CR3 if run with user GSBASE.
This results in infinitely recursing exceptions if user code does SYSENTER
with TF set if both FSGSBASE and PTI are enabled.

The old code worked if user code just set TF without SYSENTER because #DB
from user mode is special cased in idtentry and paranoid_exit doesn't run.

Fix it by cleaning up the spaghetti code.  All that paranoid_exit needs to
do is to disable IRQs, handle IRQ tracing, then restore CR3, and restore
GSBASE.  Simply do those actions in that order.

Fixes: 708078f6 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/59725ceb08977359489fbed979716949ad45f616.1562035429.git.luto@kernel.org

arch/x86/entry/entry_64.S

@@ -1256,31 +1256,32 @@ END(paranoid_entry)
 ENTRY(paranoid_exit)
 	UNWIND_HINT_REGS
 	DISABLE_INTERRUPTS(CLBR_ANY)
-	TRACE_IRQS_OFF_DEBUG
 
-	/* Handle GS depending on FSGSBASE availability */
-	ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "nop",X86_FEATURE_FSGSBASE
+	/*
+	 * The order of operations is important.  IRQ tracing requires
+	 * kernel GSBASE and CR3.  RESTORE_CR3 requires kernel GS base.
+	 *
+	 * NB to anyone to tries to optimize this code: this code does
+	 * not execute at all for exceptions coming from user mode.  Those
+	 * exceptions go through error_exit instead.
+	 */
+	TRACE_IRQS_IRETQ_DEBUG
+	RESTORE_CR3	scratch_reg=%rax save_reg=%r14
+
+	/* Handle the three GSBASE cases. */
+	ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE
 
 	/* With FSGSBASE enabled, unconditionally restore GSBASE */
 	wrgsbase	%rbx
-	jmp	.Lparanoid_exit_no_swapgs;
+	jmp	restore_regs_and_return_to_kernel
 
 .Lparanoid_exit_checkgs:
 	/* On non-FSGSBASE systems, conditionally do SWAPGS */
 	testl	%ebx, %ebx
-	jnz	.Lparanoid_exit_no_swapgs
-	TRACE_IRQS_IRETQ
-	/* Always restore stashed CR3 value (see paranoid_entry) */
-	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
-	SWAPGS_UNSAFE_STACK
-	jmp	.Lparanoid_exit_restore
-
-.Lparanoid_exit_no_swapgs:
-	TRACE_IRQS_IRETQ_DEBUG
-	/* Always restore stashed CR3 value (see paranoid_entry) */
-	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
+	jnz	restore_regs_and_return_to_kernel
 
-.Lparanoid_exit_restore:
+	/* We are returning to a context with user GSBASE. */
+	SWAPGS_UNSAFE_STACK
 	jmp	restore_regs_and_return_to_kernel
 END(paranoid_exit)