[PATCH 2/2] x86/entry: Harden return-to-user

[Peter Zijlstra]

Make the DEBUG_ENTRY check that validates CS is a user segment
unconditional and move it nearer to IRET.

PRE:
       140,026,608      cycles:k                                                      ( +-  0.01% )
       236,696,176      instructions:k            #    1.69  insn per cycle           ( +-  0.00% )

POST:
       139,957,681      cycles:k                                                      ( +-  0.01% )
       236,681,819      instructions:k            #    1.69  insn per cycle           ( +-  0.00% )

(this is with --repeat 100 and the run-to-run variance is bigger than
the difference shown)

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S |   18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -559,13 +559,6 @@ SYM_CODE_END(\asmsym)
 SYM_CODE_START_LOCAL(common_interrupt_return)
 SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
 	IBRS_EXIT
-#ifdef CONFIG_DEBUG_ENTRY
-	/* Assert that pt_regs indicates user mode. */
-	testb	$3, CS(%rsp)
-	jnz	1f
-	ud2
-1:
-#endif
 #ifdef CONFIG_XEN_PV
 	ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
 #endif
@@ -576,8 +569,14 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_
 	STACKLEAK_ERASE
 	POP_REGS
 	add	$8, %rsp	/* orig_ax */
+	UNWIND_HINT_IRET_REGS
+
+.Lswapgs_and_iret:
 	swapgs
-	jmp	.Lnative_iret
+	/* Assert that the IRET frame indicates user mode. */
+	testb	$3, 8(%rsp)
+	jnz	.Lnative_iret
+	ud2
 
 #ifdef CONFIG_PAGE_TABLE_ISOLATION
 .Lpti_restore_regs_and_return_to_usermode:
@@ -613,8 +612,7 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_
 
 	/* Restore RDI. */
 	popq	%rdi
-	swapgs
-	jmp	.Lnative_iret
+	jmp	.Lswapgs_and_iret
 #endif
 
 SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)