[tip: x86/urgent] x86/entry/64: Fix unwind hints in kernel exit path

[tip-bot2 for Josh Poimboeuf]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     1fb143634a38095b641a3a21220774799772dc4c
Gitweb:        https://git.kernel.org/tip/1fb143634a38095b641a3a21220774799772dc4c
Author:        Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate:    Sat, 25 Apr 2020 05:03:02 -05:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Sat, 25 Apr 2020 12:22:27 +02:00

x86/entry/64: Fix unwind hints in kernel exit path

In swapgs_restore_regs_and_return_to_usermode, after the stack is
switched to the trampoline stack, the existing UNWIND_HINT_REGS hint is
no longer valid, which can result in the following ORC unwinder warning:

  WARNING: can't dereference registers at 000000003aeb0cdd for ip swapgs_restore_regs_and_return_to_usermode+0x93/0xa0

For full correctness, we could try to add complicated unwind hints so
the unwinder could continue to find the registers, but when when it's
this close to kernel exit, unwind hints aren't really needed anymore and
it's fine to just use an empty hint which tells the unwinder to stop.

For consistency, also move the UNWIND_HINT_EMPTY in
entry_SYSCALL_64_after_hwframe to a similar location.

Fixes: 3e3b9293d392 ("x86/entry/64: Return to userspace from the trampoline stack")
Reported-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Reported-by: Dave Jones <dsj@xxxxxx>
Reported-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
Reported-by: Joe Mario <jmario@xxxxxxxxxx>
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Miroslav Benes <mbenes@xxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/60ea8f562987ed2d9ace2977502fe481c0d7c9a0.1587808742.git.jpoimboe@xxxxxxxxxx
---
 arch/x86/entry/entry_64.S | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 0e9504f..6b0d679 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -249,7 +249,6 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
 	 */
 syscall_return_via_sysret:
 	/* rcx and r11 are already restored (see code above) */
-	UNWIND_HINT_EMPTY
 	POP_REGS pop_rdi=0 skip_r11rcx=1
 
 	/*
@@ -258,6 +257,7 @@ syscall_return_via_sysret:
 	 */
 	movq	%rsp, %rdi
 	movq	PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
+	UNWIND_HINT_EMPTY
 
 	pushq	RSP-RDI(%rdi)	/* RSP */
 	pushq	(%rdi)		/* RDI */
@@ -637,6 +637,7 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
 	 */
 	movq	%rsp, %rdi
 	movq	PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
+	UNWIND_HINT_EMPTY
 
 	/* Copy the IRET frame to the trampoline stack. */
 	pushq	6*8(%rdi)	/* SS */