[PATCH v2] x86/entry: re-enable interrupts before exiting

[Tycho Andersen]

If the kernel oopses in an interrupt, nothing re-enables interrupts:

Aug 23 19:30:27 xpfo kernel: [   38.302714] BUG: sleeping function called from invalid context at
./include/linux/percpu-rwsem.h:33
Aug 23 19:30:27 xpfo kernel: [   38.303837] in_atomic(): 0, irqs_disabled(): 1, pid: 1970, name:
lkdtm_xpfo_test
Aug 23 19:30:27 xpfo kernel: [   38.304758] CPU: 3 PID: 1970 Comm: lkdtm_xpfo_test Tainted: G      D
4.13.0-rc5+ #228
Aug 23 19:30:27 xpfo kernel: [   38.305813] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.10.1-1ubuntu1 04/01/2014
Aug 23 19:30:27 xpfo kernel: [   38.306926] Call Trace:
Aug 23 19:30:27 xpfo kernel: [   38.307243]  dump_stack+0x63/0x8b
Aug 23 19:30:27 xpfo kernel: [   38.307665]  ___might_sleep+0xec/0x110
Aug 23 19:30:27 xpfo kernel: [   38.308139]  __might_sleep+0x45/0x80
Aug 23 19:30:27 xpfo kernel: [   38.308593]  exit_signals+0x21/0x1c0
Aug 23 19:30:27 xpfo kernel: [   38.309046]  ? blocking_notifier_call_chain+0x11/0x20
Aug 23 19:30:27 xpfo kernel: [   38.309677]  do_exit+0x98/0xbf0
Aug 23 19:30:27 xpfo kernel: [   38.310078]  ? smp_reader+0x27/0x40 [lkdtm]
Aug 23 19:30:27 xpfo kernel: [   38.310604]  ? kthread+0x10f/0x150
Aug 23 19:30:27 xpfo kernel: [   38.311045]  ? read_user_with_flags+0x60/0x60 [lkdtm]
Aug 23 19:30:27 xpfo kernel: [   38.311680]  rewind_stack_do_exit+0x17/0x20

do_exit() expects to be called in a well-defined environment, so let's
re-enable interrupts after unwinding the stack, in case they were disabled.

Note that if any spinlocks are held, etc. we'll also get the above warning,
so this isn't a silver bullet. So, let's add a C helper in case someone
wants to add fancier lock busting or if we've forgotten to unwind something
else.

Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
CC: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
I split this out from the XPFO series since it's mostly unrelated, and is
just a bug I found while working on that series.

v2: whitelist __finish_rewind_stack_do_exit() as a noreturn function in
    objtool (Josh)
---
 arch/x86/entry/common.c   | 10 ++++++++++
 arch/x86/entry/entry_32.S |  2 +-
 arch/x86/entry/entry_64.S |  2 +-
 tools/objtool/check.c     |  1 +
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 7bc105f47d21..4e9c54e0495f 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -427,3 +427,13 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
 #endif
 }
 #endif
+
+void __noreturn __finish_rewind_stack_do_exit(long code)
+{
+	/*
+	 * If we oopsed in an interrupt handler, interrupts may be off. Let's turn
+	 * them back on before going back to "normal" code.
+	 */
+	 local_irq_enable();
+	 do_exit(code);
+}
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d309f30cf7af..095b8770e3b0 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1507,6 +1507,6 @@ ENTRY(rewind_stack_do_exit)
 	movl	PER_CPU_VAR(cpu_current_top_of_stack), %esi
 	leal	-TOP_OF_KERNEL_STACK_PADDING-PTREGS_SIZE(%esi), %esp
 
-	call	do_exit
+	call	__finish_rewind_stack_do_exit
 1:	jmp 1b
 END(rewind_stack_do_exit)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1f0efdb7b629..890a66d17f31 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1672,5 +1672,5 @@ ENTRY(rewind_stack_do_exit)
 	leaq	-PTREGS_SIZE(%rax), %rsp
 	UNWIND_HINT_FUNC sp_offset=PTREGS_SIZE
 
-	call	do_exit
+	call	__finish_rewind_stack_do_exit
 END(rewind_stack_do_exit)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 5dde107083c6..f88f8f0b3f6e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -156,6 +156,7 @@ static int __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"__stack_chk_fail",
 		"panic",
 		"do_exit",
+		"__finish_rewind_stack_do_exit",
 		"do_task_dead",
 		"__module_put_and_exit",
 		"complete_and_exit",
-- 
2.19.1