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Subject[PATCH v2] x86/entry: re-enable interrupts before exiting
Date
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@tycho.ws>
CC: Josh Poimboeuf <jpoimboe@redhat.com>
---
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
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