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

[Josh Poimboeuf]

On Fri, Apr 05, 2019 at 09:35:24AM -0600, Tycho Andersen wrote:
> 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.
> 
> I've had to add back in the hack that Josh removed in 8c1f75587a18
> ("x86/entry/64: Add unwind hint annotations") with the loop after the call,
> because for whatever reason without that I get a warning:
> 
>   AS      arch/x86/entry/entry_64.o
> arch/x86/entry/entry_64.o: warning: objtool: .entry.text: unexpected end of section
> 
> It seems to actually work fine for me though, since the new helper is also
> __noreturn. Perhaps there's a better way to do this?

Unfortunately, objtool doesn't have a way to detect noreturn functions
in other objects, so they're hard-coded in a list.  You can add
__finish_rewind_stack_do_exit to the global_noreturns array in
tools/objtool/check.c.

-- 
Josh