kernel-hardening - Re: [PATCH v4 10/29] x86/die: Don't try to recover from an OOPS on a non-default stack

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Message-ID: <CALCETrVowqk90fmsg9eJG7Fvk0EMVFS0azCbJ89dioRtuXZOKQ@mail.gmail.com>
Date: Sun, 3 Jul 2016 07:25:05 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, 
	Nadav Amit <nadav.amit@...il.com>, Kees Cook <keescook@...omium.org>, 
	Brian Gerst <brgerst@...il.com>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Jann Horn <jann@...jh.net>, 
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 10/29] x86/die: Don't try to recover from an OOPS on a
 non-default stack

On Sat, Jul 2, 2016 at 11:34 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Sat, Jul 02, 2016 at 07:24:41PM +0200, Borislav Petkov wrote:
>> On Sun, Jun 26, 2016 at 02:55:32PM -0700, Andy Lutomirski wrote:
>> > It's not going to work, because the scheduler will explode if we try
>> > to schedule when running on an IST stack or similar.
>> >
>> > This will matter when we let kernel stack overflows (which are #DF)
>> > call die().
>> >
>> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> > ---
>> >  arch/x86/kernel/dumpstack.c | 3 +++
>> >  1 file changed, 3 insertions(+)
>> >
>> > diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
>> > index ef8017ca5ba9..352f022cfd5b 100644
>> > --- a/arch/x86/kernel/dumpstack.c
>> > +++ b/arch/x86/kernel/dumpstack.c
>> > @@ -245,6 +245,9 @@ void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
>> >             return;
>> >     if (in_interrupt())
>> >             panic("Fatal exception in interrupt");
>> > +   if (((current_stack_pointer() ^ (current_top_of_stack() - 1))
>> > +        & ~(THREAD_SIZE - 1)) != 0)
>>
>> Ugh, that's hard to parse. You could remove the "!= 0" at least to
>> shorten it a bit and have one less braces level.
>>
>> Or maybe even do something like that to make it a bit more readable:
>>
>>         if ((current_stack_pointer() ^ (current_top_of_stack() - 1))
>>                         &
>>              ~(THREAD_SIZE - 1))
>>                 panic("Fatal exception on non-default stack");
>>
>> Meh.
>
> A helper function would be even better.
>
> The existing 'object_is_on_stack()' can probably be used:
>
>         if (!object_is_on_stack(current_top_of_stack()))
>                 panic("...");
>
> Though that function isn't quite accurately named.  It should really
> have 'task_stack' in its name, like 'object_is_on_task_stack()'.  Or
> even better, something more concise like 'on_task_stack()'.
>

Given that the very next patch deletes this code, I vote for leaving
it alone.  Or I could fold the patches together.

--Andy

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