Re: [PATCH v7 1/4] syscalls: Restore address limit after a syscall

From: Thomas Garnier <thgarnie@google.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Martin Schwidefsky" <schwidefsky@de.ibm.com>,
	"Heiko Carstens" <heiko.carstens@de.ibm.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"David Howells" <dhowells@redhat.com>,
	"René Nyffenegger" <mail@renenyffenegger.ch>,
	"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Oleg Nesterov" <oleg@redhat.com>,
	"Stephen Smalley" <sds@tycho.nsa.gov>,
	"Pavel Tikhomirov" <ptikhomirov@virtuozzo.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Rik van Riel" <riel@redhat.com>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Brian Gerst" <brgerst@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	"Christian Borntraeger" <borntraeger@de.ibm.com>,
	"Russell King" <linux@armlinux.org.uk>,
	"Will Deacon" <will.deacon@arm.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"James Morse" <james.morse@arm.com>,
	linux-s390 <linux-s390@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Linux API" <linux-api@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	"Kernel Hardening" <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v7 1/4] syscalls: Restore address limit after a syscall
Date: Wed, 26 Apr 2017 07:09:14 -0700	[thread overview]
Message-ID: <CAJcbSZGKknsc6YdwFjd-JHnXA0wjEpU7rCYjJCezQ7bwAxMn1A@mail.gmail.com> (raw)
In-Reply-To: <20170426081229.6wnugrs7w3at4xry@gmail.com>

On Wed, Apr 26, 2017 at 1:12 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Thomas Garnier <thgarnie@google.com> wrote:
>
>> >> +#ifdef CONFIG_ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE
>> >> +/*
>> >> + * This function is called when an architecture specific implementation detected
>> >> + * an invalid address limit. The generic user-mode state checker will finish on
>> >> + * the appropriate BUG_ON.
>> >> + */
>> >> +asmlinkage void address_limit_check_failed(void)
>> >> +{
>> >> +     verify_pre_usermode_state();
>> >> +     panic("address_limit_check_failed called with a valid user-mode state");
>> >
>> > It's very unconstructive to unconditionally panic the system, just because some
>> > kernel code leaked the address limit! Do a warn-once printout and kill the current
>> > task (i.e. don't continue execution), but don't crash everything else!
>>
>> The original change did not crash the kernel for this exact reason.
>> Through reviews, there was an overall agreement that the kernel should
>> not continue in this state.
>
> Ok, I guess we can try that - but the panic message is still pretty misleading:
>
>         panic("address_limit_check_failed called with a valid user-mode state");
>
> ... so it was called with a _valid_ user-mode state, and we crash due to something
> valid? Huh?

Yes the message is accurate but I agree that it is misleading and I
will improve it. The address_limit_check_failed function is called by
assembly code on different architectures once the state was detected
as invalid. Instead of crashing at different places, we redirect to
the generic handler (verify_pre_usermode_state) that will crash on the
appropriate BUG_ON line. The address_limit_check_failed function is
not supposed to comeback, the panic call is just a safe guard.

>
> ( Also, the style rule applies to kernel messages as well: function names should
>   be referred to as "function_name()". )

Will change.

>
> Thanks,
>
>         Ingo

-- 
Thomas