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

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Date:   Wed, 26 Apr 2017 07:09:14 -0700
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Rik van Riel <riel@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v7 1/4] syscalls: Restore address limit after a syscall

On Wed, Apr 26, 2017 at 1:12 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Thomas Garnier <thgarnie@...gle.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