linux-kernel - Re: [PATCH v8 1/4] syscalls: Verify address limit before returning to user-mode

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Message-ID: <CAJcbSZENjCgn3TNDGzCBBSOXMvbTX8mBO7Trugdb5VRzhxwmHg@mail.gmail.com>
Date:   Thu, 27 Apr 2017 07:16:41 -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>,
        Dave Hansen <dave.hansen@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        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 v8 1/4] syscalls: Verify address limit before returning to user-mode

On Wed, Apr 26, 2017 at 11:49 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Thomas Garnier <thgarnie@...gle.com> wrote:
>
>> +
>> +/*
>> + * Called before coming back to user-mode. Returning to user-mode with an
>> + * address limit different than USER_DS can allow to overwrite kernel memory.
>> + */
>> +static inline void addr_limit_check_syscall(void)
>> +{
>> +     BUG_ON(!segment_eq(get_fs(), USER_DS));
>> +}
>> +
>> +#ifndef CONFIG_ADDR_LIMIT_CHECK
>> +#define __CHECK_USERMODE_SYSCALL() \
>> +     bool user_caller = segment_eq(get_fs(), USER_DS)
>> +#define __VERIFY_ADDR_LIMIT() \
>> +     if (user_caller) addr_limit_check_syscall()
>> +#else
>> +#define __CHECK_USERMODE_SYSCALL()
>> +#define __VERIFY_ADDR_LIMIT()
>> +asmlinkage void addr_limit_check_failed(void) __noreturn;
>> +#endif
>
> _Please_ harmonize all the externally exposed names and symbols.
>
> There's no reason for this mismash of names:
>
>         CONFIG_ADDR_LIMIT_CHECK
>
>         __CHECK_USERMODE_SYSCALL
>         __VERIFY_ADDR_LIMIT
>
> When we could just as easily name them consistently, along the existing pattern:
>
>         CONFIG_ADDR_LIMIT_CHECK
>
>         __SYSCALL_ADDR_LIMIT_CHECK
>         __ADDR_LIMIT_CHECK
>
> which should fit into existing nomenclature:
>
>>  #define __SYSCALL_DEFINEx(x, name, ...)                                      \
>
> But even with that fixed, the whole construct still looks pretty weird:
>
>>       {                                                               \
>> -             long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));  \
>> +             long ret;                                               \
>> +             __CHECK_USERMODE_SYSCALL();                             \
>> +             ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));       \
>> +             __ADDR_LIMIT_CHECK();                                   \
>>               __MAP(x,__SC_TEST,__VA_ARGS__);                         \
>>               __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));       \
>>               return ret;                                             \
>
> I think something like this would be more natural to read:
>
>> +             ADDR_LIMIT_CHECK_PRE();                                 \
>> +             ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));       \
>> +             ADDR_LIMIT_CHECK_POST();                                \
>
> it's a clear pre/post construct. Also note the lack of double underscores.

I think this construct makes more sense because the first macro check
if the syscall was called by user-mode. I will send an update for this
on this thread.

>
> BTW., a further simplification would be:
>
> #ifndef ADDR_LIMIT_CHECK_PRE
> # define ADDR_LIMIT_CHECK_PRE ...
> #endif
>
> This way architectures could override this generic functionality simply by
> defining the helpers. Architectures that don't do that get the generic version.

I don't think architectures need to do that. The optimizations are
embedding the checks on their architecture-specific code to make it
faster and remove the size impact. The pre/post is fine for the rest.

>
> Thanks,
>
>         Ingo



-- 
Thomas