linux-kernel - Re: [PATCH v6 4/4] arm64/syscalls: Specific usage of verify_pre_usermode

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Message-ID: <CAJcbSZGYC4cKqyYDXh9LvDctnrGZGxxTSTmtaxMAipQgZ8A-pQ@mail.gmail.com>
Date:   Wed, 5 Apr 2017 11:14:34 -0700
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Rik van Riel <riel@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Brian Gerst <brgerst@...il.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Borislav Petkov <bp@...en8.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        Linux API <linux-api@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        James Morse <james.morse@....com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v6 4/4] arm64/syscalls: Specific usage of verify_pre_usermode_state

On Wed, Apr 5, 2017 at 10:49 AM, Catalin Marinas
<catalin.marinas@....com> wrote:
> On Wed, Apr 05, 2017 at 07:36:17AM -0700, Thomas Garnier wrote:
>> On Wed, Apr 5, 2017 at 7:22 AM, Catalin Marinas <catalin.marinas@....com> wrote:
>> > On Tue, Apr 04, 2017 at 10:47:27AM -0700, Thomas Garnier wrote:
>> >> +
>> >>       ldr     x1, [tsk, #TSK_TI_FLAGS]
>> >>       and     x2, x1, #_TIF_WORK_MASK
>> >>       cbnz    x2, work_pending
>> >> @@ -779,6 +788,12 @@ finish_ret_to_user:
>> >>       kernel_exit 0
>> >>  ENDPROC(ret_to_user)
>> >>
>> >> +addr_limit_fail:
>> >> +     stp     x0, lr, [sp,#-16]!
>> >> +     bl      asm_verify_pre_usermode_state
>> >> +     ldp     x0, lr, [sp],#16
>> >> +     ret     lr
>> >
>> > Where is this supposed to return? What is the value of lr when branching
>> > to addr_limit_fail?
>>
>> It is not supposed to return. Do you think I should remove stp, ldp,
>> ret and jut add a brk 0x100 or jmp/call a break/bug function?
>
> Can you not just make addr_limit_fail a C function which never returns
> (similar to what we to with bad_mode() on arm64)? Since addr_limit_fail
> is only called when the segment is not the right one, I don't really see
> why you need another call to asm_verify_pre_usermode_state() to do a
> similar check again. Just panic in addr_limit_fail (unless I
> misunderstood what you are trying to achieve).

Calling asm_verify_pre_usermode_state has the advantage of having a
clear BUG_ON for the error (versus a panic description).

What do you think about replacing asm_verify_pre_usermode_state by a
"address_limit_fail" function that still calls
verify_pre_usermode_state but panic afterwards (because it should
never return)?

The assembly code would be easier to understand and in case of error
the BUG_ON is clear for the user.

>
> --
> Catalin



-- 
Thomas