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

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Message-ID: <20170407161159.GB20645@e104818-lin.cambridge.arm.com>
Date:   Fri, 7 Apr 2017 17:11:59 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Will Deacon <will.deacon@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        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>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Brian Gerst <brgerst@...il.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>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        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>,
        Ingo Molnar <mingo@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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 05, 2017 at 11:14:34AM -0700, Thomas Garnier wrote:
> 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.

It looks fine to me, though I'd have to see the patch.

-- 
Catalin