Date: Wed, 5 Apr 2017 18:49:22 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Thomas Garnier <thgarnie@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>, "H . Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@kernel.org>,
        Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Will Deacon <will.deacon@arm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
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        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v6 4/4] arm64/syscalls: Specific usage of
 verify_pre_usermode_state
Message-ID: <20170405174921.GB2752@e104818-lin.cambridge.arm.com>
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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@arm.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).

-- 
Catalin