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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
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