Date: Wed, 5 Apr 2017 15:22:25 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Thomas Garnier <thgarnie@google.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Dave Hansen <dave.hansen@intel.com>, Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Al Viro <viro@zeniv.linux.org.uk>, David Howells <dhowells@redhat.com>,
        =?iso-8859-1?Q?Ren=E9?= Nyffenegger <mail@renenyffenegger.ch>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
        Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        Kees Cook <keescook@chromium.org>, Rik van Riel <riel@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Russell King <linux@armlinux.org.uk>,
        Will Deacon <will.deacon@arm.com>, Mark Rutland <mark.rutland@arm.com>,
        James Morse <james.morse@arm.com>, linux-s390@vger.kernel.org,
        kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 4/4] arm64/syscalls: Specific usage of
 verify_pre_usermode_state
Message-ID: <20170405142225.GA9072@e104818-lin.cambridge.arm.com>
References: <20170404174727.35478-1-thgarnie@google.com>
 <20170404174727.35478-4-thgarnie@google.com>
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On Tue, Apr 04, 2017 at 10:47:27AM -0700, Thomas Garnier wrote:
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 43512d4d7df2..6d598e7051c3 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -744,6 +744,10 @@ ENDPROC(cpu_switch_to)
>  ret_fast_syscall:
>  	disable_irq				// disable interrupts
>  	str	x0, [sp, #S_X0]			// returned x0
> +	ldr	x2, [tsk, #TSK_TI_ADDR_LIMIT]	// check addr limit change
> +	mov	x1, #TASK_SIZE_64
> +	cmp	x2, x1
> +	b.ne	addr_limit_fail

KERNEL_DS is set to the maximum address (-1UL), so it would be easier to
check against this here and avoid a "mov". Even simpler if you'd check
against bit 63 of the address for KERNEL_DS:

	ldr	x1, [tsk, TSK_TI_ADDR_LIMIT]	// check addr limit change
	tbnz	x1, #63, addr_limit_fail	// KERNEL_DS is -1UL

>  	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for syscall tracing
>  	and	x2, x1, #_TIF_SYSCALL_WORK
>  	cbnz	x2, ret_fast_syscall_trace
> @@ -771,6 +775,11 @@ work_pending:
>   */
>  ret_to_user:
>  	disable_irq				// disable interrupts
> +	ldr	x2, [tsk, #TSK_TI_ADDR_LIMIT]	// check addr limit change
> +	mov	x1, #TASK_SIZE_64
> +	cmp	x2, x1
> +	b.ne	addr_limit_fail

Same here.

> +
>  	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?

-- 
Catalin