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

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Date:   Wed, 5 Apr 2017 15:22:25 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Thomas Garnier <thgarnie@...gle.com>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        René Nyffenegger <mail@...enyffenegger.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Rik van Riel <riel@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>, linux-s390@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 4/4] arm64/syscalls: Specific usage of
 verify_pre_usermode_state

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