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Subject: Re: [PATCH -next] x86/xen: Fix read buffer overflow
To:     YueHaibing <yuehaibing@huawei.com>, boris.ostrovsky@oracle.com,
        sstabellini@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com
Cc:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        x86@kernel.org
References: <20181218081910.18080-1-yuehaibing@huawei.com>
From:   Juergen Gross <jgross@suse.com>
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Date:   Tue, 18 Dec 2018 09:31:26 +0100
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On 18/12/2018 09:19, YueHaibing wrote:
> Fix smatch warning:
> 
> arch/x86/xen/enlighten_pv.c:649 get_trap_addr() error:
>  buffer overflow 'early_idt_handler_array' 32 <= 32
> 
> Fixes: 42b3a4cb5609 ("x86/xen: Support early interrupts in xen pv guests")
> Signed-off-by: YueHaibing <yuehaibing@huawei.com>
> ---
>  arch/x86/xen/enlighten_pv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 2f6787f..81f200d 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -646,7 +646,7 @@ static bool __ref get_trap_addr(void **addr, unsigned int ist)
>  
>  	if (nr == ARRAY_SIZE(trap_array) &&
>  	    *addr >= (void *)early_idt_handler_array[0] &&
> -	    *addr < (void *)early_idt_handler_array[NUM_EXCEPTION_VECTORS]) {
> +	    *addr < (void *)early_idt_handler_array[NUM_EXCEPTION_VECTORS - 1]) {
>  		nr = (*addr - (void *)early_idt_handler_array[0]) /
>  		     EARLY_IDT_HANDLER_SIZE;
>  		*addr = (void *)xen_early_idt_handler_array[nr];
> 

No, this patch is wrong.

early_idt_handler_array is a 2-dimensional array:

const char
early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_SIZE];

So above code doesn't do an out of bounds array access, but checks for
*addr being in the array or outside of it (note the "<" used for the
test).


Juergen