linux-kernel - Re: [PATCH -next] x86/xen: Fix read buffer overflow

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Date:   Tue, 18 Dec 2018 18:42:17 +0800
From:   YueHaibing <yuehaibing@...wei.com>
To:     Juergen Gross <jgross@...e.com>, <boris.ostrovsky@...cle.com>,
        <sstabellini@...nel.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
        <bp@...en8.de>, <hpa@...or.com>
CC:     <linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        <x86@...nel.org>
Subject: Re: [PATCH -next] x86/xen: Fix read buffer overflow

On 2018/12/18 16:31, Juergen Gross wrote:
> 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@...wei.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).

Thank you for your explanation.

> 
> 
> Juergen
> 
> .
>