Re: [Xen-devel] [PATCH -next] x86/xen: Fix read buffer overflow

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/12/2018 10:42, YueHaibing wrote:
> 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@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).
> Thank you for your explanation.

This looks like a smatch bug.  I'd feed it back upstream.

It is explicitly permitted in the C spec to construct a pointer to
one-past-the-end of an array, for the purposes of a < comparison.

I'm not entirely sure where the "32 <= 32" statement is coming from.

~Andrew