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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 12/18/18 6:28 AM, Andrew Cooper wrote:
> 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@xxxxxxxxxx>
>>>> ---
>>>>  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.

+Dan

>
> 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


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