Re: [PATCH] xen: hypercall: fix out-of-bounds memcpy

[Arnd Bergmann]

On Sat, Feb 3, 2018 at 12:33 AM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> On 02/02/2018 10:32 AM, Arnd Bergmann wrote:
>> The legacy hypercall handlers were originally added with
>> a comment explaining that "copying the argument structures in
>> HYPERVISOR_event_channel_op() and HYPERVISOR_physdev_op() into the local
>> variable is sufficiently safe" and only made sure to not write
>> past the end of the argument structure, the checks in linux/string.h
>> disagree with that, when link-time optimizations are used:
>>
>> In function 'memcpy',
>>     inlined from 'pirq_query_unmask' at drivers/xen/fallback.c:53:2,
>>     inlined from '__startup_pirq' at drivers/xen/events/events_base.c:529:2,
>>     inlined from 'restore_pirqs' at drivers/xen/events/events_base.c:1439:3,
>>     inlined from 'xen_irq_resume' at drivers/xen/events/events_base.c:1581:2:
>> include/linux/string.h:350:3: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
>>    __read_overflow2();
>>    ^
>> make[3]: *** [ccLujFNx.ltrans15.ltrans.o] Error 1
>> make[3]: Target 'all' not remade because of errors.
>> lto-wrapper: fatal error: make returned 2 exit status
>> compilation terminated.
>> ld: error: lto-wrapper failed
>>
>> This changes the functions so that each argument is accessed with
>> exactly the correct length based on the command code.
>>
>> Fixes: cf47a83fb06e ("xen/hypercall: fix hypercall fallback code for very old hypervisors")
>> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
>> ---
>>  drivers/xen/fallback.c | 94 ++++++++++++++++++++++++++++----------------------
>>  1 file changed, 53 insertions(+), 41 deletions(-)
>>

>>       default:
>> -             WARN_ON(rc != -ENOSYS);
>> -             break;
>> +             return -ENOSYS;
>>       }
>>
>> +     memcpy(&op.u, arg, len);
>> +     rc = _hypercall1(int, event_channel_op_compat, &op);
>> +     memcpy(arg, &op.u, len);
>
>
> We don't copy back for all commands, only those that are COPY_BACK.

Not sure what you mean. Is it harmful to copy back the data for the others
in any way? Otherwise I wouldn't micro-optimize this.

        Arnd