Re: [PATCH] q800: fix segfault with invalid MacROM

[BALATON Zoltan <balaton@eik.bme.hu>]

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On Fri, 7 Jan 2022, Laurent Vivier wrote:
> Le 07/01/2022 à 09:15, Mark Cave-Ayland a écrit :
>> On 06/01/2022 12:22, Laurent Vivier wrote:
>> 
>>> "qemu-system-m68k -M q800 -bios /dev/null" crahses with a segfault
>>> in q800_init().
>>> This happens because the code doesn't check that rom_ptr() returned
>>> a non-NULL pointer .
>>> 
>>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/756
>>> Reported-by: Peter Maydell <peter.maydell@linaro.org>
>>> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
>>> ---
>>>   hw/m68k/q800.c | 8 +++++++-
>>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/hw/m68k/q800.c b/hw/m68k/q800.c
>>> index e4c7c9b88ad0..6261716c8f7e 100644
>>> --- a/hw/m68k/q800.c
>>> +++ b/hw/m68k/q800.c
>>> @@ -672,10 +672,16 @@ static void q800_init(MachineState *machine)
>>>           /* Remove qtest_enabled() check once firmware files are in the 
>>> tree */
>>>           if (!qtest_enabled()) {
>>> -            if (bios_size < 0 || bios_size > MACROM_SIZE) {
>>> +            if (bios_size == -1) {
>>>                   error_report("could not load MacROM '%s'", bios_name);
>>>                   exit(1);
>>>               }
>>> +            if (bios_size != MACROM_SIZE) {
>>> +                error_report("Invalid size for MacROM '%s': %d bytes,"
>>> +                             " expected %d bytes", bios_name, bios_size,
>>> +                             MACROM_SIZE);
>>> +                exit(1);
>>> +            }
>>>               ptr = rom_ptr(MACROM_ADDR, MACROM_SIZE);
>>>               stl_phys(cs->as, 0, ldl_p(ptr));    /* reset initial SP */
>> 
>> The patch does fix the issue, but it seems a little odd that you can't use 
>> -bios path/to/m68k-binary to boot with an arbitrary sized binary which 
>> could be useful for reproducers such as 
>> https://gitlab.com/qemu-project/qemu/-/issues/360.
>> 
>> How easy would it be to add the extra rom_ptr() NULL check instead?
>> 
>
> I was thinking that a smaller binary can be padded to 1 MB for use because on 
> a real hardware the size of the ROM cannot be arbitrary.
>
> But it seems reasonable to check only for the NULL pointer rather than the 
> size, I'm going to send a v2.

Instead of adding !rom_ptr as well, isn't it enough to change to
bios_size <= 0 
in the existing check?

Regards,
BALATON Zoltan