Re: [PATCH 2/4] hw/misc/bcm2835_property: Avoid overflow in OTP access properties

[Peter Maydell]

On Wed, 24 Jul 2024 at 08:06, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> Hi Peter,
>
> On 23/7/24 15:10, Peter Maydell wrote:
> > Coverity points out that in our handling of the property
> > RPI_FWREQ_SET_CUSTOMER_OTP we have a potential overflow.  This
> > happens because we read start_num and number from the guest as
> > unsigned 32 bit integers, but then the variable 'n' we use as a loop
> > counter as we iterate from start_num to start_num + number is only an
> > "int".  That means that if the guest passes us a very large start_num
> > we will interpret it as negative.  This will result in an assertion
> > failure inside bcm2835_otp_set_row(), which checks that we didn't
> > pass it an invalid row number.
> >
> > A similar issue applies to all the properties for accessing OTP rows
> > where we are iterating through with a start and length read from the
> > guest.
> >
> > Use uint32_t for the loop counter to avoid this problem. Because in
> > all cases 'n' is only used as a loop counter, we can do this as
> > part of the for(), restricting its scope to exactly where we need it.
> >
> > Resolves: Coverity CID 1549401
> > Cc: qemu-stable@nongnu.org
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>
> > ---
> >   hw/misc/bcm2835_property.c | 9 ++++-----
> >   1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/hw/misc/bcm2835_property.c b/hw/misc/bcm2835_property.c
> > index e28fdca9846..7eb623b4e90 100644
> > --- a/hw/misc/bcm2835_property.c
> > +++ b/hw/misc/bcm2835_property.c
> > @@ -30,7 +30,6 @@ static void 
> > bcm2835_property_mbox_push(BCM2835PropertyState *s, uint32_t value)
> >       uint32_t tot_len;
> >       size_t resplen;
> >       uint32_t tmp;
> > -    int n;
> >       uint32_t start_num, number, otp_row;
> >
> >       /*
> > @@ -337,7 +336,7 @@ static void 
> > bcm2835_property_mbox_push(BCM2835PropertyState *s, uint32_t value)
> >
> >               resplen = 8 + 4 * number;
> >
> > -            for (n = start_num; n < start_num + number &&
> > +            for (uint32_t n = start_num; n < start_num + number &&
> >                    n < BCM2835_OTP_CUSTOMER_OTP_LEN; n++) {
>
> I find not making the counter size explicit and use 'unsigned'
> simpler, since using 32-bit in particular doesn't bring much here.
> Is there a reason I'm missing?

I just wanted to match the types between n and start_num and
number (where the latter two should be uint32_t because we load
them from the guest as 32-bit values). Otherwise we're relying
on "unsigned" being at least 32 bit -- it is, but if we need
it to be 32 bit then why not use the type that is guaranteed
and says specifically that it's 32 bits ?

thanks
-- PMM