hw/net/smc91c111: Don't allow data register access to overrun buffer

hw/net/smc91c111: Don't allow data register access to overrun buffer

For accesses to the 91c111 data register, the address within the
packet's data frame is determined by a combination of the pointer
register and the offset used to access the data register, so that you
can access data at effectively wider than byte width.  The pointer
register's pointer field is 11 bits wide, which is exactly the size
to index a 2048-byte data frame.

We weren't quite getting the logic right for ensuring that we end up
with a pointer value to use in the s->data[][] array that isn't out
of bounds:

 * we correctly mask when getting the initial pointer value
 * for the "autoincrement the pointer register" case, we
   correctly mask after adding 1 so that the pointer register
   wraps back around at the 2048 byte mark
 * but for the non-autoincrement case where we have to add the
   low 2 bits of the data register offset, we don't account
   for the possibility that the pointer register is 0x7ff
   and the addition should wrap

Fix this bug by factoring out the "get the p value to use as an array
index" into a function, making it use FIELD macro names rather than
hard-coded constants, and having a utility function that does "add a
value and wrap it" that we can use both for the "autoincrement" and
"add the offset bits" codepaths.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2758
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250228191652.1957208-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 700d3d6dd41de3bd3f1153e3cfe00b93f99b1441)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>