[XS] Change in coreboot[main]: lib/ramdetect: Limit probe size to avoid uintptr_t overflow

[Alper Nebi Yasak (Code Review)]

Alper Nebi Yasak has uploaded a new patch set (#2). ( https://review.coreboot.org/c/coreboot/+/81366?usp=email )

Change subject: lib/ramdetect: Limit probe size to avoid uintptr_t overflow
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lib/ramdetect: Limit probe size to avoid uintptr_t overflow

The probe_mb() function used in DRAM size detection and its variants
used for QEMU mainboards use the following address calculation which can
overflow on 32-bit architectures:

uintptr_t addr = dram_start + (size * MiB) - sizeof(uint32_t);

This can be observed on 32-bit RISC-V QEMU virtual machines where DRAM
starts at 2 GiB. The process starts bisecting RAM with an initial size
of 2 GiB (half of uintptr_t maximum) which makes the probed address just
below 4 GiB due to the last term. If we have less than 2 GiB of DRAM,
that probe fails and everything goes fine. Otherwise, following
iterations overflow and probe the [1 GiB, 2 GiB) address range mistaking
it for [3 GiB, 4 GiB) of DRAM. Somehow all of those succeed and we get a
wrong result of 4095 MiB.

Make sure we don't pass sizes to probe_mb() that will result in these
overflows. We could check for overflow in the calculation instead, but
we already have other similar checks in probe_ramsize() and the primary
user of this function (cbmem_top()) eventually returns uintptr_t anyway.

Change-Id: I65d0e97fff8708a034d1fd82fda32f0aecfb3382
Signed-off-by: Alper Nebi Yasak 
---
M src/lib/ramdetect.c
1 file changed, 3 insertions(+), 2 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/66/81366/2