[XS] Change in coreboot[main]: mb/dell/optiplex_9020: Fix UB in package power calculation

[Nico Huber (Code Review)]

Nico Huber has submitted this change. ( https://review.coreboot.org/c/coreboot/+/83736?usp=email )

Change subject: mb/dell/optiplex_9020: Fix UB in package power calculation
......................................................................

mb/dell/optiplex_9020: Fix UB in package power calculation

Fix potential undefined behaviour in the `get_pkg_power()` function:
- If `rapl_power_unit == 0`, `pkg_power_info / rapl_power_unit` is
  invalid
- If `rapl_power_unit > 7`, the result of the shift doesn't fit into a
  `uint8_t`

Signed-off-by: Mate Kukri 
Change-Id: I48ef59c4fbeb0a55675ac24da31e6e0b194cb58d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83736
Reviewed-by: Angel Pons 
Reviewed-by: Nico Huber 
Tested-by: build bot (Jenkins) 
Reviewed-by: Elyes Haouas 
---
M src/mainboard/dell/optiplex_9020/mainboard.c
1 file changed, 3 insertions(+), 5 deletions(-)

Approvals:
  build bot (Jenkins): Verified
  Angel Pons: Looks good to me, approved
  Elyes Haouas: Looks good to me, approved
  Nico Huber: Looks good to me, approved

diff --git a/src/mainboard/dell/optiplex_9020/mainboard.c b/src/mainboard/dell/optiplex_9020/mainboard.c
index 6630a12..1cb850c4 100644
--- a/src/mainboard/dell/optiplex_9020/mainboard.c
+++ b/src/mainboard/dell/optiplex_9020/mainboard.c
@@ -303,11 +303,9 @@

static uint16_t get_pkg_power(void)
 {
-	uint8_t rapl_power_unit = rdmsr(0x606).lo & 0xf;
-	if (rapl_power_unit)
-		rapl_power_unit = 2 << (rapl_power_unit - 1);
-	uint16_t pkg_power_info = rdmsr(0x614).lo & 0x7fff;
-	if (pkg_power_info / rapl_power_unit > 0x41)
+	const unsigned int pkg_power = rdmsr(0x614).lo & 0x7fff;
+	const unsigned int power_unit = 1 << (rdmsr(0x606).lo & 0xf);
+	if (pkg_power / power_unit > 65)
    	return 32;
    else
    	return 16;