RISCV: PMP: Correct pmpcfg register indexing for multi-register configs

The PMP initialization and thread context switching logic was incorrectly
assuming that the configuration for all global PMP entries was stored
and restored from the first PMP configuration register, pmpcfg0
(i.e., pmp_cfg[0]).

On RISC-V architectures, each pmpcfg register controls multiple PMP
entries (e.g., 4 entries on RV32). When the number of PMP entries exceeds
this, the configuration data spans across multiple pmpcfg registers
(pmpcfg0, pmpcfg1, etc.). Failure to save and restore the contents of the
correct pmpcfg register leads to an incorrect PMP setup, potentially
compromising memory protection boundaries.

This patch fixes the indexing by calculating the correct pmpcfg array
index using integer division (index / PMPCFG_STRIDE), where PMPCFG_STRIDE
represents the number of PMP entries per cfg register. The fixes are in:

1) z_riscv_pmp_init: When saving the configuration of the last global PMP
   entry, the content is now read from pmp_cfg[index / PMPCFG_STRIDE]
   instead of always from pmp_cfg[0].

2) z_riscv_pmp_thread_init: When restoring the global PMP configuration
   for a new thread, the saved value is now written back to
   pmp_cfg[global_pmp_end_index / PMPCFG_STRIDE].

Signed-off-by: Firas Sammoura <[email protected]>

Author: fsammoura1980

arch/riscv/core/pmp.c

@@ -432,12 +432,12 @@ void z_riscv_pmp_init(void)
 	/* Make sure secondary CPUs produced the same values */
 	if (global_pmp_end_index != 0) {
 		__ASSERT(global_pmp_end_index == index, "");
-		__ASSERT(global_pmp_cfg[0] == pmp_cfg[0], "");
+		__ASSERT(global_pmp_cfg[0] == pmp_cfg[index / PMPCFG_STRIDE], "");
 		__ASSERT(global_pmp_last_addr == pmp_addr[index - 1], "");
 	}
 #endif
 
-	global_pmp_cfg[0] = pmp_cfg[0];
+	global_pmp_cfg[0] = pmp_cfg[index / PMPCFG_STRIDE];
 	global_pmp_last_addr = pmp_addr[index - 1];
 	global_pmp_end_index = index;
 
@@ -457,9 +457,9 @@ static inline unsigned int z_riscv_pmp_thread_init(unsigned long *pmp_addr,
 	ARG_UNUSED(index_limit);
 
 	/*
-	 * Retrieve pmpcfg0 partial content from global entries.
+	 * Retrieve the pmpcfg register with partial content from global entries.
 	 */
-	pmp_cfg[0] = global_pmp_cfg[0];
+	pmp_cfg[global_pmp_end_index / PMPCFG_STRIDE] = global_pmp_cfg[0];
 
 	/*
 	 * Retrieve the pmpaddr value matching the last global PMP slot.