lib: sbi_pmu: fix integer overflow in pmu_ctr_idx_validate

pmu_ctr_idx_validate() checks whether counter indices are in range
using cbase + sbi_fls(cmask) < total_ctrs.  Both operands are unsigned
long, so a crafted cbase close to ULONG_MAX causes the addition to wrap
around to a small value that passes the comparison.

Once validation is bypassed, sbi_pmu_ctr_cfg_match() with the
SKIP_MATCH flag uses the overflowed index directly as an array subscript
into phs->active_events[], producing an out-of-bounds read in M-mode.
Through the firmware-event code path, the same overflowed index reaches
fw_counters_data[] and fw_counters_started, giving an attacker OOB
write-zero and OOB bit-set primitives in M-mode memory.

Fix pmu_ctr_idx_validate() by checking for unsigned overflow before the
comparison, and add a secondary bounds check on cidx_first in the
SKIP_MATCH path so that even if validation is somehow bypassed in the
future, the array access remains bounded.

Signed-off-by: liutong <liutong@iscas.ac.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20260624035049.1753003-1-liutong@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>

lib/sbi/sbi_pmu.c

@@ -223,8 +223,16 @@ static int pmu_ctr_validate(struct sbi_pmu_hart_state *phs,
 
 static bool pmu_ctr_idx_validate(unsigned long cbase, unsigned long cmask)
 {
-	/* Do a basic sanity check of counter base & mask */
+	unsigned long last;
-	return cmask && cbase + sbi_fls(cmask) < total_ctrs;
+
+	if (!cmask)
+		return false;
+
+	last = sbi_fls(cmask);
+	if (cbase > -1UL - last)
+		return false;
+
+	return (cbase + last) < total_ctrs;
 }
 
 int sbi_pmu_ctr_fw_read(unsigned long cidx, uint64_t *cval, bool high_bits)
@@ -915,6 +923,9 @@ int sbi_pmu_ctr_cfg_match(unsigned long cidx_base, unsigned long cidx_mask,
 		 */
 		unsigned long cidx_first = cidx_base + sbi_ffs(cidx_mask);
 
+		if (cidx_first >= total_ctrs)
+			return SBI_EINVAL;
+
 		if (phs->active_events[cidx_first] == SBI_PMU_EVENT_IDX_INVALID)
 			return SBI_EINVAL;
 		ctr_idx = cidx_first;