ACPI: GTDT: Relax sanity checking on Platform Timers array count

Perhaps unsurprisingly there are some platforms where the GTDT isn't
quite right and the Platforms Timer array overflows the length of the
overall table.

While the recently-added sanity checking isn't wrong, it makes it
impossible to boot the kernel on offending platforms. Try to hobble
along and limit the Platform Timer count to the bounds of the table.

Cc: Marc Zyngier <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Zheng Zengkai <[email protected]>
Cc: [email protected]
Fixes: 263e22d ("ACPI: GTDT: Tighten the check for the array of platform timer structures")
Signed-off-by: Oliver Upton <[email protected]>
Acked-by: Marc Zyngier <[email protected]>
Reviewed-by: Lorenzo Pieralisi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: oupton

drivers/acpi/arm64/gtdt.c

@@ -163,7 +163,7 @@ int __init acpi_gtdt_init(struct acpi_table_header *table,
 {
 	void *platform_timer;
 	struct acpi_table_gtdt *gtdt;
-	int cnt = 0;
+	u32 cnt = 0;
 
 	gtdt = container_of(table, struct acpi_table_gtdt, header);
 	acpi_gtdt_desc.gtdt = gtdt;
@@ -188,13 +188,17 @@ int __init acpi_gtdt_init(struct acpi_table_header *table,
 		cnt++;
 
 	if (cnt != gtdt->platform_timer_count) {
+		cnt = min(cnt, gtdt->platform_timer_count);
+		pr_err(FW_BUG "limiting Platform Timer count to %d\n", cnt);
+	}
+
+	if (!cnt) {
 		acpi_gtdt_desc.platform_timer = NULL;
-		pr_err(FW_BUG "invalid timer data.\n");
-		return -EINVAL;
+		return 0;
 	}
 
 	if (platform_timer_count)
-		*platform_timer_count = gtdt->platform_timer_count;
+		*platform_timer_count = cnt;
 
 	return 0;
 }