x86/boot: Reject overlong acpi_rsdp= values

cmdline_find_option() returns the full length of the acpi_rsdp= value even if it is truncated. However, get_cmdline_acpi_rsdp() only checks whether acpi_rsdp= is present and does not reject overlong values that do not fit in the buffer.

Reject overlong values and warn to prevent boot_kstrtoul() from parsing a truncated value and thus from silently using the wrong RSDP address.

---
Changes in v2:
- Warn on overlong acpi_rsdp= values (Boris)
- v1: https://lore.kernel.org/r/[email protected]/

---
 arch/x86/boot/compressed/acpi.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index f196b1d1ddf8..baaad01d2074 100644
@@ -184,10 +184,15 @@ static unsigned long get_cmdline_acpi_rsdp(void)
 	char val[MAX_ADDR_LEN] = { };
 	int ret;
 
-	ret = cmdline_find_option("acpi_rsdp", val, MAX_ADDR_LEN);
+	ret = cmdline_find_option("acpi_rsdp", val, sizeof(val));
 	if (ret < 0)
 		return 0;
 
+	if (ret >= sizeof(val)) {
+		warn("acpi_rsdp= value too long; ignoring\n");
+		return 0;
+	}
+
 	if (boot_kstrtoul(val, 16, &addr))
 		return 0;
 #endif