x86/boot: Reject too long acpi_rsdp= values

cmdline_find_option() returns the full length of the parsed acpi_rsdp=
value. get_cmdline_acpi_rsdp() then silently truncates values which do
not fit in the val[] buffer.

Prevent boot_kstrtoul() from parsing a truncated value and then the
kernel from silently using the wrong RSDP address, see discussion in
Link:.

Issue a warning so that the user is aware that s/he supplied a malformed
value and can get feedback instead of silent crashes.

  [ bp: Make commit message more precise. ]

Fixes: 3c98e71 ("x86/boot: Add "acpi_rsdp=" early parsing")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20260617130417.36651-4-thorsten.blum@linux.dev

Author: toblux

arch/x86/boot/compressed/acpi.c

@@ -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");
+		return 0;
+	}
+
 	if (boot_kstrtoul(val, 16, &addr))
 		return 0;
 #endif