iommu/amd: Avoid stack buffer overflow from kernel cmdline

While the kernel command line is considered trusted in most environments,
avoid writing 1 byte past the end of "acpiid" if the "str" argument is
maximum length.

Reported-by: Simcha Kosman <simcha.kosman@cyberark.com>
Closes: https://lore.kernel.org/all/AS8P193MB2271C4B24BCEDA31830F37AE84A52@AS8P193MB2271.EURP193.PROD.OUTLOOK.COM
Fixes: b6b26d8 ("iommu/amd: Add a length limitation for the ivrs_acpihid command-line parameter")
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Ankit Soni <Ankit.Soni@amd.com>
Link: https://lore.kernel.org/r/20250804154023.work.970-kees@kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

Author: kees

drivers/iommu/amd/init.c

@@ -3638,7 +3638,7 @@ static int __init parse_ivrs_acpihid(char *str)
 {
 	u32 seg = 0, bus, dev, fn;
 	char *hid, *uid, *p, *addr;
-	char acpiid[ACPIID_LEN] = {0};
+	char acpiid[ACPIID_LEN + 1] = { }; /* size with NULL terminator */
 	int i;
 
 	addr = strchr(str, '@');
@@ -3664,7 +3664,7 @@ static int __init parse_ivrs_acpihid(char *str)
 	/* We have the '@', make it the terminator to get just the acpiid */
 	*addr++ = 0;
 
-	if (strlen(str) > ACPIID_LEN + 1)
+	if (strlen(str) > ACPIID_LEN)
 		goto not_found;
 
 	if (sscanf(str, "=%s", acpiid) != 1)