iscsi_ibft: Fix UBSAN shift-out-of-bounds warning in ibft_attr_show_nic()

jira VULN-56208
cve CVE-2025-21993
commit-author Chengen Du <chengen.du@canonical.com>
commit 07e0d99

When performing an iSCSI boot using IPv6, iscsistart still reads the
/sys/firmware/ibft/ethernetX/subnet-mask entry. Since the IPv6 prefix
length is 64, this causes the shift exponent to become negative,
triggering a UBSAN warning. As the concept of a subnet mask does not
apply to IPv6, the value is set to ~0 to suppress the warning message.

	Signed-off-by: Chengen Du <chengen.du@canonical.com>
	Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 07e0d99)
	Signed-off-by: Pratham Patel <ppatel@ciq.com>

Author: thefossguy-ciq

drivers/firmware/iscsi_ibft.c

@@ -316,7 +316,10 @@ static ssize_t ibft_attr_show_nic(void *data, int type, char *buf)
 		str += sprintf_ipaddr(str, nic->ip_addr);
 		break;
 	case ISCSI_BOOT_ETH_SUBNET_MASK:
-		val = cpu_to_be32(~((1 << (32-nic->subnet_mask_prefix))-1));
+		if (nic->subnet_mask_prefix > 32)
+			val = cpu_to_be32(~0);
+		else
+			val = cpu_to_be32(~((1 << (32-nic->subnet_mask_prefix))-1));
 		str += sprintf(str, "%pI4", &val);
 		break;
 	case ISCSI_BOOT_ETH_ORIGIN: