[PATCH] scsi: target: iscsi: validate CHAP_R length before base64 decode

chap_server_compute_hash() allocates client_digest as
kzalloc(chap->digest_size) and then, for BASE64-encoded responses,
passes chap_r directly to chap_base64_decode() without checking whether
the input length could produce more than digest_size bytes of output.

chap_base64_decode() writes to the destination unconditionally as long
as there is input to consume. With MAX_RESPONSE_LENGTH set to 128 and
the "0b" prefix stripped by extract_param(), up to 127 base64 characters
can reach the decoder. 127 characters decode to 95 bytes. For SHA-256
(digest_size=32) this overflows client_digest by 63 bytes; for MD5
(digest_size=16) the overflow is 79 bytes.

The length check at line 344 fires after the write has already happened.

The HEX branch in the same switch statement already validates the length
up front. Apply the same approach to the BASE64 branch: reject any input
whose maximum decoded length exceeds digest_size before calling the
decoder.

The formula (digest_size * 4 + 2) / 3 is the ceiling of digest_size *
4/3, i.e. the maximum number of base64 characters that can decode to
exactly digest_size bytes.

Fixes: 1e5733883421 ("scsi: target: iscsi: Support base64 in CHAP")
Cc: [email protected]
Signed-off-by: Alexandru Hossu <[email protected]>
---
 drivers/target/iscsi/iscsi_target_auth.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/target/iscsi/iscsi_target_auth.c b/drivers/target/iscsi/iscsi_target_auth.c
index c46c69a..653be1a 100644
--- a/drivers/target/iscsi/iscsi_target_auth.c
+++ b/drivers/target/iscsi/iscsi_target_auth.c
@@ -341,6 +341,10 @@ static int chap_server_compute_hash(
 		}
 		break;
 	case BASE64:
+		if (strlen(chap_r) > (chap->digest_size * 4 + 2) / 3) {
+			pr_err("Malformed CHAP_R: base64 payload too long\n");
+			goto out;
+		}
 		if (chap_base64_decode(client_digest, chap_r, strlen(chap_r)) !=
 		    chap->digest_size) {
 			pr_err("Malformed CHAP_R: invalid BASE64\n");
-- 
2.54.0