reject non canonical encoding length.

fix #28.

Signed-off-by: Yang, Longlong <[email protected]>

spdmlib/src/crypto/x509v3.rs

@@ -25,7 +25,6 @@ const ASN1_TAG_SEQUENCE: u8 =
     ASN1_TAG_CLASS_UNIVERSAL_MASK | ASN1_FORM_CONSTRUCTED_MASK | ASN1_TAG_NUMBER_SEQUENCE;
 const ASN1_TAG_EXPLICIT_EXTENSION: u8 = 0xA3;
 const ASN1_TAG_EXTN_VALUE: u8 = 0x04;
-const ASN1_LENGTH_MULTI_OCTET_MASK: u8 = 0x80;
 
 const X509V3_VERSION: u8 = 2;
 const OID_RSA_SHA256RSA: &[u8] = &[0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01, 0x0bu8];
@@ -263,19 +262,43 @@ fn check_length(data: &[u8]) -> SpdmResult<(usize, usize)> {
     let len = data.len();
     if len < 1 {
         Err(SPDM_STATUS_VERIF_FAIL)
-    } else if data[0] & ASN1_LENGTH_MULTI_OCTET_MASK == 0 {
-        Ok((data[0] as usize, 1))
     } else {
-        let length_count = data[0] - ASN1_LENGTH_MULTI_OCTET_MASK;
-        if len < (length_count as usize + 1) || length_count == 0 || length_count > 8 {
-            Err(SPDM_STATUS_VERIF_FAIL)
-        } else {
-            let mut length = [0u8; 8];
-            for (i, b) in data[1..length_count as usize + 1].iter().rev().enumerate() {
-                length[i] = *b;
+        let length_byte0 = data[0];
+        let (length, byte_comsumed) = match length_byte0 {
+            n if (n & 0x80) == 0 => (n as usize, 1),
+            0x81 => {
+                if len < 2 {
+                    return Err(SPDM_STATUS_VERIF_FAIL);
+                } else {
+                    let second_byte = data[1] as usize;
+                    if second_byte < 0x80 {
+                        return Err(SPDM_STATUS_VERIF_FAIL); // Not the canonical encoding.
+                    } else {
+                        (second_byte, 2)
+                    }
+                }
             }
-            Ok((usize::from_le_bytes(length), length_count as usize + 1))
-        }
+            0x82 => {
+                if len < 3 {
+                    return Err(SPDM_STATUS_VERIF_FAIL);
+                } else {
+                    let second_byte = data[1] as usize;
+                    let third_byte = data[2] as usize;
+
+                    let combined = (second_byte << 8) | third_byte;
+                    if combined < 0x100 {
+                        return Err(SPDM_STATUS_VERIF_FAIL); // Not the canonical encoding.
+                    } else {
+                        (combined, 3)
+                    }
+                }
+            }
+            _ => {
+                return Err(SPDM_STATUS_VERIF_FAIL); // spdm-rs don't support longer lengths.
+            }
+        };
+
+        Ok((length, byte_comsumed))
     }
 }
 
@@ -998,13 +1021,13 @@ mod tests {
     #[test]
     fn test_case0_check_length() {
         let l1 = [0x03];
-        let l2 = [0x81, 0x12];
+        let l2 = [0x81, 0x82];
         let l3 = [0x82, 0x01, 0xD7];
         let l1_wrong = [0x80];
         let l2_wrong = [0x81];
         let l3_wrong = [0x82, 0x01];
         assert_eq!(check_length(&l1), Ok((3, 1)));
-        assert_eq!(check_length(&l2), Ok((0x12, 2)));
+        assert_eq!(check_length(&l2), Ok((0x82, 2)));
         assert_eq!(check_length(&l3), Ok((0x1D7, 3)));
         assert_eq!(check_length(&l1_wrong), Err(SPDM_STATUS_VERIF_FAIL));
         assert_eq!(check_length(&l2_wrong), Err(SPDM_STATUS_VERIF_FAIL));