[pacman-dev] [Patch 1/1] Fix CVE-2016-5434 (DoS/loop and out of boundary read)

[Nils Freydank]

This is a rewrite of Tobias Stoeckmann’s patch from June 2016[1] using
functions instead of macros. (Thanks to Tobias for explanations of his patch.) 
A short question on Freenode IRC showed that macros are generally discouraged 
and functions should be used.

The patch introduces size_t length_check() and applies it to 
libalpm/signing.c and signing.h.

Feedback is very appreciated.


[1] Original patch:
https://lists.archlinux.org/pipermail/pacman-dev/2016-June/021148.html
CVE request (and assignment):
http://seclists.org/oss-sec/2016/q2/526

diff --git a/lib/libalpm/signing.c b/lib/libalpm/signing.c
index 95cb3280..f7a9c9a8 100644
--- a/lib/libalpm/signing.c
+++ b/lib/libalpm/signing.c
@@ -986,6 +986,19 @@ int SYMEXPORT alpm_siglist_cleanup(alpm_siglist_t 
*siglist)
 	return 0;
 }
 
+/* Check to avoid out of boundary reads */
+size_t length_check(size_t length, size_t position, size_t a,
+		alpm_handle_t *handle, const char *identifier)
+{
+	if( a == 0 || length - position <= a)
+	{
+	_alpm_log(handle, ALPM_LOG_ERROR, \
+	_("%s: signature format error"), identifier);
+	return -1;
+	}
+	else return 0;
+}
+
 /**
  * Extract the Issuer Key ID from a signature
  * @param sig PGP signature
@@ -1021,19 +1034,22 @@ int SYMEXPORT alpm_extract_keyid(alpm_handle_t 
*handle, const char *identifier,
 		}
 
 		switch(sig[pos] & 0x03) {
-			case 0:
-				blen = sig[pos + 1];
-				pos = pos + 2;
+			case 0: if( length_check(len, pos, 1, handle, identifier) ){ return 
-1; }
+				else blen = sig[pos + 1];
+				if( length_check(len, pos, 2, handle, identifier) ){ return -1; }
+				else pos = pos + 2;
 				break;
-
 			case 1:
-				blen = (sig[pos + 1] << 8) | sig[pos + 2];
-				pos = pos + 3;
+				if( length_check(len, pos, 2, handle, identifier) ){ return -1; }
+				else blen = (sig[pos + 1] << 8) | sig[pos + 2];
+				if( length_check(len, pos, 3, handle, identifier) ){ return -1; }
+				else pos = pos + 3;
 				break;
 
-			case 2:
-				blen = (sig[pos + 1] << 24) | (sig[pos + 2] << 16) | (sig[pos + 3] 
<< 8) | sig[pos + 4];
-				pos = pos + 5;
+			case 2:	if( length_check(len, pos, 4, handle, identifier) ){ return 
-1; }
+				else blen = (sig[pos + 1] << 24) | (sig[pos + 2] << 16) | (sig[pos 
+ 3] << 8) | sig[pos + 4];
+				if( length_check(len, pos, 5, handle, identifier) ){ return -1; }
+				else pos = pos + 5;
 				break;
 
 			case 3:
@@ -1057,25 +1073,38 @@ int SYMEXPORT alpm_extract_keyid(alpm_handle_t 
*handle, const char *identifier,
 			return -1;
 		}
 
-		pos = pos + 4;
+		if( length_check(len, pos, 4, handle, identifier) ){ return -1; }
+		else pos = pos + 4;
 
-		hlen = (sig[pos] << 8) | sig[pos + 1];
-		pos = pos + hlen + 2;
+		if( length_check(len, pos, 1, handle, identifier) ){ return -1; }
+		else hlen = (sig[pos] << 8) | sig[pos + 1];
 
-		ulen = (sig[pos] << 8) | sig[pos + 1];
-		pos = pos + 2;
+		if( length_check(len, pos, 2, handle, identifier) ){ return -1; }
+		else pos = pos + hlen + 2;
+
+		if( length_check(len, pos, 1, handle, identifier) ){ return -1; }
+		else ulen = (sig[pos] << 8) | sig[pos + 1];
+
+		if( length_check(len, pos, 2, handle, identifier) ){ return -1; }
+		else pos = pos + 2;
 
 		spos = pos;
 
+		if( length_check(len, pos, ulen, handle, identifier) ){ return -1; }
+		else
 		while(spos < pos + ulen) {
 			if(sig[spos] < 192) {
 				slen = sig[spos];
-				spos = spos + 1;
+				if( length_check(len + ulen, spos, 1, handle, identifier) ){ 
return -1; }
+				else spos = spos + 1;
 			} else if(sig[spos] < 255) {
-				slen = (sig[spos] << 8) | sig[spos + 1];
-				spos = spos + 2;
+				if( length_check(pos + ulen, spos, 1, handle, identifier) ){ 
return -1; }
+				else slen = (sig[spos] << 8) | sig[spos + 1];
+				if( length_check(pos + ulen, spos, 2, handle, identifier) ){ 
return -1; }
+				else spos = spos + 2;
 			} else {
-				slen = (sig[spos + 1] << 24) | (sig[spos + 2] << 16) | (sig[spos + 
3] << 8) | sig[spos + 4];
+				if( length_check(len, pos, 4, handle, identifier) ){ return -1; }
+				else slen = (sig[spos + 1] << 24) | (sig[spos + 2] << 16) | 
(sig[spos + 3] << 8) | sig[spos + 4];
 				spos = spos + 5;
 			}
 
@@ -1083,17 +1112,20 @@ int SYMEXPORT alpm_extract_keyid(alpm_handle_t 
*handle, const char *identifier,
 				/* issuer key ID */
 				char key[17];
 				size_t i;
+				if( length_check(pos + ulen, spos, 8, handle, identifier) ){return 
-1; }
+				else{
 				for (i = 0; i < 8; i++) {
 					sprintf(&key[i * 2], "%02X", sig[spos + i + 1]);
 				}
+				}
 				*keys = alpm_list_add(*keys, strdup(key));
 				break;
 			}
-
-			spos = spos + slen;
+			if( length_check(pos + ulen + 1, spos, slen, handle, identifier) )
{return -1; }
+			else spos = spos + slen;
 		}
-
-		pos = pos + (blen - hlen - 8);
+		if( length_check( blen, hlen, 8, handle, identifier ) ){return -1;}
+		else pos = pos + (blen - hlen - 8);
 	}
 
 	return 0;
diff --git a/lib/libalpm/signing.h b/lib/libalpm/signing.h
index 3507f584..a67bd900 100644
--- a/lib/libalpm/signing.h
+++ b/lib/libalpm/signing.h
@@ -36,4 +36,6 @@ int _alpm_key_import(alpm_handle_t *handle, const char 
*fpr);
 
 #endif /* ALPM_SIGNING_H */
 
+size_t length_check(size_t length, size_t position, size_t a,
+		alpm_handle_t *handle, const char *identifier);


-- 
GPG fingerprint: '766B 8122 1342 6912 3401 492A 8B54 D7A3 FF3C DB17'
Nils Freydank / Holgersson
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