Fallback to detached signatures during keyring check

Pacman has a 'key in keyring' verification step that makes sure the signatures
have a valid keyid. Currently pacman parses embedded package signatures only.

Add a fallback to detached signatures. If embedded signature is missing then it
tries to read corresponding *.sig file and get keyid from there.

Verification:
  debug: found cached pkg: /var/cache/pacman/pkg/glib-networking-2.64.3-1-x86_64.pkg.tar.zst
  debug: found detached signature /var/cache/pacman/pkg/glib-networking-2.64.3-1-x86_64.pkg.tar.zst.sig with size 310
  debug: found signature key: A5E9288C4FA415FA
  debug: looking up key A5E9288C4FA415FA locally
  debug: key lookup success, key exists

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Allan McRae <allan@archlinux.org>

lib/libalpm/alpm.h

@@ -1405,6 +1405,17 @@ alpm_db_t *alpm_pkg_get_db(alpm_pkg_t *pkg);
  */
 const char *alpm_pkg_get_base64_sig(alpm_pkg_t *pkg);
 
+/** Extracts package signature either from embedded package signature
+ * or if it is absent then reads data from detached signature file.
+ * @param pkg a pointer to package.
+ * @param sig output parameter for signature data. Callee function allocates
+ * a buffer needed for the signature data. Caller is responsible for
+ * freeing this buffer.
+ * @param sig_len output parameter for the signature data length.
+ * @return 0 on success, negative number on error.
+ */
+int alpm_pkg_get_sig(alpm_pkg_t *pkg, unsigned char **sig, size_t *sig_len);
+
 /** Returns the method used to validate a package during install.
  * @param pkg a pointer to package
  * @return an enum member giving the validation method

lib/libalpm/package.c

@@ -268,6 +268,46 @@ const char SYMEXPORT *alpm_pkg_get_base64_sig(alpm_pkg_t *pkg)
 	return pkg->base64_sig;
 }
 
+int SYMEXPORT alpm_pkg_get_sig(alpm_pkg_t *pkg, unsigned char **sig, size_t *sig_len)
+{
+	if(pkg != NULL) {
+		RET_ERR(pkg->handle, ALPM_ERR_WRONG_ARGS, -1);
+	}
+
+	if(pkg->base64_sig) {
+		int ret = alpm_decode_signature(pkg->base64_sig, sig, sig_len);
+		if(ret != 0) {
+			RET_ERR(pkg->handle, ALPM_ERR_SIG_INVALID, -1);
+		}
+		return 0;
+	} else {
+		char *pkgpath = NULL, *sigpath = NULL;
+		alpm_errno_t err;
+		int ret = -1;
+
+		pkgpath = _alpm_filecache_find(pkg->handle, pkg->filename);
+		if(!pkgpath) {
+			GOTO_ERR(pkg->handle, ALPM_ERR_PKG_NOT_FOUND, cleanup);
+		}
+		sigpath = _alpm_sigpath(pkg->handle, pkgpath);
+		if(!sigpath || _alpm_access(pkg->handle, NULL, sigpath, R_OK)) {
+			GOTO_ERR(pkg->handle, ALPM_ERR_SIG_MISSING, cleanup);
+		}
+		err = _alpm_read_file(sigpath, sig, sig_len);
+		if(err == ALPM_ERR_OK) {
+			_alpm_log(pkg->handle, ALPM_LOG_DEBUG, "found detached signature %s with size %ld\n",
+				sigpath, *sig_len);
+		} else {
+			GOTO_ERR(pkg->handle, err, cleanup);
+		}
+		ret = 0;
+cleanup:
+		FREE(pkgpath);
+		FREE(sigpath);
+		return ret;
+	}
+}
+
 const char SYMEXPORT *alpm_pkg_get_arch(alpm_pkg_t *pkg)
 {
 	ASSERT(pkg != NULL, return NULL);

lib/libalpm/sync.c

@@ -880,18 +880,18 @@ static int check_keyring(alpm_handle_t *handle)
 		}
 
 		level = alpm_db_get_siglevel(alpm_pkg_get_db(pkg));
-		if((level & ALPM_SIG_PACKAGE) && pkg->base64_sig) {
-			unsigned char *decoded_sigdata = NULL;
-			size_t data_len;
-			int decode_ret = alpm_decode_signature(pkg->base64_sig,
-					&decoded_sigdata, &data_len);
-			if(decode_ret == 0) {
+		if((level & ALPM_SIG_PACKAGE)) {
+			unsigned char *sig = NULL;
+			size_t sig_len;
+			int ret = alpm_pkg_get_sig(pkg, &sig, &sig_len);
+			if(ret == 0) {
 				alpm_list_t *keys = NULL;
-				if(alpm_extract_keyid(handle, pkg->name, decoded_sigdata,
-							data_len, &keys) == 0) {
+				if(alpm_extract_keyid(handle, pkg->name, sig,
+							sig_len, &keys) == 0) {
 					alpm_list_t *k;
 					for(k = keys; k; k = k->next) {
 						char *key = k->data;
+						_alpm_log(handle, ALPM_LOG_DEBUG, "found signature key: %s\n", key);
 						if(!alpm_list_find(errors, key, key_cmp) &&
 								_alpm_key_in_keychain(handle, key) == 0) {
 							keyinfo = malloc(sizeof(struct keyinfo_t));
@@ -905,8 +905,8 @@ static int check_keyring(alpm_handle_t *handle)
 					}
 					FREELIST(keys);
 				}
-				free(decoded_sigdata);
 			}
+			free(sig);
 		}
 	}
 

lib/libalpm/util.c

@@ -1489,3 +1489,40 @@ void _alpm_alloc_fail(size_t size)
 {
 	fprintf(stderr, "alloc failure: could not allocate %zu bytes\n", size);
 }
+
+/** This functions reads file content.
+ *
+ * Memory buffer is allocated by the callee function. It is responsibility
+ * of the caller to free the buffer.
+ *
+ * @param filepath filepath to read
+ * @param data pointer to output buffer
+ * @param data_len size of the output buffer
+ * @return error code for the operation
+ */
+alpm_errno_t _alpm_read_file(const char *filepath, unsigned char **data, size_t *data_len)
+{
+	struct stat st;
+	FILE *fp;
+
+	if((fp = fopen(filepath, "rb")) == NULL) {
+		return ALPM_ERR_NOT_A_FILE;
+	}
+
+	if(fstat(fileno(fp), &st) != 0) {
+		fclose(fp);
+		return ALPM_ERR_NOT_A_FILE;
+	}
+	*data_len = st.st_size;
+
+	MALLOC(*data, *data_len, fclose(fp); return ALPM_ERR_MEMORY);
+
+	if(fread(*data, *data_len, 1, fp) != 1) {
+		FREE(*data);
+		fclose(fp);
+		return ALPM_ERR_SYSTEM;
+	}
+
+	fclose(fp);
+	return ALPM_ERR_OK;
+}

lib/libalpm/util.h

@@ -155,6 +155,7 @@ int _alpm_fnmatch_patterns(alpm_list_t *patterns, const char *string);
 int _alpm_fnmatch(const void *pattern, const void *string);
 void *_alpm_realloc(void **data, size_t *current, const size_t required);
 void *_alpm_greedy_grow(void **data, size_t *current, const size_t required);
+alpm_errno_t _alpm_read_file(const char *filepath, unsigned char **data, size_t *data_len);
 
 #ifndef HAVE_STRSEP
 char *strsep(char **, const char *);