Commit message: Skip 0-length ACL fields

Currently, it is possible to create an archive that crashes bsdtar
with a malformed ACL:

Program received signal SIGSEGV, Segmentation fault.
archive_acl_from_text_l (acl=<optimised out>, text=0x7e2e92 "", want_type=<optimised out>, sc=<optimised out>) at libarchive/archive_acl.c:1726
1726				switch (*s) {
(gdb) p n
$1 = 1
(gdb) p field[n]
$2 = {start = 0x0, end = 0x0}

Stop this by checking that the length is not zero before beginning
the switch statement.

I am pretty sure this is the bug mentioned in the qsym paper [1],
and I was able to replicate it with a qsym + AFL + afl-rb setup.

[1] https://www.usenix.org/conference/usenixsecurity18/presentation/yun
CVE: CVE-2018-1000879
CWE: CWE-476
Vulnerability description: libarchive version commit 379867ecb330b3a952fb7bfa7bffb7bbd5547205 onwards (release v3.3.0 onwards) contains a CWE-476: NULL Pointer Dereference vulnerability in ACL parser - libarchive/archive_acl.c, archive_acl_from_text_l() that can result in Crash/DoS. This attack appear to be exploitable via the victim must open a specially crafted archive file.

Project: libarchive
File: archive_acl.c

archive_acl_from_text_l(struct archive_acl *acl, const char *text,
    int want_type, struct archive_string_conv *sc)
{
	struct {
		const char *start;
		const char *end;
	} field[6], name;

	const char *s, *st;
	int numfields, fields, n, r, sol, ret;
	int type, types, tag, permset, id;
	size_t len;
	char sep;

	switch (want_type) {
	case ARCHIVE_ENTRY_ACL_TYPE_POSIX1E:
		want_type = ARCHIVE_ENTRY_ACL_TYPE_ACCESS;
		__LA_FALLTHROUGH;
	case ARCHIVE_ENTRY_ACL_TYPE_ACCESS:
	case ARCHIVE_ENTRY_ACL_TYPE_DEFAULT:
		numfields = 5;
		break;
	case ARCHIVE_ENTRY_ACL_TYPE_NFS4:
		numfields = 6;
		break;
	default:
		return (ARCHIVE_FATAL);
	}

	ret = ARCHIVE_OK;
	types = 0;

	while (text != NULL &&  *text != '\0') {
		/*
		 * Parse the fields out of the next entry,
		 * advance 'text' to start of next entry.
		 */
		fields = 0;
		do {
			const char *start, *end;
			next_field(&text, &start, &end, &sep);
			if (fields < numfields) {
				field[fields].start = start;
				field[fields].end = end;
			}
			++fields;
		} while (sep == ':');

		/* Set remaining fields to blank. */
		for (n = fields; n < numfields; ++n)
			field[n].start = field[n].end = NULL;

		if (field[0].start != NULL && *(field[0].start) == '#') {
			/* Comment, skip entry */
			continue;
		}

		n = 0;
		sol = 0;
		id = -1;
		permset = 0;
		name.start = name.end = NULL;

		if (want_type != ARCHIVE_ENTRY_ACL_TYPE_NFS4) {
			/* POSIX.1e ACLs */
			/*
			 * Default keyword "default:user::rwx"
			 * if found, we have one more field
			 *
			 * We also support old Solaris extension:
			 * "defaultuser::rwx" is the default ACL corresponding
			 * to "user::rwx", etc. valid only for first field
			 */
			s = field[0].start;
			len = field[0].end - field[0].start;
			if (*s == 'd' && (len == 1 || (len >= 7
			    && memcmp((s + 1), "efault", 6) == 0))) {
				type = ARCHIVE_ENTRY_ACL_TYPE_DEFAULT;
				if (len > 7)
					field[0].start += 7;
				else
					n = 1;
			} else
				type = want_type;

			/* Check for a numeric ID in field n+1 or n+3. */
			isint(field[n + 1].start, field[n + 1].end, &id);
			/* Field n+3 is optional. */
			if (id == -1 && fields > (n + 3))
				isint(field[n + 3].start, field[n + 3].end,
				    &id);

			tag = 0;
			s = field[n].start;
 			st = field[n].start + 1;
 			len = field[n].end - field[n].start;
 
			if (len == 0) {
				ret = ARCHIVE_WARN;
				continue;
			}

 			switch (*s) {
 			case 'u':
 				if (len == 1 || (len == 4
				    && memcmp(st, "ser", 3) == 0))
					tag = ARCHIVE_ENTRY_ACL_USER_OBJ;
				break;
			case 'g':
				if (len == 1 || (len == 5
				    && memcmp(st, "roup", 4) == 0))
					tag = ARCHIVE_ENTRY_ACL_GROUP_OBJ;
				break;
			case 'o':
				if (len == 1 || (len == 5
				    && memcmp(st, "ther", 4) == 0))
					tag = ARCHIVE_ENTRY_ACL_OTHER;
				break;
			case 'm':
				if (len == 1 || (len == 4
				    && memcmp(st, "ask", 3) == 0))
					tag = ARCHIVE_ENTRY_ACL_MASK;
				break;
			default:
					break;
			}

			switch (tag) {
			case ARCHIVE_ENTRY_ACL_OTHER:
			case ARCHIVE_ENTRY_ACL_MASK:
				if (fields == (n + 2)
				    && field[n + 1].start < field[n + 1].end
				    && ismode(field[n + 1].start,
				    field[n + 1].end, &permset)) {
					/* This is Solaris-style "other:rwx" */
					sol = 1;
				} else if (fields == (n + 3) &&
				    field[n + 1].start < field[n + 1].end) {
					/* Invalid mask or other field */
					ret = ARCHIVE_WARN;
					continue;
				}
				break;
			case ARCHIVE_ENTRY_ACL_USER_OBJ:
			case ARCHIVE_ENTRY_ACL_GROUP_OBJ:
				if (id != -1 ||
				    field[n + 1].start < field[n + 1].end) {
					name = field[n + 1];
					if (tag == ARCHIVE_ENTRY_ACL_USER_OBJ)
						tag = ARCHIVE_ENTRY_ACL_USER;
					else
						tag = ARCHIVE_ENTRY_ACL_GROUP;
				}
				break;
			default:
				/* Invalid tag, skip entry */
				ret = ARCHIVE_WARN;
				continue;
			}

			/*
			 * Without "default:" we expect mode in field 3
			 * Exception: Solaris other and mask fields
			 */
			if (permset == 0 && !ismode(field[n + 2 - sol].start,
			    field[n + 2 - sol].end, &permset)) {
				/* Invalid mode, skip entry */
				ret = ARCHIVE_WARN;
				continue;
			}
		} else {
			/* NFS4 ACLs */
			s = field[0].start;
			len = field[0].end - field[0].start;
			tag = 0;

			switch (len) {
			case 4:
				if (memcmp(s, "user", 4) == 0)
					tag = ARCHIVE_ENTRY_ACL_USER;
				break;
			case 5:
				if (memcmp(s, "group", 5) == 0)
					tag = ARCHIVE_ENTRY_ACL_GROUP;
				break;
			case 6:
				if (memcmp(s, "owner@", 6) == 0)
					tag = ARCHIVE_ENTRY_ACL_USER_OBJ;
				else if (memcmp(s, "group@", 6) == 0)
					tag = ARCHIVE_ENTRY_ACL_GROUP_OBJ;
				break;
			case 9:
				if (memcmp(s, "everyone@", 9) == 0)
					tag = ARCHIVE_ENTRY_ACL_EVERYONE;
				break;
			default:
				break;
			}

			if (tag == 0) {
				/* Invalid tag, skip entry */
				ret = ARCHIVE_WARN;
				continue;
			} else if (tag == ARCHIVE_ENTRY_ACL_USER ||
			    tag == ARCHIVE_ENTRY_ACL_GROUP) {
				n = 1;
				name = field[1];
				isint(name.start, name.end, &id);
			} else
				n = 0;

			if (!is_nfs4_perms(field[1 + n].start,
			    field[1 + n].end, &permset)) {
				/* Invalid NFSv4 perms, skip entry */
				ret = ARCHIVE_WARN;
				continue;
			}
			if (!is_nfs4_flags(field[2 + n].start,
			    field[2 + n].end, &permset)) {
				/* Invalid NFSv4 flags, skip entry */
				ret = ARCHIVE_WARN;
				continue;
			}
			s = field[3 + n].start;
			len = field[3 + n].end - field[3 + n].start;
			type = 0;
			if (len == 4) {
				if (memcmp(s, "deny", 4) == 0)
					type = ARCHIVE_ENTRY_ACL_TYPE_DENY;
			} else if (len == 5) {
				if (memcmp(s, "allow", 5) == 0)
					type = ARCHIVE_ENTRY_ACL_TYPE_ALLOW;
				else if (memcmp(s, "audit", 5) == 0)
					type = ARCHIVE_ENTRY_ACL_TYPE_AUDIT;
				else if (memcmp(s, "alarm", 5) == 0)
					type = ARCHIVE_ENTRY_ACL_TYPE_ALARM;
			}
			if (type == 0) {
				/* Invalid entry type, skip entry */
				ret = ARCHIVE_WARN;
				continue;
			}
			isint(field[4 + n].start, field[4 + n].end,
			    &id);
		}

		/* Add entry to the internal list. */
		r = archive_acl_add_entry_len_l(acl, type, permset,
		    tag, id, name.start, name.end - name.start, sc);
		if (r < ARCHIVE_WARN)
			return (r);
		if (r != ARCHIVE_OK)
			ret = ARCHIVE_WARN;
		types |= type;
	}

	/* Reset ACL */
	archive_acl_reset(acl, types);

	return (ret);
}