fusefs: Handle buggy servers' LISTXATTR response

The fuse protocol requires server to respond to LISTXATTR with a
NUL-terminated string.  If they don't, report an error rather than
attempt to scan through uninitialized memory for a NUL.

Approved by:	so
Security:	FreeBSD-SA-26:20.fusefs
Security:	CVE-2026-45252
admbugs:	1039
Reported by:	Joshua Rogers
Sponsored by:	ConnectWise

sys/fs/fuse/fuse_ipc.h

@@ -240,6 +240,7 @@ struct fuse_data {
 #define FSESS_WARN_INODE_MISMATCH 0x4000000 /* ino != nodeid */
 #define	FSESS_SETXATTR_EXT	  0x8000000 /* extended fuse_setxattr_in */
 #define FSESS_AUTO_UNMOUNT	  0x10000000 /* perform unmount when server dies */
+#define FSESS_WARN_LSEXTATTR_NUL 0x20000000 /* Non nul-terminated xattr list */
 #define FSESS_MNTOPTS_MASK	( \
 	FSESS_DAEMON_CAN_SPY | FSESS_PUSH_SYMLINKS_IN | \
 	FSESS_DEFAULT_PERMISSIONS | FSESS_INTR | FSESS_AUTO_UNMOUNT)

sys/fs/fuse/fuse_vnops.c

@@ -2978,8 +2978,8 @@ fuse_vnop_setextattr(struct vop_setextattr_args *ap)
  * bsd_list, bsd_list_len - output list compatible with bsd vfs
  */
 static int
-fuse_xattrlist_convert(char *prefix, const char *list, int list_len,
-    char *bsd_list, int *bsd_list_len)
+fuse_xattrlist_convert(struct fuse_data *data, char *prefix, const char *list,
+    int list_len, char *bsd_list, int *bsd_list_len)
 {
 	int len, pos, dist_to_next, prefix_len;
 
@@ -2988,7 +2988,14 @@ fuse_xattrlist_convert(char *prefix, const char *list, int list_len,
 	prefix_len = strlen(prefix);
 
 	while (pos < list_len && list[pos] != '\0') {
-		dist_to_next = strlen(&list[pos]) + 1;
+		dist_to_next = strnlen(&list[pos], list_len - pos - 1) + 1;
+		if (list[pos + dist_to_next - 1] != '\0') {
+			fuse_warn(data, FSESS_WARN_LSEXTATTR_NUL,
+				"The FUSE server returned a non nul-terminated "
+				"LISTXATTR response.");
+			return (EXTERROR(EIO,
+				"The FUSE server returned a malformed list"));
+		}
 		if (bcmp(&list[pos], prefix, prefix_len) == 0 &&
 		    list[pos + prefix_len] == extattr_namespace_separator) {
 			len = dist_to_next -
@@ -3044,6 +3051,7 @@ fuse_vnop_listextattr(struct vop_listextattr_args *ap)
 	struct fuse_listxattr_in *list_xattr_in;
 	struct fuse_listxattr_out *list_xattr_out;
 	struct mount *mp = vnode_mount(vp);
+	struct fuse_data *data = fuse_get_mpdata(mp);
 	struct thread *td = ap->a_td;
 	struct ucred *cred = ap->a_cred;
 	char *prefix;
@@ -3124,8 +3132,6 @@ fuse_vnop_listextattr(struct vop_listextattr_args *ap)
 	linux_list = fdi.answ;
 	/* FUSE doesn't allow the server to return more data than requested */
 	if (fdi.iosize > linux_list_len) {
-		struct fuse_data *data = fuse_get_mpdata(mp);
-
 		fuse_warn(data, FSESS_WARN_LSEXTATTR_LONG,
 			"server returned "
 			"more extended attribute data than requested; "
@@ -3142,7 +3148,7 @@ fuse_vnop_listextattr(struct vop_listextattr_args *ap)
 	 * FreeBSD's format before giving it to the user.
 	 */
 	bsd_list = malloc(linux_list_len, M_TEMP, M_WAITOK);
-	err = fuse_xattrlist_convert(prefix, linux_list, linux_list_len,
+	err = fuse_xattrlist_convert(data, prefix, linux_list, linux_list_len,
 	    bsd_list, &bsd_list_len);
 	if (err != 0)
 		goto out;

tests/sys/fs/fusefs/xattr.cc

@@ -492,6 +492,79 @@ TEST_F(ListxattrSig, erange_forever)
 	ASSERT_TRUE(WIFSIGNALED(status));
 }
 
+/*
+ * A buggy or malicious server returns a list that isn't nul-terminated.  The
+ * kernel should handle it gracefully.
+ */
+TEST_F(Listxattr, not_nul_terminated)
+{
+	uint64_t ino = 42;
+	int ns = EXTATTR_NAMESPACE_USER;
+	char *data;
+	const char expected[4] = {3, 'f', 'o', 'o'};
+	const char first[255] = "user.foo\0system.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
+	const uint8_t badlist[9] = {'u', 's', 'e', 'r', '.', 'f', 'o', 'o', 'd'};
+	Sequence seq;
+
+	EXPECT_LOOKUP(FUSE_ROOT_ID, RELPATH)
+	.WillRepeatedly(Invoke(
+		ReturnImmediate([=](auto in __unused, auto& out) {
+		SET_OUT_HEADER_LEN(out, entry);
+		out.body.entry.attr.mode = S_IFREG | 0644;
+		out.body.entry.nodeid = ino;
+		out.body.entry.attr.nlink = 1;
+		out.body.entry.attr_valid = UINT64_MAX;
+		out.body.entry.entry_valid = UINT64_MAX;
+	})));
+
+	/* 
+	 * On the first LISTXATTRS call, return a big attribute just to fill
+	 * the heap with non-NUL data.
+	 */
+	expect_listxattr(ino, 0,
+		ReturnImmediate([&](auto in __unused, auto& out) {
+			out.body.listxattr.size = sizeof(first);
+			SET_OUT_HEADER_LEN(out, listxattr);
+		}), &seq
+	);
+	expect_listxattr(ino, sizeof(first),
+		ReturnImmediate([&](auto in __unused, auto& out) {
+			memcpy((void*)out.body.bytes, first, sizeof(first));
+			out.header.len = sizeof(fuse_out_header) + sizeof(first);
+		}), &seq
+	);
+	/*
+	 * On the second LISTXATTRS call, return a malformed list with no NUL
+	 * termination.  The heap might still be full of the data from the
+	 * first call.
+	 */
+	expect_listxattr(ino, 0,
+		ReturnImmediate([&](auto in __unused, auto& out) {
+			out.body.listxattr.size = sizeof(badlist);
+			SET_OUT_HEADER_LEN(out, listxattr);
+		}), &seq
+	);
+	expect_listxattr(ino, sizeof(badlist),
+		ReturnImmediate([&](auto in __unused, auto& out) {
+			memset((void*)out.body.bytes, 'x', sizeof(first));
+			memcpy((void*)out.body.bytes, badlist, sizeof(badlist));
+			out.header.len = sizeof(fuse_out_header) + sizeof(badlist);
+		}), &seq
+	);
+
+	data = new char[1024];
+
+	ASSERT_EQ(static_cast<ssize_t>(sizeof(expected)),
+		extattr_list_file(FULLPATH, ns, data, sizeof(data)))
+		<< strerror(errno);
+	/*
+	 * Receiving this malformed list, the kernel should log it to dmesg and
+	 * report an IO error to the caller.
+	 */
+	ASSERT_EQ(-1, extattr_list_file(FULLPATH, ns, data, sizeof(data)));
+	EXPECT_EQ(EIO, errno);
+}
+
 /*
  * Get the size of the list that it would take to list no extended attributes
  */