syscall/receiver: honour a relative alt-basis dir on a daemon receiver (#915)

The symlink-race hardening routed the receiver's basis open through
secure_relative_open(), which rejects any '..' -- so a sibling
--link-dest=../01 on a use-chroot=no daemon was silently ignored and every file
re-transferred (#915/#928, a regression from 3.4.1).

Narrow the confinement to the sanitizing daemon (am_daemon && !am_chrooted) and
re-anchor it at the module root, the real trust boundary: secure_relative_open()
prefixes the cwd's module-relative path (from rsync's logical curr_dir[], a
guaranteed lexical prefix of module_dir) and resolves beneath module_dir, so
RESOLVE_BENEATH permits an in-module '..' climb while still rejecting one that
escapes the module.  secure_basis_open() opens with a bare do_open() in the
non-sanitizing cases.  t_stub.c gains weak curr_dir[]/curr_dir_len for the
helpers (via #pragma weak on non-GNU compilers, where rsync.h erases
__attribute__).

Two tests: link-dest-relative-basis asserts the in-module '..' is honoured;
link-dest-module-escape asserts a --link-dest=../../OUTSIDE climb that leaves
the module is refused (not hard-linked to an outside file).  See upstream
PR #930.

Thanks to @fufu65 (#915) and @JetAppsClark (#928) for the reports.

(cherry picked from commit 948edffb43)

receiver.c

@@ -99,6 +99,27 @@ static int updating_basis_or_equiv;
  * Anything else is a straight pass-through that preserves the strict contract. */
 static int secure_basis_open(const char *basedir, const char *relpath, int flags, mode_t mode)
 {
+	extern int am_daemon, am_chrooted;
+
+	/* The confined resolver is only needed for the sanitizing daemon
+	 * (am_daemon && !am_chrooted, i.e. use_secure_symlinks).  Local /
+	 * remote-shell mode has no module boundary, and "use chroot = yes" makes
+	 * the kernel root the boundary, so there an alt-dest basis like
+	 * --link-dest=../01 must resolve against the cwd as a bare open did before
+	 * the hardening (confining it would reject the legitimate sibling "..",
+	 * #915). */
+	if (!am_daemon || am_chrooted) {
+		if (basedir) {
+			char fullpath[MAXPATHLEN];
+			if (pathjoin(fullpath, sizeof fullpath, basedir, relpath) >= sizeof fullpath) {
+				errno = ENAMETOOLONG;
+				return -1;
+			}
+			return do_open(fullpath, flags, mode);
+		}
+		return do_open(relpath, flags, mode);
+	}
+
 	if (!basedir && relpath && *relpath == '/') {
 		const char *slash = strrchr(relpath, '/');
 		const char *leaf = slash + 1;
@@ -859,7 +880,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 				basedir = basis_dir[0];
 				fnamecmp = fname;
 				fnamecmp_type = FNAMECMP_BASIS_DIR_LOW;
-				fd1 = secure_relative_open(basedir, fnamecmp, O_RDONLY, 0);
+				fd1 = secure_basis_open(basedir, fnamecmp, O_RDONLY, 0);
 			}
 		}
 

syscall.c

@@ -1761,13 +1761,68 @@ static int secure_relative_open_resolve_beneath(const char *basedir, const char
 }
 #endif
 
+/* The logical current directory (maintained by change_dir() in util1.c).
+ * Defined here -- rather than in util1.c -- so the test helpers that link
+ * syscall.o but not util1.o (tls, trimslash) get the definition without a
+ * weak-symbol fallback, which is not portable to PE/COFF targets (Cygwin). */
+char curr_dir[MAXPATHLEN];
+unsigned int curr_dir_len;
+
 int secure_relative_open(const char *basedir, const char *relpath, int flags, mode_t mode)
 {
+	extern int am_daemon, am_chrooted;
+	extern char *module_dir;
+	extern unsigned int module_dirlen;
+	char modrel_buf[MAXPATHLEN];
+	int reanchored = 0;
+
 	if (!relpath || relpath[0] == '/') {
 		// must be a relative path
 		errno = EINVAL;
 		return -1;
 	}
+
+	/* Sanitizing daemon only (am_daemon && !am_chrooted).  Here we have chdir'd
+	 * into a sub-dir of the module (the transfer destination), so a relative
+	 * alt-dest like "../01" may legitimately climb to a sibling that is still
+	 * inside the module (#915).  Confining beneath the cwd would reject that
+	 * climb.  Re-anchor at the module root -- the real trust boundary -- by
+	 * prefixing the cwd's module-relative path (from rsync's logical curr_dir[],
+	 * a guaranteed lexical prefix of module_dir, unlike getcwd()) and resolving
+	 * beneath module_dir; RESOLVE_BENEATH then allows in-module climbs and still
+	 * rejects escapes.  Only for paths that contain "..".  module_dirlen is 0 for
+	 * a `path = /` module (clientserver.c), so we gate on module_dir, not its
+	 * length, to cover that case too -- the prefix check below treats
+	 * module_dirlen 0 as "module root is /". */
+	if (am_daemon && !am_chrooted
+	 && module_dir && module_dir[0] == '/'
+	 && (basedir == NULL || basedir[0] != '/')
+	 && (path_has_dotdot_component(relpath)
+	  || (basedir && path_has_dotdot_component(basedir)))) {
+		const char *p;
+		int n;
+		if (curr_dir_len >= module_dirlen
+		 && strncmp(curr_dir, module_dir, module_dirlen) == 0
+		 && (curr_dir[module_dirlen] == '\0' || curr_dir[module_dirlen] == '/')) {
+			for (p = curr_dir + module_dirlen; *p == '/'; p++) {}
+			if (basedir)
+				n = snprintf(modrel_buf, sizeof modrel_buf, "%s%s%s/%s",
+					     p, *p ? "/" : "", basedir, relpath);
+			else
+				n = snprintf(modrel_buf, sizeof modrel_buf, "%s%s%s",
+					     p, *p ? "/" : "", relpath);
+			if (n < 0 || n >= (int)sizeof modrel_buf) {
+				errno = ENAMETOOLONG;
+				return -1;
+			}
+			basedir = module_dir;	/* absolute, operator-trusted anchor */
+			relpath = modrel_buf;
+			reanchored = 1;
+		}
+		/* else: cwd not under module root as expected -- fall through to the
+		 * front-door rejection below (fail safe). */
+	}
+
 	/* Reject any path with a literal ".." component (bare "..",
 	 * "../foo", "foo/..", "foo/../bar", "subdir/.."). The previous
 	 * substring-based check caught only "../" prefix and "/../"
@@ -1776,14 +1831,19 @@ int secure_relative_open(const char *basedir, const char *relpath, int flags, mo
 	 * and pre-5.6 Linux. RESOLVE_BENEATH on Linux/FreeBSD/macOS
 	 * catches some of these in-kernel with EXDEV, but the front
 	 * door must reject them consistently with EINVAL across all
-	 * platforms so callers can rely on the validation. */
-	if (path_has_dotdot_component(relpath)) {
-		errno = EINVAL;
-		return -1;
-	}
-	if (basedir && basedir[0] != '/' && path_has_dotdot_component(basedir)) {
-		errno = EINVAL;
-		return -1;
+	 * platforms so callers can rely on the validation.  Skipped for a
+	 * re-anchored path: its ".." is deliberate, stays within the module,
+	 * and is adjudicated by RESOLVE_BENEATH below (the portable fallback
+	 * re-rejects it -- see there). */
+	if (!reanchored) {
+		if (path_has_dotdot_component(relpath)) {
+			errno = EINVAL;
+			return -1;
+		}
+		if (basedir && basedir[0] != '/' && path_has_dotdot_component(basedir)) {
+			errno = EINVAL;
+			return -1;
+		}
 	}
 
 #ifdef __linux__
@@ -1800,6 +1860,21 @@ int secure_relative_open(const char *basedir, const char *relpath, int flags, mo
 	return secure_relative_open_resolve_beneath(basedir, relpath, flags, mode);
 #endif
 
+	/* Portable fallback only (no kernel RESOLVE_BENEATH): the per-component
+	 * O_NOFOLLOW walk below can't adjudicate ".." safely, so reject it here --
+	 * even for a re-anchored path.  This re-breaks --link-dest=../01 on
+	 * openat2/O_RESOLVE_BENEATH-less platforms (NetBSD/OpenBSD/Solaris/Cygwin/
+	 * pre-5.6 Linux), trading function for safety; on the kernel paths above
+	 * RESOLVE_BENEATH already allowed the in-module climb. */
+	if (path_has_dotdot_component(relpath)) {
+		errno = EINVAL;
+		return -1;
+	}
+	if (basedir && basedir[0] != '/' && path_has_dotdot_component(basedir)) {
+		errno = EINVAL;
+		return -1;
+	}
+
 #if !defined(O_NOFOLLOW) || !defined(O_DIRECTORY) || !defined(AT_FDCWD)
 	// really old system, all we can do is live with the risks
 	if (!basedir) {

t_stub.c

@@ -39,6 +39,8 @@ int open_noatime = 0;
 size_t max_alloc = 0; /* max_alloc is needed when combined with util2.o */
 char *partial_dir;
 char *module_dir;
+/* curr_dir[]/curr_dir_len (read by secure_relative_open) are defined in
+ * syscall.c, which every helper links -- no stub needed here. */
 filter_rule_list daemon_filter_list;
 
  void rprintf(UNUSED(enum logcode code), const char *format, ...)

util1.c

@@ -41,8 +41,8 @@ extern filter_rule_list daemon_filter_list;
 
 int sanitize_paths = 0;
 
-char curr_dir[MAXPATHLEN];
-unsigned int curr_dir_len;
+extern char curr_dir[MAXPATHLEN];   /* defined in syscall.c */
+extern unsigned int curr_dir_len;
 int curr_dir_depth; /* This is only set for a sanitizing daemon. */
 
 /* Set a fd into nonblocking mode. */