From e59ef9939d3f0ccc8f9bab51442989a81be0c914 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Sat, 23 Nov 2024 12:28:13 +1100
Subject: [PATCH 3/4] receiver: use secure_relative_open() for basis file

this prevents attacks where the basis file is manipulated by a
malicious sender to gain information about files outside the
destination tree
---
 receiver.c | 42 ++++++++++++++++++++++++++----------------
 1 file changed, 26 insertions(+), 16 deletions(-)

Index: rsync-3.1.3/receiver.c
===================================================================
--- rsync-3.1.3.orig/receiver.c
+++ rsync-3.1.3/receiver.c
@@ -537,6 +537,8 @@ int recv_files(int f_in, int f_out, char
 		delayed_bits = bitbag_create(cur_flist->used + 1);
 
 	while (1) {
+		const char *basedir = NULL;
+
 		cleanup_disable();
 
 		/* This call also sets cur_flist. */
@@ -706,27 +708,29 @@ int recv_files(int f_in, int f_out, char
 					exit_cleanup(RERR_PROTOCOL);
 				}
 				if (file->dirname) {
-					pathjoin(fnamecmpbuf, sizeof fnamecmpbuf, file->dirname, xname);
-					fnamecmp = fnamecmpbuf;
-				} else
-					fnamecmp = xname;
+					basedir = file->dirname;
+				}
+				fnamecmp = xname;
 				break;
 			default:
 				if (fnamecmp_type > FNAMECMP_FUZZY && fnamecmp_type-FNAMECMP_FUZZY <= basis_dir_cnt) {
 					fnamecmp_type -= FNAMECMP_FUZZY + 1;
 					if (file->dirname) {
-						stringjoin(fnamecmpbuf, sizeof fnamecmpbuf,
-							   basis_dir[fnamecmp_type], "/", file->dirname, "/", xname, NULL);
-					} else
-						pathjoin(fnamecmpbuf, sizeof fnamecmpbuf, basis_dir[fnamecmp_type], xname);
+						pathjoin(fnamecmpbuf, sizeof fnamecmpbuf, basis_dir[fnamecmp_type], file->dirname);
+						basedir = fnamecmpbuf;
+					} else {
+						basedir = basis_dir[fnamecmp_type];
+					}
+					fnamecmp = xname;
 				} else if (fnamecmp_type >= basis_dir_cnt) {
 					rprintf(FERROR,
 						"invalid basis_dir index: %d.\n",
 						fnamecmp_type);
 					exit_cleanup(RERR_PROTOCOL);
-				} else
-					pathjoin(fnamecmpbuf, sizeof fnamecmpbuf, basis_dir[fnamecmp_type], fname);
-				fnamecmp = fnamecmpbuf;
+				} else {
+					basedir = basis_dir[fnamecmp_type];
+					fnamecmp = fname;
+				}
 				break;
 			}
 			if (!fnamecmp || (daemon_filter_list.head
@@ -749,7 +753,7 @@ int recv_files(int f_in, int f_out, char
 		}
 
 		/* open the file */
-		fd1 = do_open(fnamecmp, O_RDONLY, 0);
+		fd1 = secure_relative_open(basedir, fnamecmp, O_RDONLY, 0);
 
 		if (fd1 == -1 && protocol_version < 29) {
 			if (fnamecmp != fname) {
@@ -759,13 +763,19 @@ int recv_files(int f_in, int f_out, char
 
 			if (fd1 == -1 && basis_dir[0]) {
 				/* pre-29 allowed only one alternate basis */
-				pathjoin(fnamecmpbuf, sizeof fnamecmpbuf,
-					 basis_dir[0], fname);
-				fnamecmp = fnamecmpbuf;
-				fd1 = do_open(fnamecmp, O_RDONLY, 0);
+				basedir = basis_dir[0];
+				fnamecmp = fname;
+				fd1 = secure_relative_open(basedir, fnamecmp, O_RDONLY, 0);
 			}
 		}
 
+		if (basedir) {
+			// for the following code we need the full
+			// path name as a single string
+			pathjoin(fnamecmpbuf, sizeof fnamecmpbuf, basedir, fnamecmp);
+			fnamecmp = fnamecmpbuf;
+		}
+
 		updating_basis_or_equiv = inplace
 		    && (fnamecmp == fname || fnamecmp_type == FNAMECMP_BACKUP);