Prevent path traversal in pg_basebackup and pg_rewind

pg_rewind and pg_basebackup could be fed paths from rogue endpoints that
could overwrite the contents of the client when received, achieving path
traversal.

There were two areas in the tree that were sensitive to this problem:
- pg_basebackup, through the astreamer code, where no validation was
performed before building an output path when streaming tar data.  This
is an issue in v15 and newer versions.
- pg_rewind file operations for paths received through libpq, for all
the stable branches supported.

In order to address this problem, this commit adds a helper function in
path.c, that reuses path_is_relative_and_below_cwd() after applying
canonicalize_path().  This can be used to validate the paths received
from a connection point.  A path is considered invalid if any of the two
following conditions is satisfied:
- The path is absolute.
- The path includes a direct parent-directory reference.

Reported-by: XlabAI Team of Tencent Xuanwu Lab
Reported-by: Valery Gubanov <valerygubanov95@gmail.com>
Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 14
Security: CVE-2026-6475

src/bin/pg_basebackup/bbstreamer_file.c

@@ -215,6 +215,10 @@ bbstreamer_extractor_content(bbstreamer *streamer, bbstreamer_member *member,
 		case BBSTREAMER_MEMBER_HEADER:
 			Assert(mystreamer->file == NULL);
 
+			if (!path_is_safe_for_extraction(member->pathname))
+				pg_fatal("tar member has unsafe path name: \"%s\"",
+						 member->pathname);
+
 			/* Prepend basepath. */
 			snprintf(mystreamer->filename, sizeof(mystreamer->filename),
 					 "%s/%s", mystreamer->basepath, member->pathname);
@@ -233,6 +237,14 @@ bbstreamer_extractor_content(bbstreamer *streamer, bbstreamer_member *member,
 
 				if (mystreamer->link_map)
 					linktarget = mystreamer->link_map(linktarget);
+
+				if (!is_absolute_path(linktarget) &&
+					!path_is_safe_for_extraction(member->linktarget))
+				{
+					pg_fatal("link target has unsafe path name: \"%s\"",
+							 member->linktarget);
+				}
+
 				extract_link(mystreamer->filename, linktarget);
 			}
 			else

src/bin/pg_basebackup/bbstreamer_tar.c

@@ -295,6 +295,9 @@ bbstreamer_tar_header(bbstreamer_tar_parser *mystreamer)
 	strlcpy(member->pathname, &buffer[0], MAXPGPATH);
 	if (member->pathname[0] == '\0')
 		pg_fatal("tar member has empty name");
+	if (!path_is_safe_for_extraction(member->pathname))
+		pg_fatal("tar member has unsafe path name: \"%s\"",
+				 member->pathname);
 	member->size = read_tar_number(&buffer[124], 12);
 	member->mode = read_tar_number(&buffer[100], 8);
 	member->uid = read_tar_number(&buffer[108], 8);

src/bin/pg_rewind/file_ops.c

@@ -48,6 +48,9 @@ open_target_file(const char *path, bool trunc)
 {
 	int			mode;
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target file path is unsafe for open: \"%s\"", path);
+
 	if (dry_run)
 		return;
 
@@ -188,6 +191,9 @@ remove_target_file(const char *path, bool missing_ok)
 {
 	char		dstpath[MAXPGPATH];
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target file path is unsafe for removal: \"%s\"", path);
+
 	if (dry_run)
 		return;
 
@@ -208,6 +214,9 @@ truncate_target_file(const char *path, off_t newsize)
 	char		dstpath[MAXPGPATH];
 	int			fd;
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target file path is unsafe for truncation: \"%s\"", path);
+
 	if (dry_run)
 		return;
 
@@ -230,6 +239,10 @@ create_target_dir(const char *path)
 {
 	char		dstpath[MAXPGPATH];
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target directory path is unsafe for directory creation: \"%s\"",
+				 path);
+
 	if (dry_run)
 		return;
 
@@ -244,6 +257,10 @@ remove_target_dir(const char *path)
 {
 	char		dstpath[MAXPGPATH];
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target directory path is unsafe for directory removal: \"%s\"",
+				 path);
+
 	if (dry_run)
 		return;
 
@@ -258,6 +275,9 @@ create_target_symlink(const char *path, const char *link)
 {
 	char		dstpath[MAXPGPATH];
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target symlink path is unsafe for creation: \"%s\"", path);
+
 	if (dry_run)
 		return;
 
@@ -272,6 +292,9 @@ remove_target_symlink(const char *path)
 {
 	char		dstpath[MAXPGPATH];
 
+	if (!path_is_safe_for_extraction(path))
+		pg_fatal("target symlink path is unsafe for removal: \"%s\"", path);
+
 	if (dry_run)
 		return;
 

src/include/port.h

@@ -58,6 +58,7 @@ extern void make_native_path(char *path);
 extern void cleanup_path(char *path);
 extern bool path_contains_parent_reference(const char *path);
 extern bool path_is_relative_and_below_cwd(const char *path);
+extern bool path_is_safe_for_extraction(const char *path);
 extern bool path_is_prefix_of_path(const char *path1, const char *path2);
 extern char *make_absolute_path(const char *path);
 extern const char *get_progname(const char *argv0);

src/port/path.c

@@ -626,6 +626,23 @@ path_is_relative_and_below_cwd(const char *path)
 		return true;
 }
 
+/*
+ * Detect whether a path is safe for use during archive extraction.
+ *
+ * This applies canonicalize_path(), then it checks that the path does
+ * not contain any parent directory references.
+ */
+bool
+path_is_safe_for_extraction(const char *path)
+{
+	char		buf[MAXPGPATH];
+
+	strlcpy(buf, path, sizeof(buf));
+	canonicalize_path(buf);
+
+	return path_is_relative_and_below_cwd(buf);
+}
+
 /*
  * Detect whether path1 is a prefix of path2 (including equality).
  *