syscall: use openat2(RESOLVE_BENEATH) on Linux for secure_relative_open

The CVE fix in commit c35e283 made secure_relative_open() walk every
component of relpath with O_NOFOLLOW. That blocks every symlink in the
path, which is stricter than the threat model required: legitimate
directory symlinks within the destination tree (e.g. when using -K /
--copy-dirlinks) are also rejected, breaking delta transfers with
"failed verification -- update discarded".  See issue #715.

On Linux 5.6+, openat2(RESOLVE_BENEATH | RESOLVE_NO_MAGICLINKS) gives
us exactly what we want: the kernel rejects any resolution that would
escape the starting directory (via "..", absolute paths, or symlinks
pointing outside dirfd) while still following symlinks that resolve
within it. /proc magic-links are blocked too.

Use openat2 first; fall back to the existing per-component O_NOFOLLOW
walk on ENOSYS (kernel < 5.6). The lexical "../" checks at the head
of the function are kept as defense in depth. The Linux gate is
plain #ifdef __linux__: the runtime ENOSYS fallback covers the only
case that actually matters (header present + old kernel), and any
Linux build environment without linux/openat2.h will fail with a
clear "no such file" error rather than silently disabling the
protection.

Verified manually that openat2(RESOLVE_BENEATH) blocks all four
escape patterns (absolute symlink, ../ symlink, lexical .., absolute
path) while allowing direct and within-tree symlinks. The new
testsuite/symlink-dirlink-basis.test (taken from PR #864 by Samuel
Henrique) exercises the issue #715 regression and passes; full
make check passes 47/47.

Test: testsuite/symlink-dirlink-basis.test (8 scenarios)
Fixes: https://github.com/RsyncProject/rsync/issues/715
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

diff --git a/syscall.c b/syscall.c
index ec0e0708a7403ab4de852e3c86d5eb9eb179f44b..66c6d29c76e1985d226d9498a1a43f6fb80f7f01 100644 (file)
--- a/syscall.c
+++ b/syscall.c
@@ -33,6 +33,11 @@
 #include <sys/syscall.h>
 #endif
 
+#ifdef __linux__
+#include <sys/syscall.h>
+#include <linux/openat2.h>
+#endif
+
 #include "ifuncs.h"
 
 extern int dry_run;
@@ -720,12 +725,49 @@ int do_open_nofollow(const char *pathname, int flags)
 /*
   open a file relative to a base directory. The basedir can be NULL,
   in which case the current working directory is used. The relpath
-  must be a relative path, and the relpath must not contain any
-  elements in the path which follow symlinks (ie. like O_NOFOLLOW, but
-  applies to all path components, not just the last component)
-
-  The relpath must also not contain any ../ elements in the path
+  must be a relative path. The kernel must guarantee that resolution
+  cannot escape basedir (or the cwd, when basedir is NULL): no ".."
+  jumps above the start, no symlinks pointing outside, no absolute
+  paths, no /proc magic-link tricks.
+
+  Symlinks *within* basedir are followed normally — earlier rsync
+  versions rejected every symlink with O_NOFOLLOW on each component,
+  which broke legitimate directory symlinks on the receiver side
+  (https://github.com/RsyncProject/rsync/issues/715). The escape
+  prevention is handled by the kernel via openat2(RESOLVE_BENEATH)
+  on Linux 5.6+; older systems fall back to the per-component
+  O_NOFOLLOW walk below.
+
+  The relpath must also not contain any ../ elements in the path.
 */
+
+#ifdef __linux__
+static int secure_relative_open_linux(const char *basedir, const char *relpath, int flags, mode_t mode)
+{
+       struct open_how how;
+       int dirfd, retfd;
+
+       memset(&how, 0, sizeof how);
+       how.flags = flags;
+       how.mode = mode;
+       how.resolve = RESOLVE_BENEATH | RESOLVE_NO_MAGICLINKS;
+
+       if (basedir == NULL) {
+               dirfd = AT_FDCWD;
+       } else {
+               dirfd = openat(AT_FDCWD, basedir, O_RDONLY | O_DIRECTORY);
+               if (dirfd == -1)
+                       return -1;
+       }
+
+       retfd = syscall(SYS_openat2, dirfd, relpath, &how, sizeof how);
+
+       if (dirfd != AT_FDCWD)
+               close(dirfd);
+       return retfd;
+}
+#endif
+
 int secure_relative_open(const char *basedir, const char *relpath, int flags, mode_t mode)
 {
        if (!relpath || relpath[0] == '/') {
@@ -739,6 +781,16 @@ int secure_relative_open(const char *basedir, const char *relpath, int flags, mo
                return -1;
        }
 
+#ifdef __linux__
+       {
+               int fd = secure_relative_open_linux(basedir, relpath, flags, mode);
+               /* ENOSYS = kernel < 5.6 doesn't have the syscall even though
+                * glibc/kernel-headers do; fall through to the portable path. */
+               if (fd != -1 || errno != ENOSYS)
+                       return fd;
+       }
+#endif
+
 #if !defined(O_NOFOLLOW) || !defined(O_DIRECTORY) || !defined(AT_FDCWD)
        // really old system, all we can do is live with the risks
        if (!basedir) {

diff --git a/testsuite/symlink-dirlink-basis.test b/testsuite/symlink-dirlink-basis.test
new file mode 100755 (executable)
index 0000000..9065dd8
--- /dev/null
+++ b/testsuite/symlink-dirlink-basis.test
@@ -0,0 +1,247 @@
+#!/bin/sh
+
+# Test that updating a file through a directory symlink works when using
+# -K (--copy-dirlinks). This is a regression test for:
+#   https://github.com/RsyncProject/rsync/issues/715
+#
+# The CVE fix in commit c35e283 introduced secure_relative_open() which
+# uses O_NOFOLLOW on all path components, breaking legitimate directory
+# symlinks on the receiver side. The fix splits the path into basedir
+# (dirname, symlinks followed) and basename (O_NOFOLLOW) so that
+# directory symlinks are traversed while the final file component is
+# still protected.
+#
+# The regression only manifests when delta matching is triggered (i.e.,
+# the sender finds matching blocks in the old file). Small files with
+# completely different content are transferred in full and don't trigger
+# the bug. We use a large file with a small modification to ensure
+# delta transfer is used.
+#
+# In addition to the original regression, this test covers edge cases
+# in the fix itself:
+#   - --backup with directory symlinks (finish_transfer pointer identity)
+#   - --partial-dir with protocol < 29 (fnamecmp != partialptr guard)
+#   - --inplace with directory symlinks (updating_basis_or_equiv check)
+#   - Files without a dirname (top-level files, no split needed)
+
+. "$suitedir/rsync.fns"
+
+RSYNC_RSH="$scratchdir/src/support/lsh.sh"
+export RSYNC_RSH
+
+# $HOME is set to $scratchdir by rsync.fns
+# localhost: destination will cd to $HOME (i.e., $scratchdir)
+
+# Helper: create a large file suitable for delta transfers.
+# ~32KB is large enough for rsync's block matching to find matches.
+make_testfile() {
+    dd if=/dev/urandom of="$1" bs=1024 count=32 2>/dev/null \
+       || test_fail "failed to create test file $1"
+}
+
+# Set up source tree
+srcbase="$tmpdir/src"
+
+######################################################################
+# Test 1: Basic directory symlink update (the original issue #715)
+######################################################################
+
+mkdir -p "$HOME/real-dir"
+ln -s real-dir "$HOME/dir"
+
+mkdir -p "$srcbase/dir"
+make_testfile "$srcbase/dir/file"
+
+# First transfer (initial): should create the file through the symlink
+(cd "$srcbase" && $RSYNC -KRlptv --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 1: initial transfer failed"
+
+if [ ! -f "$HOME/real-dir/file" ]; then
+    test_fail "test 1: initial transfer did not create file through symlink"
+fi
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 1: initial transfer content mismatch"
+
+# Small modification to trigger delta transfer
+echo "appended update" >> "$srcbase/dir/file"
+sleep 1
+touch "$srcbase/dir/file"
+
+# Second transfer (update): was failing with "failed verification"
+(cd "$srcbase" && $RSYNC -KRlptv --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 1: update through directory symlink failed"
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 1: update transfer content mismatch"
+
+######################################################################
+# Test 2: Compression (-z) as in the original reproducer
+######################################################################
+
+echo "another line" >> "$srcbase/dir/file"
+sleep 1
+touch "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptzv --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 2: compressed update through directory symlink failed"
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 2: compressed update content mismatch"
+
+######################################################################
+# Test 3: Nested directory symlinks (nested/sub/data.txt where
+#          "nested" is a symlink to "nested_real")
+######################################################################
+
+mkdir -p "$HOME/nested_real/sub"
+ln -s nested_real "$HOME/nested"
+
+mkdir -p "$srcbase/nested/sub"
+make_testfile "$srcbase/nested/sub/data.txt"
+
+(cd "$srcbase" && $RSYNC -KRlptv --rsync-path="$RSYNC" nested/sub/data.txt localhost:) \
+    || test_fail "test 3: initial nested transfer failed"
+
+echo "appended nested" >> "$srcbase/nested/sub/data.txt"
+sleep 1
+touch "$srcbase/nested/sub/data.txt"
+
+(cd "$srcbase" && $RSYNC -KRlptv --rsync-path="$RSYNC" nested/sub/data.txt localhost:) \
+    || test_fail "test 3: update through nested directory symlink failed"
+
+diff "$srcbase/nested/sub/data.txt" "$HOME/nested_real/sub/data.txt" >/dev/null \
+    || test_fail "test 3: nested update content mismatch"
+
+######################################################################
+# Test 4: --backup with directory symlinks
+#
+# Exercises the finish_transfer() "fnamecmp == fname" pointer
+# comparison that determines whether to update fnamecmp to the
+# backup name. If broken, --backup would reference a renamed file
+# for xattr handling.
+######################################################################
+
+# Reset destination
+rm -f "$HOME/real-dir/file" "$HOME/real-dir/file~"
+
+make_testfile "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 4: initial transfer for backup test failed"
+
+echo "backup update" >> "$srcbase/dir/file"
+sleep 1
+touch "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --backup --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 4: update with --backup through directory symlink failed"
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 4: backup update content mismatch"
+
+if [ ! -f "$HOME/real-dir/file~" ]; then
+    test_fail "test 4: backup file was not created"
+fi
+
+######################################################################
+# Test 5: --inplace with directory symlinks
+#
+# Exercises the updating_basis_or_equiv check which uses
+# "fnamecmp == fname". With --inplace, rsync writes directly to
+# the destination file instead of a temp file.
+######################################################################
+
+rm -f "$HOME/real-dir/file" "$HOME/real-dir/file~"
+
+make_testfile "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --inplace --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 5: initial inplace transfer failed"
+
+echo "inplace update" >> "$srcbase/dir/file"
+sleep 1
+touch "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --inplace --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 5: inplace update through directory symlink failed"
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 5: inplace update content mismatch"
+
+######################################################################
+# Test 6: Top-level file (no dirname, no split needed)
+#
+# Ensures the dirname/basename split is not attempted for files
+# at the top level (file->dirname is NULL).
+######################################################################
+
+make_testfile "$srcbase/topfile"
+mkdir -p "$HOME"
+
+(cd "$srcbase" && $RSYNC -Rlptv --rsync-path="$RSYNC" topfile localhost:) \
+    || test_fail "test 6: initial top-level transfer failed"
+
+echo "toplevel update" >> "$srcbase/topfile"
+sleep 1
+touch "$srcbase/topfile"
+
+(cd "$srcbase" && $RSYNC -Rlptv --rsync-path="$RSYNC" topfile localhost:) \
+    || test_fail "test 6: top-level update failed"
+
+diff "$srcbase/topfile" "$HOME/topfile" >/dev/null \
+    || test_fail "test 6: top-level update content mismatch"
+
+######################################################################
+# Test 7: --partial-dir with protocol < 29
+#
+# For protocol < 29, fnamecmp_type stays FNAMECMP_FNAME even when
+# fnamecmp is set to partialptr. The dirname/basename split must
+# NOT trigger in this case (guarded by "fnamecmp == fname").
+######################################################################
+
+rm -f "$HOME/real-dir/file"
+make_testfile "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --protocol=28 --partial-dir=.rsync-partial \
+    --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 7: initial proto28 partial-dir transfer failed"
+
+echo "partial-dir update" >> "$srcbase/dir/file"
+sleep 1
+touch "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --protocol=28 --partial-dir=.rsync-partial \
+    --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 7: proto28 partial-dir update through dirlink failed"
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 7: proto28 partial-dir update content mismatch"
+
+######################################################################
+# Test 8: Protocol < 29 basic directory symlink update
+#
+# Exercises the protocol < 29 code path and its fallback logic
+# (clearing basedir on retry).
+######################################################################
+
+rm -f "$HOME/real-dir/file"
+make_testfile "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --protocol=28 \
+    --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 8: initial proto28 transfer failed"
+
+echo "proto28 update" >> "$srcbase/dir/file"
+sleep 1
+touch "$srcbase/dir/file"
+
+(cd "$srcbase" && $RSYNC -KRlptv --protocol=28 \
+    --rsync-path="$RSYNC" dir/file localhost:) \
+    || test_fail "test 8: proto28 update through directory symlink failed"
+
+diff "$srcbase/dir/file" "$HOME/real-dir/file" >/dev/null \
+    || test_fail "test 8: proto28 update content mismatch"
+
+# The script would have aborted on error, so getting here means we've won.
+exit 0