Restrict following symlinks to directories by ownership (CVE-2017-7500)

Only follow directory symlinks owned by target directory owner or root.
This prevents privilege escalation from user-writable directories via
directory symlinks to privileged directories on package upgrade, while
still allowing admin to arrange disk usage with symlinks.

The rationale is that if you can create symlinks owned by user X you *are*
user X (or root), and if you also own directory Y you can do whatever with
it already, including change permissions. So when you create a symlink to
that directory, the link ownership acts as a simple stamp of authority that
you indeed want rpm to treat this symlink as it were the directory that
you own. Such a permission can only be given by you or root, which
is just the way we want it. Plus it's almost ridiculously simple as far
as rules go, compared to trying to calculate something from the
source vs destination directory permissions etc.

In the normal case, the user arranging diskspace with symlinks is indeed
root so nothing changes, the only real change here is to links created by
non-privileged users which should be few and far between in practise.
Unfortunately our test-suite runs as a regular user via fakechroot and
thus the testcase for this fails under the new rules. Adjust the testcase
to get the ownership straight and add a second case for the illegal
behavior, basically the same as the old one but with different expectations.

lib/fsm.c

@@ -650,7 +650,7 @@ static int fsmUtime(const char *path, mode_t mode, time_t mtime)
     return rc;
 }
 
-static int fsmVerify(const char *path, rpmfi fi)
+static int fsmVerify(const char *path, rpmfi fi, const struct stat *fsb)
 {
     int rc;
     int saveerrno = errno;
@@ -675,11 +675,14 @@ static int fsmVerify(const char *path, rpmfi fi)
     } else if (S_ISDIR(mode)) {
         if (S_ISDIR(dsb.st_mode)) return 0;
         if (S_ISLNK(dsb.st_mode)) {
+	    uid_t luid = dsb.st_uid;
             rc = fsmStat(path, 0, &dsb);
             if (rc == RPMERR_ENOENT) rc = 0;
             if (rc) return rc;
             errno = saveerrno;
-            if (S_ISDIR(dsb.st_mode)) return 0;
+	    /* Only permit directory symlinks by target owner and root */
+            if (S_ISDIR(dsb.st_mode) && (luid == 0 || luid == fsb->st_uid))
+		    return 0;
         }
     } else if (S_ISLNK(mode)) {
         if (S_ISLNK(dsb.st_mode)) {
@@ -921,7 +924,7 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
 	    }
 	    /* Assume file does't exist when tmp suffix is in use */
 	    if (!suffix) {
-		rc = fsmVerify(fpath, fi);
+		rc = fsmVerify(fpath, fi, &sb);
 	    } else {
 		rc = (action == FA_TOUCH) ? 0 : RPMERR_ENOENT;
 	    }

tests/data/SPECS/replacetest.spec

@@ -1,5 +1,6 @@
 %{!?filetype: %global filetype file}
 %{?fixit: %global havepretrans 1}
+%{!?user: %global user root}
 
 Name:		replacetest%{?sub:-%{sub}}
 Version:	%{ver}
@@ -43,5 +44,5 @@ rm -rf $RPM_BUILD_ROOT
 %endif
 
 %files
-%defattr(-,root,root,-)
+%defattr(-,%{user},%{user},-)
 /opt/*

tests/rpmreplace.at

@@ -398,6 +398,40 @@ tf="${RPMTEST}"/opt/foo
 rm -rf "${RPMTEST}"/opt/*
 rm -rf "${TOPDIR}"
 
+runroot rpmbuild --quiet -bb \
+        --define "ver 1.0" \
+	--define "filetype datadir" \
+	--define "filedata README1" \
+	--define "user $(id -u -n)" \
+          /data/SPECS/replacetest.spec
+
+runroot rpmbuild --quiet -bb \
+        --define "ver 2.0" \
+	--define "filetype datadir" \
+	--define "filedata README2" \
+	--define "user $(id -u -n)" \
+          /data/SPECS/replacetest.spec
+
+mkdir "${RPMTEST}"/opt/f00f
+ln -s f00f "${RPMTEST}"/opt/foo
+runroot rpm -U /build/RPMS/noarch/replacetest-1.0-1.noarch.rpm
+test -L "${tf}" && test -d "${tf}" && runroot rpm -U /build/RPMS/noarch/replacetest-2.0-1.noarch.rpm
+test -L "${tf}" && test -d "${tf}"
+],
+[0],
+[],
+[])
+AT_CLEANUP
+
+AT_SETUP([upgrade invalid locally symlinked directory])
+AT_KEYWORDS([install])
+AT_CHECK([
+RPMDB_CLEAR
+RPMDB_INIT
+tf="${RPMTEST}"/opt/foo
+rm -rf "${RPMTEST}"/opt/*
+rm -rf "${TOPDIR}"
+
 runroot rpmbuild --quiet -bb \
         --define "ver 1.0" \
 	--define "filetype datadir" \
@@ -414,7 +448,7 @@ mkdir "${RPMTEST}"/opt/f00f
 ln -s f00f "${RPMTEST}"/opt/foo
 runroot rpm -U /build/RPMS/noarch/replacetest-1.0-1.noarch.rpm
 test -L "${tf}" && test -d "${tf}" && runroot rpm -U /build/RPMS/noarch/replacetest-2.0-1.noarch.rpm
-test -L "${tf}" && test -d "${tf}"
+test -d "${tf}"
 ],
 [0],
 [],