From 96ec957e281220f8e137a2d5eb23b83a6377d556 Mon Sep 17 00:00:00 2001
From: Panu Matilainen <pmatilai@redhat.com>
Date: Thu, 10 Feb 2022 14:32:43 +0200
Subject: [PATCH] Validate intermediate symlinks during installation,
 CVE-2021-35939

Conflict:adapt context; adapt configure.ac because c223d84fbf,f8b8e86ae3,
9fe75561f9 is not merged.
Reference:https://github.com/rpm-software-management/rpm/commit/96ec957e281220f8e137a2d5eb23b83a6377d556

Whenever directory changes during unpacking, walk the entire tree from
starting from / and validate any symlinks crossed, fail the install
on invalid links.

This is the first of step of many towards securing our file operations
against local tamperers and besides plugging that one CVE, paves the way
for the next step by adding the necessary directory fd tracking.
This also bumps the rpm OS requirements to a whole new level by requiring
the *at() family of calls from POSIX-1.2008.

This necessarily does a whole lot of huffing and puffing we previously
did not do. It should be possible to cache secure (ie root-owned)
directory structures to avoid validating everything a million times
but for now, just keeping things simple.
---
 INSTALL      |   2 +
 configure.ac |   3 +-
 lib/fsm.c    | 144 ++++++++++++++++++++++++++++++++++++++++++++++++---
 3 files changed, 142 insertions(+), 7 deletions(-)

diff --git a/INSTALL b/INSTALL
index 677ef88c4..961a160e0 100644
--- a/INSTALL
+++ b/INSTALL
@@ -103,6 +103,8 @@ option to configure).  For GCC, OpenMP 4.5 is fully supported since GCC 6.1,
 of GPG, available from
     http://www.gnupg.org/
 
+Rpm requires a POSIX.1-2008 level operating system.
+
 To compile RPM:
 --------------
 
diff --git a/configure.ac b/configure.ac
index 3ee340726..0099e5f34 100644
--- a/configure.ac
+++ b/configure.ac
@@ -580,7 +580,8 @@ AC_CHECK_FUNCS([secure_getenv __secure_getenv])
 
 AC_CHECK_FUNCS(
    [mkstemp getcwd basename dirname realpath setenv unsetenv regcomp lchown \
-    utimes getline],
+    utimes getline \
+    openat mkdirat fstatat ],
    [], [AC_MSG_ERROR([function required by rpm])])
 
 AC_LIBOBJ(fnmatch)
diff --git a/lib/fsm.c b/lib/fsm.c
index 911898347..b6b152ab1 100644
--- a/lib/fsm.c
+++ b/lib/fsm.c
@@ -8,6 +8,7 @@
 
 #include <utime.h>
 #include <errno.h>
+#include <fcntl.h>
 #if WITH_CAP
 #include <sys/capability.h>
 #endif
@@ -20,6 +21,7 @@
 #include "rpmio/rpmio_internal.h"	/* fdInit/FiniDigest */
 #include "lib/fsm.h"
 #include "lib/rpmte_internal.h"	/* XXX rpmfs */
+#include "lib/rpmfi_internal.h" /* rpmfiSetOnChdir */
 #include "lib/rpmplugins.h"	/* rpm plugins hooks */
 #include "lib/rpmug.h"
 
@@ -406,17 +408,118 @@ static int fsmRmdir(const char *path)
     return rc;
 }
 
-static int fsmMkdir(const char *path, mode_t mode)
+static int fsmMkdir(int dirfd, const char *path, mode_t mode)
 {
-    int rc = mkdir(path, (mode & 07777));
+    int rc = mkdirat(dirfd, path, (mode & 07777));
     if (_fsm_debug)
-	rpmlog(RPMLOG_DEBUG, " %8s (%s, 0%04o) %s\n", __func__,
-	       path, (unsigned)(mode & 07777),
+	rpmlog(RPMLOG_DEBUG, " %8s (%d %s, 0%04o) %s\n", __func__,
+	       dirfd, path, (unsigned)(mode & 07777),
 	       (rc < 0 ? strerror(errno) : ""));
     if (rc < 0)	rc = RPMERR_MKDIR_FAILED;
     return rc;
 }
 
+static int fsmOpenat(int dirfd, const char *path, int flags)
+{
+    struct stat lsb, sb;
+    int sflags = flags | O_NOFOLLOW;
+    int fd = openat(dirfd, path, sflags);
+
+    /*
+     * Only ever follow symlinks by root or target owner. Since we can't
+     * open the symlink itself, the order matters: we stat the link *after*
+     * opening the target, and if the link ownership changed between the calls
+     * it could've only been the link owner or root.
+     */
+    if (fd < 0 && errno == ELOOP && flags != sflags) {
+	int ffd = openat(dirfd, path, flags);
+	if (ffd >= 0 && fstatat(dirfd, path, &lsb, AT_SYMLINK_NOFOLLOW) == 0) {
+	    if (fstat(ffd, &sb) == 0) {
+		if (lsb.st_uid == 0 || lsb.st_uid == sb.st_uid) {
+		    fd = ffd;
+		} else {
+		    close(ffd);
+		}
+	    }
+	}
+    }
+    return fd;
+}
+
+static int fsmDoMkDir(rpmPlugins plugins, int dirfd, const char *dn,
+			int owned, mode_t mode)
+{
+    int rc;
+    rpmFsmOp op = (FA_CREATE);
+    if (!owned)
+	op |= FAF_UNOWNED;
+
+    /* Run fsm file pre hook for all plugins */
+    rc = rpmpluginsCallFsmFilePre(plugins, NULL, dn, mode, op);
+
+    if (!rc)
+	rc = fsmMkdir(dirfd, dn, mode);
+
+    if (!rc) {
+	rc = rpmpluginsCallFsmFilePrepare(plugins, NULL, dn, dn, mode, op);
+    }
+
+    /* Run fsm file post hook for all plugins */
+    rpmpluginsCallFsmFilePost(plugins, NULL, dn, mode, op, rc);
+
+    if (!rc) {
+	rpmlog(RPMLOG_DEBUG,
+		"%s directory created with perms %04o\n",
+		dn, (unsigned)(mode & 07777));
+    }
+
+    return rc;
+}
+
+static int ensureDir(rpmPlugins plugins, const char *p, int owned, int create)
+{
+    char *path = xstrdup(p);
+    char *dp = path;
+    char *sp = NULL, *bn;
+    int oflags = O_RDONLY;
+
+    int dirfd = fsmOpenat(-1, "/", oflags);
+    int fd = dirfd; /* special case of "/" */
+
+    while ((bn = strtok_r(dp, "/", &sp)) != NULL) {
+	struct stat sb;
+	fd = fsmOpenat(dirfd, bn, oflags);
+
+	if (fd < 0 && errno == ENOENT && create) {
+	    mode_t mode = S_IFDIR | (_dirPerms & 07777);
+	    if (fsmDoMkDir(plugins, dirfd, bn, owned, mode) == 0) {
+		fd = fsmOpenat(dirfd, bn, oflags|O_NOFOLLOW);
+	    }
+	}
+
+	if (fd >= 0 && fstat(fd, &sb) == 0 && !S_ISDIR(sb.st_mode)) {
+	    close(fd);
+	    errno = ENOTDIR;
+	    fd = -1;
+	}
+
+	close(dirfd);
+	if (fd >= 0) {
+	    dirfd = fd;
+	} else {
+	    dirfd = -1;
+	    rpmlog(RPMLOG_ERR, _("failed to open dir %s of %s: %s\n"),
+				bn, p, strerror(errno));
+	    break;
+	}
+
+	dp = NULL;
+    }
+
+    free(path);
+    return dirfd;
+}
+
 static int fsmMkfifo(const char *path, mode_t mode)
 {
     int rc = mkfifo(path, (mode & 07777));
@@ -507,7 +610,7 @@ static int fsmMkdirs(rpmfiles files, rpmfs fs, rpmPlugins plugins)
 		rc = rpmpluginsCallFsmFilePre(plugins, NULL, dn, mode, op);
 
 		if (!rc)
-		    rc = fsmMkdir(dn, mode);
+		    rc = fsmMkdir(-1, dn, mode);
 
 		if (!rc) {
 		    rc = rpmpluginsCallFsmFilePrepare(plugins, NULL, dn, dn,
@@ -874,6 +977,21 @@ static void setFileState(rpmfs fs, int i)
     }
 }
 
+struct diriter_s {
+    int dirfd;
+};
+
+static int onChdir(rpmfi fi, void *data)
+{
+    struct diriter_s *di = data;
+
+    if (di->dirfd >= 0) {
+	close(di->dirfd);
+	di->dirfd = -1;
+    }
+    return 0;
+}
+
 int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
               rpmpsm psm, char ** failedFile)
 {
@@ -890,6 +1008,7 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
     char *tid = NULL;
     struct filedata_s *fdata = xcalloc(fc, sizeof(*fdata));
     struct filedata_s *firstlink = NULL;
+    struct diriter_s di = { -1 };
 
     /* transaction id used for temporary path suffix while installing */
     rasprintf(&tid, ";%08x", (unsigned)rpmtsGetTid(ts));
@@ -932,6 +1051,7 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
         rc = RPMERR_BAD_MAGIC;
         goto exit;
     }
+    rpmfiSetOnChdir(fi, onChdir, &di);
 
     /* Detect and create directories not explicitly in package. */
     if (!rc)
@@ -946,6 +1066,16 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
 	    if (!fp->suffix) {
 		rc = fsmBackup(fi, fp->action);
 	    }
+
+	    if (di.dirfd == -1) {
+		di.dirfd = ensureDir(plugins, rpmfiDN(fi), 0,
+				    (fp->action == FA_CREATE));
+		if (di.dirfd == -1) {
+		    rc = RPMERR_OPEN_FAILED;
+		    break;
+		}
+	    }
+
 	    /* Assume file does't exist when tmp suffix is in use */
 	    if (!fp->suffix) {
 		rc = fsmVerify(fp->fpath, fi);
@@ -980,7 +1110,7 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
                     mode_t mode = fp->sb.st_mode;
                     mode &= ~07777;
                     mode |=  00700;
-                    rc = fsmMkdir(fp->fpath, mode);
+                    rc = fsmMkdir(di.dirfd, fp->fpath, mode);
                 }
             } else if (S_ISLNK(fp->sb.st_mode)) {
 		if (rc == RPMERR_ENOENT) {
@@ -1022,6 +1152,8 @@ int rpmPackageFilesInstall(rpmts ts, rpmte te, rpmfiles files,
 	fp->stage = FILE_UNPACK;
     }
     fi = rpmfiFree(fi);
+    close(di.dirfd);
+    di.dirfd = -1;
 
     if (!rc && fx < 0 && fx != RPMERR_ITER_END)
 	rc = fx;
-- 
2.27.0

