File flatpak-CVE-2024-42472-part01-3caeb16c.patch of Package flatpak.35425
From 3caeb16c31a3ed62d744e2aaf01d684f7991051a
From: Alexander Larsson <alexl@redhat.com>
Date: Mon, 3 Jun 2024 12:22:30 +0200
Subject: [PATCH] Don't follow symlinks when mounting persisted directories
References: CVE-2024-42472
References: bsc#1229157
Upstream: Backport from upstream
These directories are in a location under application control, so we
can't trust them to not be a symlink outside of the files accessibe to
the application.
Continue to treat --persist=/foo as --persist=foo for backwards compat,
since this is how it (accidentally) worked before, but print a warning.
Don't allow ".." elements in persist paths: these would not be useful
anyway, and are unlikely to be in use, however they could potentially
be used to confuse the persist path handling.
This partially addresses CVE-2024-42472. If only one instance of the
malicious or compromised app is run at a time, the vulnerability
is avoided. If two instances can run concurrently, there is a
time-of-check/time-of-use issue remaining, which can only be resolved
with changes to bubblewrap; this will be resolved in a separate commit,
because the bubblewrap dependency might be more difficult to provide in
LTS distributions.
Helps: CVE-2024-42472, GHSA-7hgv-f2j8-xw87
[smcv: Make whitespace consistent]
[smcv: Use g_warning() if unable to create --persist paths]
[smcv: Use stat() to detect symlinks and warn about them]
Co-authored-by: Simon McVittie <smcv@collabora.com>
Signed-off-by: Simon McVittie <smcv@collabora.com>
---
common/flatpak-context.c | 109 +++++++++++++++++++++++++++++++++++++--
1 file changed, 105 insertions(+), 4 deletions(-)
--- flatpak-1.10.8/common/flatpak-context.c
+++ flatpak-1.10.8_new/common/flatpak-context.c
@@ -2616,6 +2616,100 @@
return flags;
}
+/* This creates zero or more directories unders base_fd+basedir, each
+ * being guaranteed to either exist and be a directory (no symlinks)
+ * or be created as a directory. The last directory is opened
+ * and the fd is returned.
+ */
+static gboolean
+mkdir_p_open_nofollow_at (int base_fd,
+ const char *basedir,
+ int mode,
+ const char *subdir,
+ int *out_fd,
+ GError **error)
+{
+ glnx_autofd int parent_fd = -1;
+
+ if (g_path_is_absolute (subdir))
+ {
+ const char *skipped_prefix = subdir;
+
+ while (*skipped_prefix == '/')
+ skipped_prefix++;
+
+ g_warning ("--persist=\"%s\" is deprecated, treating it as --persist=\"%s\"", subdir, skipped_prefix);
+ subdir = skipped_prefix;
+ }
+
+ g_autofree char *subdir_dirname = g_path_get_dirname (subdir);
+
+ if (strcmp (subdir_dirname, ".") == 0)
+ {
+ /* It is ok to open basedir with follow=true */
+ if (!glnx_opendirat (base_fd, basedir, TRUE, &parent_fd, error))
+ return FALSE;
+ }
+ else if (strcmp (subdir_dirname, "..") == 0)
+ {
+ return glnx_throw (error, "'..' not supported in --persist paths");
+ }
+ else
+ {
+ if (!mkdir_p_open_nofollow_at (base_fd, basedir, mode,
+ subdir_dirname, &parent_fd, error))
+ return FALSE;
+ }
+
+ g_autofree char *subdir_basename = g_path_get_basename (subdir);
+
+ if (strcmp (subdir_basename, ".") == 0)
+ {
+ *out_fd = glnx_steal_fd (&parent_fd);
+ return TRUE;
+ }
+ else if (strcmp (subdir_basename, "..") == 0)
+ {
+ return glnx_throw (error, "'..' not supported in --persist paths");
+ }
+
+ if (!glnx_shutil_mkdir_p_at (parent_fd, subdir_basename, mode, NULL, error))
+ return FALSE;
+
+ int fd = openat (parent_fd, subdir_basename, O_PATH | O_NONBLOCK | O_DIRECTORY | O_CLOEXEC | O_NOCTTY | O_NOFOLLOW);
+ if (fd == -1)
+ {
+ int saved_errno = errno;
+ struct stat stat_buf;
+
+ /* If it's a symbolic link, that could be a user trying to offload
+ * large data to another filesystem, but it could equally well be
+ * a malicious or compromised app trying to exploit GHSA-7hgv-f2j8-xw87.
+ * Produce a clearer error message in this case.
+ * Unfortunately the errno we get in this case is ENOTDIR, so we have
+ * to ask again to find out whether it's really a symlink. */
+ if (saved_errno == ENOTDIR &&
+ fstatat (parent_fd, subdir_basename, &stat_buf, AT_SYMLINK_NOFOLLOW) == 0 &&
+ S_ISLNK (stat_buf.st_mode))
+ return glnx_throw (error, "Symbolic link \"%s\" not allowed to avoid sandbox escape", subdir_basename);
+
+ return glnx_throw_errno_prefix (error, "openat(%s)", subdir_basename);
+ }
+
+ *out_fd = fd;
+ return TRUE;
+}
+
+//This function is copied from GLIB higher version Since CVE-2024-42472's fix will use it.
+//GLIB_AVAILABLE_STATIC_INLINE_IN_2_70
+static inline int
+g_steal_fd (int *fd_ptr)
+{
+ int fd = *fd_ptr;
+ *fd_ptr = -1;
+ return fd;
+}
+
void
flatpak_context_append_bwrap_filesystem (FlatpakContext *context,
FlatpakBwrap *bwrap,
@@ -2643,12 +2737,30 @@
while (g_hash_table_iter_next (&iter, &key, NULL))
{
const char *persist = key;
- g_autofree char *src = g_build_filename (g_get_home_dir (), ".var/app", app_id, persist, NULL);
+ g_autofree char *appdir = g_build_filename (g_get_home_dir (), ".var/app", app_id, NULL);
g_autofree char *dest = g_build_filename (g_get_home_dir (), persist, NULL);
+ g_autoptr(GError) local_error = NULL;
+
+ if (g_mkdir_with_parents (appdir, 0755) != 0)
+ {
+ g_warning ("Unable to create directory %s", appdir);
+ continue;
+ }
+
+ /* Don't follow symlinks from the persist directory, as it is under user control */
+ glnx_autofd int src_fd = -1;
+ if (!mkdir_p_open_nofollow_at (AT_FDCWD, appdir, 0755,
+ persist, &src_fd,
+ &local_error))
+ {
+ g_warning ("Failed to create persist path %s: %s", persist, local_error->message);
+ continue;
+ }
- g_mkdir_with_parents (src, 0755);
+ g_autofree char *src_via_proc = g_strdup_printf ("/proc/self/fd/%d", src_fd);
- flatpak_bwrap_add_bind_arg (bwrap, "--bind", src, dest);
+ flatpak_bwrap_add_fd (bwrap, g_steal_fd (&src_fd));
+ flatpak_bwrap_add_bind_arg (bwrap, "--bind", src_via_proc, dest);
}
}
