9pfs: local: lremovexattr: don't follow symlinks
The local_lremovexattr() callback is vulnerable to symlink attacks because
it calls lremovexattr() which follows symbolic links in all path elements
but the rightmost one.
This patch introduces a helper to emulate the non-existing fremovexattrat()
function: it is implemented with /proc/self/fd which provides a trusted
path that can be safely passed to lremovexattr().
local_lremovexattr() is converted to use this helper and opendir_nofollow().
This partly fixes CVE-2016-9602.
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/hw/9pfs/9p-xattr.c b/hw/9pfs/9p-xattr.c
index a0167dd..eec160b 100644
--- a/hw/9pfs/9p-xattr.c
+++ b/hw/9pfs/9p-xattr.c
@@ -234,17 +234,43 @@
return local_setxattr_nofollow(ctx, path, name, value, size, flags);
}
-int pt_removexattr(FsContext *ctx, const char *path, const char *name)
+static ssize_t fremovexattrat_nofollow(int dirfd, const char *filename,
+ const char *name)
{
- char *buffer;
+ char *proc_path = g_strdup_printf("/proc/self/fd/%d/%s", dirfd, filename);
int ret;
- buffer = rpath(ctx, path);
- ret = lremovexattr(path, name);
- g_free(buffer);
+ ret = lremovexattr(proc_path, name);
+ g_free(proc_path);
return ret;
}
+ssize_t local_removexattr_nofollow(FsContext *ctx, const char *path,
+ const char *name)
+{
+ char *dirpath = g_path_get_dirname(path);
+ char *filename = g_path_get_basename(path);
+ int dirfd;
+ ssize_t ret = -1;
+
+ dirfd = local_opendir_nofollow(ctx, dirpath);
+ if (dirfd == -1) {
+ goto out;
+ }
+
+ ret = fremovexattrat_nofollow(dirfd, filename, name);
+ close_preserve_errno(dirfd);
+out:
+ g_free(dirpath);
+ g_free(filename);
+ return ret;
+}
+
+int pt_removexattr(FsContext *ctx, const char *path, const char *name)
+{
+ return local_removexattr_nofollow(ctx, path, name);
+}
+
ssize_t notsup_getxattr(FsContext *ctx, const char *path, const char *name,
void *value, size_t size)
{
