ovl: don't follow redirects if redirect_dir=off
Overlayfs is following redirects even when redirects are disabled. If this
is unintentional (probably the majority of cases) then this can be a
problem. E.g. upper layer comes from untrusted USB drive, and attacker
crafts a redirect to enable read access to otherwise unreadable
directories.
If "redirect_dir=off", then turn off following as well as creation of
redirects. If "redirect_dir=follow", then turn on following, but turn off
creation of redirects (which is what "redirect_dir=off" does now).
This is a backward incompatible change, so make it dependent on a config
option.
Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 8caa607..e6a5f491 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -156,6 +156,40 @@
root of the overlay. Finally the directory is moved to the new
location.
+There are several ways to tune the "redirect_dir" feature.
+
+Kernel config options:
+
+- OVERLAY_FS_REDIRECT_DIR:
+ If this is enabled, then redirect_dir is turned on by default.
+- OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW:
+ If this is enabled, then redirects are always followed by default. Enabling
+ this results in a less secure configuration. Enable this option only when
+ worried about backward compatibility with kernels that have the redirect_dir
+ feature and follow redirects even if turned off.
+
+Module options (can also be changed through /sys/module/overlay/parameters/*):
+
+- "redirect_dir=BOOL":
+ See OVERLAY_FS_REDIRECT_DIR kernel config option above.
+- "redirect_always_follow=BOOL":
+ See OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW kernel config option above.
+- "redirect_max=NUM":
+ The maximum number of bytes in an absolute redirect (default is 256).
+
+Mount options:
+
+- "redirect_dir=on":
+ Redirects are enabled.
+- "redirect_dir=follow":
+ Redirects are not created, but followed.
+- "redirect_dir=off":
+ Redirects are not created and only followed if "redirect_always_follow"
+ feature is enabled in the kernel/module config.
+- "redirect_dir=nofollow":
+ Redirects are not created and not followed (equivalent to "redirect_dir=off"
+ if "redirect_always_follow" feature is not enabled).
+
Non-directories
---------------