proc: prevent stacking filesystems on top

[ Upstream commit e54ad7f1ee263ffa5a2de9c609d58dfa27b21cd9 ]

This prevents stacking filesystems (ecryptfs and overlayfs) from using
procfs as lower filesystem.  There is too much magic going on inside
procfs, and there is no good reason to stack stuff on top of procfs.

(For example, procfs does access checks in VFS open handlers, and
ecryptfs by design calls open handlers from a kernel thread that doesn't
drop privileges or so.)

Signed-off-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

diff --git a/fs/proc/root.c b/fs/proc/root.c
index 68feb0f70e6358f83f5a5281bf3c64d16cad2015..c3e1bc595e6db67e68399459eeb0376fc9e3b169 100644 (file)
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -121,6 +121,13 @@ static struct dentry *proc_mount(struct file_system_type *fs_type,
        if (IS_ERR(sb))
                return ERR_CAST(sb);
 
+       /*
+        * procfs isn't actually a stacking filesystem; however, there is
+        * too much magic going on inside it to permit stacking things on
+        * top of it
+        */
+       sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
+
        if (!proc_parse_options(options, ns)) {
                deactivate_locked_super(sb);
                return ERR_PTR(-EINVAL);