Commit aeaa4a79 authored Jul 23, 2016 by Eric W. Biederman

fs: Call d_automount with the filesystems creds



Seth Forshee reported a mount regression in nfs autmounts
with "fs: Add user namespace member to struct super_block".

It turns out that the assumption that current->cred is something
reasonable during mount while necessary to improve support of
unprivileged mounts is wrong in the automount path.

To fix the existing filesystems override current->cred with the
init_cred before calling d_automount and restore current->cred after
d_automount completes.

To support unprivileged mounts would require a more nuanced cred
selection, so fail on unprivileged mounts for the time being.  As none
of the filesystems that currently set FS_USERNS_MOUNT implement
d_automount this check is only good for preventing future problems.

Fixes: 6e4eab57 ("fs: Add user namespace member to struct super_block")
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

parent 81754357

fs/namei.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -36,6 +36,7 @@
		#include <linux/posix_acl.h>
		#include <linux/hash.h>
		#include <linux/bitops.h>
		#include <linux/init_task.h>
		#include <asm/uaccess.h>

		#include "internal.h"
		@@ -1099,6 +1100,7 @@ static int follow_automount(struct path path, struct nameidata nd,
		bool *need_mntput)
		{
		struct vfsmount *mnt;
		const struct cred *old_cred;
		int err;

		if (!path->dentry->d_op \|\| !path->dentry->d_op->d_automount)
		@@ -1120,11 +1122,16 @@ static int follow_automount(struct path path, struct nameidata nd,
		path->dentry->d_inode)
		return -EISDIR;

		if (path->dentry->d_sb->s_user_ns != &init_user_ns)
		return -EACCES;

		nd->total_link_count++;
		if (nd->total_link_count >= 40)
		return -ELOOP;

		old_cred = override_creds(&init_cred);
		mnt = path->dentry->d_op->d_automount(path);
		revert_creds(old_cred);
		if (IS_ERR(mnt)) {
		/*
		* The filesystem is allowed to return -EISDIR here to indicate

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