Commit 11092db5 authored by James Bottomley's avatar James Bottomley Committed by Ard Biesheuvel
Browse files

efivarfs: fix NULL dereference on resume 



LSMs often inspect the path.mnt of files in the security hooks, and this
causes a NULL deref in efivarfs_pm_notify() because the path is
constructed with a NULL path.mnt.

Fix by obtaining from vfs_kern_mount() instead, and being very careful
to ensure that deactivate_super() (potentially triggered by a racing
userspace umount) is not called directly from the notifier, because it
would deadlock when efivarfs_kill_sb() tried to unregister the notifier
chain.

[ Al notes:
	Umm...  That's probably safe, but not as a long-term solution -
	it's too intimately dependent upon fs/super.c internals. The
	reasons why you can't run into ->s_umount deadlock here are
	non-trivial... ]

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarJames Bottomley <James.Bottomley@HansenPartnership.com>
Link: https://lore.kernel.org/r/e54e6a2f-1178-4980-b771-4d9bafc2aa47@tnxip.de
Link: https://lore.kernel.org/r/3e998bf87638a442cbc6864cdcd3d8d9e08ce3e3.camel@HansenPartnership.com


Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
parent dec12778

fs/efivarfs/super.c

+48 −2
Original line number Diff line number Diff line
@@ -474,12 +474,25 @@ static int efivarfs_check_missing(efi_char16_t *name16, efi_guid_t vendor, 
	return err;

}



static void efivarfs_deactivate_super_work(struct work_struct *work)

{

	struct super_block *s = container_of(work, struct super_block,

					     destroy_work);

	/*

	 * note: here s->destroy_work is free for reuse (which

	 * will happen in deactivate_super)

	 */

	deactivate_super(s);

}



static struct file_system_type efivarfs_type;



static int efivarfs_pm_notify(struct notifier_block *nb, unsigned long action,

			      void *ptr)

{

	struct efivarfs_fs_info *sfi = container_of(nb, struct efivarfs_fs_info,

						    pm_nb);

	struct path path = { .mnt = NULL, .dentry = sfi->sb->s_root, };

	struct path path;

	struct efivarfs_ctx ectx = {

		.ctx = {

			.actor	= efivarfs_actor,
@@ -487,6 +500,7 @@ static int efivarfs_pm_notify(struct notifier_block *nb, unsigned long action, 
		.sb = sfi->sb,

	};

	struct file *file;

	struct super_block *s = sfi->sb;

	static bool rescan_done = true;



	if (action == PM_HIBERNATION_PREPARE) {
@@ -499,11 +513,43 @@ static int efivarfs_pm_notify(struct notifier_block *nb, unsigned long action, 
	if (rescan_done)

		return NOTIFY_DONE;



	/* ensure single superblock is alive and pin it */

	if (!atomic_inc_not_zero(&s->s_active))

		return NOTIFY_DONE;



	pr_info("efivarfs: resyncing variable state\n");



	/* O_NOATIME is required to prevent oops on NULL mnt */

	path.dentry = sfi->sb->s_root;



	/*

	 * do not add SB_KERNMOUNT which a single superblock could

	 * expose to userspace and which also causes MNT_INTERNAL, see

	 * below

	 */

	path.mnt = vfs_kern_mount(&efivarfs_type, 0,

				  efivarfs_type.name, NULL);

	if (IS_ERR(path.mnt)) {

		pr_err("efivarfs: internal mount failed\n");

		/*

		 * We may be the last pinner of the superblock but

		 * calling efivarfs_kill_sb from within the notifier

		 * here would deadlock trying to unregister it

		 */

		INIT_WORK(&s->destroy_work, efivarfs_deactivate_super_work);

		schedule_work(&s->destroy_work);

		return PTR_ERR(path.mnt);

	}



	/* path.mnt now has pin on superblock, so this must be above one */

	atomic_dec(&s->s_active);



	file = kernel_file_open(&path, O_RDONLY | O_DIRECTORY | O_NOATIME,

				current_cred());

	/*

	 * safe even if last put because no MNT_INTERNAL means this

	 * will do delayed deactivate_super and not deadlock

	 */

	mntput(path.mnt);

	if (IS_ERR(file))

		return NOTIFY_DONE;