[PATCH] efivarfs: fix NULL dereference on resume

James Bottomley
2025-03-18T03:06:03Z

LSMs often inspect the path.mnt of files in the security hooks which
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 caused 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.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
---
fs/efivarfs/super.c | 50 +++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 48 insertions(+), 2 deletions(-)

diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
index 6eae8cf655c1..81b3c6b7e100 100644
--- a/fs/efivarfs/super.c
+++ b/fs/efivarfs/super.c
@@ -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;

--
2.43.0