crash_dump: release keyring reference at the correct time

restore_dm_crypt_keys_to_thread_keyring() gets a reference to the user
keyring before restoring the saved dm-crypt keys.

The same keyring reference is then passed to add_key_to_keyring() for each
saved key, but add_key_to_keyring() drops that reference on every call.
This is only balanced when exactly one key is restored. With multiple
keys, the keyring reference is dropped too many times and may trigger a
refcount underflow or use-after-free.

When more than five keys are restored, a refcount underflow/use-after-free
warning can be triggered.

The early error paths after lookup_user_key() also return without dropping
the keyring reference.

Keep ownership of the keyring reference in
restore_dm_crypt_keys_to_thread_keyring(), drop it once on all exit paths,
and make add_key_to_keyring() only use the reference without consuming it.

Fixes: 62f17d9df692 ("crash_dump: retrieve dm crypt keys in kdump kernel")
Signed-off-by: Guangshuo Li <lgs201920130244@gmail.com>
Reviewed-and-tested-by: Coiby Xu <Coiby.Xu@gmail.com>
Acked-by: Baoquan He <baoquan.he@linux.dev>
Reviewed-by: Bradley Morgan <include@grrlz.net>
Link: https://patch.msgid.link/20260704112509.3717884-1-lgs201920130244@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

0 files changed, 0 insertions, 0 deletions