[PATCH] keys: set persistent keyring timeout before destination linking

[Karl Mehltretter <kmehltretter@gmail.com>]

keyring_alloc() links a newly created persistent keyring into the
namespace's hidden register with TIME64_MAX expiry.
key_get_persistent() sets the configured timeout only after linking the
keyring into the caller's destination.

If the destination rejects the link, the registered keyring retains
infinite expiry. It is exempt from key quota and can remain until namespace
teardown or reboot even though the syscall returned an error. At most one
such keyring exists per UID per namespace. Repeated failures for the same
UID therefore strand only one small allocation.

This is reachable from unprivileged userspace: KEYCTL_RESTRICT_KEYRING on
the destination makes key_link() return -EPERM via restrict_link_reject(),
after key_create_persistent() has already registered the keyring.

Set the timeout immediately after creating and registering the keyring. A
later permission or destination-link failure then leaves a collectible
persistent keyring. The successful path may refresh the same timeout again
without changing its semantics.

Fixes: f36f8c75ae2e ("KEYS: Add per-user_namespace registers for persistent per-UID kerberos caches")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Tested with QEMU 10.2.1 TCG. The reproducer set
/proc/sys/kernel/keys/persistent_keyring_expiry to 60 seconds, restricted
the destination with KEYCTL_RESTRICT_KEYRING, and then called
KEYCTL_GET_PERSISTENT. No LSM policy was loaded.

                      syscall result  /proc/keys expiry
  i386 baseline       -EPERM          perm
  i386 patched        -EPERM          1m
  x86_64 patched      -EPERM          1m

Full kernel builds completed for i386 and ARM926, and for x86_64 with
CONFIG_PROVE_LOCKING=y. The x86_64 reproducer completed without lockdep
reports, warnings, or bugs.

 security/keys/persistent.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/security/keys/persistent.c b/security/keys/persistent.c
index 97af230aa4b22..0eca8c5758434 100644
--- a/security/keys/persistent.c
+++ b/security/keys/persistent.c
@@ -63,6 +63,12 @@ static key_ref_t key_create_persistent(struct user_namespace *ns, kuid_t uid,
 	if (IS_ERR(persistent))
 		return ERR_CAST(persistent);
 
+	/* Set the expiry now: if the caller then fails to link the keyring to
+	 * its destination, the register is left holding a collectible key
+	 * rather than a permanent, quota-exempt one.
+	 */
+	key_set_timeout(persistent, persistent_keyring_expiry);
+
 	return make_key_ref(persistent, true);
 }
 

base-commit: 08dbfad3f5040f5bdb6c529da20d6d4e81fefd72
-- 
2.53.0