linux-kernel - [PATCH] KEYS: Invert FINAL

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Message-ID: <301015.1748434697@warthog.procyon.org.uk>
Date: Wed, 28 May 2025 13:18:17 +0100
From: David Howells <dhowells@...hat.com>
To: torvalds@...ux-foundation.org
cc: dhowells@...hat.com, Herbert Xu <herbert@...dor.apana.org.au>,
    Jarkko Sakkinen <jarkko@...nel.org>, keyrings@...r.kernel.org,
    linux-security-module@...r.kernel.org, linux-crypto@...r.kernel.org,
    linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] KEYS: Invert FINAL_PUT bit

Hi Linus,

Could you apply this, please?  There shouldn't be any functional change,
rather it's a switch to using combined bit-barrier ops and lesser barriers.
A better way to do this might be to provide set_bit_release(), but the end
result would be much the same.

Thanks,
David
---
From: Herbert Xu <herbert@...dor.apana.org.au>

KEYS: Invert FINAL_PUT bit

Invert the FINAL_PUT bit so that test_bit_acquire and clear_bit_unlock
can be used instead of smp_mb.

Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
Signed-off-by: David Howells <dhowells@...hat.com>
Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>
cc: keyrings@...r.kernel.org
cc: linux-security-module@...r.kernel.org
cc: linux-crypto@...r.kernel.org
cc: linux-integrity@...r.kernel.org
---
 include/linux/key.h |    2 +-
 security/keys/gc.c  |    4 ++--
 security/keys/key.c |    5 +++--
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/key.h b/include/linux/key.h
index ba05de8579ec..81b8f05c6898 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -236,7 +236,7 @@ struct key {
 #define KEY_FLAG_ROOT_CAN_INVAL	7	/* set if key can be invalidated by root without permission */
 #define KEY_FLAG_KEEP		8	/* set if key should not be removed */
 #define KEY_FLAG_UID_KEYRING	9	/* set if key is a user or user session keyring */
-#define KEY_FLAG_FINAL_PUT	10	/* set if final put has happened on key */
+#define KEY_FLAG_USER_ALIVE	10	/* set if final put has not happened on key yet */
 
 	/* the key type and key description string
 	 * - the desc is used to match a key against search criteria
diff --git a/security/keys/gc.c b/security/keys/gc.c
index f27223ea4578..748e83818a76 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -218,8 +218,8 @@ static void key_garbage_collector(struct work_struct *work)
 		key = rb_entry(cursor, struct key, serial_node);
 		cursor = rb_next(cursor);
 
-		if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) {
-			smp_mb(); /* Clobber key->user after FINAL_PUT seen. */
+		if (!test_bit_acquire(KEY_FLAG_USER_ALIVE, &key->flags)) {
+			/* Clobber key->user after final put seen. */
 			goto found_unreferenced_key;
 		}
 
diff --git a/security/keys/key.c b/security/keys/key.c
index 7198cd2ac3a3..3bbdde778631 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -298,6 +298,7 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 	key->restrict_link = restrict_link;
 	key->last_used_at = ktime_get_real_seconds();
 
+	key->flags |= 1 << KEY_FLAG_USER_ALIVE;
 	if (!(flags & KEY_ALLOC_NOT_IN_QUOTA))
 		key->flags |= 1 << KEY_FLAG_IN_QUOTA;
 	if (flags & KEY_ALLOC_BUILT_IN)
@@ -658,8 +659,8 @@ void key_put(struct key *key)
 				key->user->qnbytes -= key->quotalen;
 				spin_unlock_irqrestore(&key->user->lock, flags);
 			}
-			smp_mb(); /* key->user before FINAL_PUT set. */
-			set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
+			/* Mark key as safe for GC after key->user done. */
+			clear_bit_unlock(KEY_FLAG_USER_ALIVE, &key->flags);
 			schedule_work(&key_gc_work);
 		}
 	}