linux-kernel - [PATCH 13/15] KEYS: load key flags and expiry time atomically in key

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Message-Id: <150783945342.10956.11064832686161197456.stgit@warthog.procyon.org.uk>
Date:   Thu, 12 Oct 2017 21:17:33 +0100
From:   David Howells <dhowells@...hat.com>
To:     ebiggers@...gle.com
Cc:     keyrings@...r.kernel.org, dhowells@...hat.com,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 13/15] KEYS: load key flags and expiry time atomically in key_validate()

From: Eric Biggers <ebiggers@...gle.com>

In key_validate(), load the flags and expiry time once atomically, since
these can change concurrently if key_validate() is called without the
key semaphore held.  And we don't want to get inconsistent results if a
variable is referenced multiple times.  For example, key->expiry was
referenced in both 'if (key->expiry)' and in 'if (now.tv_sec >=
key->expiry)', making it theoretically possible to see a spurious
EKEYEXPIRED while the expiration time was being removed, i.e. set to 0.

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 security/keys/permission.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/security/keys/permission.c b/security/keys/permission.c
index 732cc0beffdf..a72b4dd70c8a 100644
--- a/security/keys/permission.c
+++ b/security/keys/permission.c
@@ -88,7 +88,8 @@ EXPORT_SYMBOL(key_task_permission);
  */
 int key_validate(const struct key *key)
 {
-	unsigned long flags = key->flags;
+	unsigned long flags = READ_ONCE(key->flags);
+	time_t expiry = READ_ONCE(key->expiry);
 
 	if (flags & (1 << KEY_FLAG_INVALIDATED))
 		return -ENOKEY;
@@ -99,9 +100,9 @@ int key_validate(const struct key *key)
 		return -EKEYREVOKED;
 
 	/* check it hasn't expired */
-	if (key->expiry) {
+	if (expiry) {
 		struct timespec now = current_kernel_time();
-		if (now.tv_sec >= key->expiry)
+		if (now.tv_sec >= expiry)
 			return -EKEYEXPIRED;
 	}