linux-kernel - Re: [PATCH] KEYS: fix refcount

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Date:   Wed, 31 May 2017 13:20:44 +0100
From:   David Howells <dhowells@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     dhowells@...hat.com, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Windsor <dwindsor@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        James Morris <james.l.morris@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] KEYS: fix refcount_inc() on zero

Mark Rutland <mark.rutland@....com> wrote:

> This patch uses refcount_inc_not_zero() to perform the peek and
> increment atomically.

I think the helper is unnecessary.  Better to adjust the comment if you really
want to explain it.  Anyone editing the code should be that this is inside a
critical section.

> A helper with lockdep annotation is added to document why this is safe.

This doesn't explain why this is safe.

> +	/*
> +	 * Pretend it doesn't exist if it is awaiting deletion. This races with
> +	 * key_put(), but we can peek at the key until we drop key_serial_lock.
>  	 */

With your change, there is no race with key_put() - so the second sentence is
unnecessary.

I've adjusted your patch - see attached.

David
---
commit f66bf831c45306ebbc28aecd407e238983457251
Author: Mark Rutland <mark.rutland@....com>
Date:   Fri May 26 18:37:34 2017 +0100

    KEYS: fix refcount_inc() on zero
    
    If a key's refcount is dropped to zero between key_lookup() peeking at
    the refcount and subsequently attempting to increment it, refcount_inc()
    will see a zero refcount.  Here, refcount_inc() will WARN_ONCE(), and
    will *not* increment the refcount, which will remain zero.
    
    Once key_lookup() drops key_serial_lock, it is possible for the key to
    be freed behind our back.
    
    This patch uses refcount_inc_not_zero() to perform the peek and increment
    atomically.
    
    Fixes: fff292914d3a2f1e ("security, keys: convert key.usage from atomic_t to refcount_t")
    Signed-off-by: Mark Rutland <mark.rutland@....com>
    Signed-off-by: David Howells <dhowells@...hat.com>
    Cc: David Windsor <dwindsor@...il.com>
    Cc: Elena Reshetova <elena.reshetova@...el.com>
    Cc: Hans Liljestrand <ishkamiel@...il.com>
    Cc: James Morris <james.l.morris@...cle.com>
    Cc: Kees Cook <keescook@...omium.org>
    Cc: Peter Zijlstra <peterz@...radead.org>

diff --git a/security/keys/key.c b/security/keys/key.c
index 455c04d80bbb..d84ee2a87da6 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -660,14 +660,11 @@ struct key *key_lookup(key_serial_t id)
 	goto error;
 
 found:
-	/* pretend it doesn't exist if it is awaiting deletion */
-	if (refcount_read(&key->usage) == 0)
-		goto not_found;
-
-	/* this races with key_put(), but that doesn't matter since key_put()
-	 * doesn't actually change the key
+	/* A key is allowed to be looked up only if someone still owns a
+	 * reference to it - otherwise it's awaiting the gc.
 	 */
-	__key_get(key);
+	if (!refcount_inc_not_zero(&key->usage))
+		goto not_found;
 
 error:
 	spin_unlock(&key_serial_lock);