Re: [PATCH] Keys: Remove redundant initialization of cred

From: David Howells <dhowells@redhat.com>
To: Yang Li <yang.lee@linux.alibaba.com>
Cc: dhowells@redhat.com, jarkko@kernel.org, jmorris@namei.org,
	serge@hallyn.com, nathan@kernel.org, ndesaulniers@google.com,
	keyrings@vger.kernel.org, linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH] Keys: Remove redundant initialization of cred
Date: Tue, 11 May 2021 11:50:14 +0100	[thread overview]
Message-ID: <2499883.1620730214@warthog.procyon.org.uk> (raw)
In-Reply-To: <1620467481-110575-1-git-send-email-yang.lee@linux.alibaba.com>

Yang Li <yang.lee@linux.alibaba.com> wrote:

> -	const struct cred *cred = current_cred();
> +	const struct cred *cred;

Good catch, but it's probably the wrong fix.

In that function, there is:

	const struct cred *cred = current_cred();
	...
	cred = get_current_cred();
	keyring = keyring_alloc(desc, cred->fsuid, cred->fsgid, cred, ...);
	put_cred(cred);
	...
	sprintf(uid_str, "%d", from_kuid(&init_user_ns, cred->fsuid));
	...

So we get the creds again, but with a ref, and then drop after calling
keyring_alloc()... and then access cred again, which is dodgy - but we get
away with it because cred is still pinned by our task_struct.

I think what is actually needed is to remove the get_current_cred() and the
put_cred() calls, in which case, you want this:

	Fixes: d84f4f992cbd ("CRED: Inaugurate COW credentials")

David