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Message-ID: <CAHmME9p1VDaLL_0VRQU1Q3fxFtwvFNgfFQPOOGOuRyW8kY=41A@mail.gmail.com>
Date: Tue, 6 Jun 2017 23:31:31 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Eric Biggers <ebiggers3@...il.com>
Cc: keyrings@...r.kernel.org, David Howells <dhowells@...hat.com>,
Herbert Xu <herbert@...dor.apana.org.au>, Kirill Marinushkin <k.marinushkin@...il.com>,
security@...nel.org, stable@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
LKML <linux-kernel@...r.kernel.org>, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] security/keys: rewrite all of big_key crypto
On Tue, Jun 6, 2017 at 10:58 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> No need to select CRYPTO_AEAD; it's already selected by CRYPTO_GCM.
Ack.
>
> Actually I just noticed another bug, which I suppose you might as well fix too.
> Because different big_keys may be added or read concurrently, and each is
> encrypted using a different encryption key, it's not safe to use a global
> 'crypto_aead'. Instead, a separate crypto_aead must be created for each key, or
> for each encryption/decryption, or else a mutex needs to be held during each
> rekeying and encryption/decryption. For what it's worth, I recently fixed a
> similar bug for the "encrypted" key type:
> https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=keys-fixes&id=69307a05d4d58f4c29aa7e9d83dc59d63e28c382
I noticed that too, but I cynically supposed the flaw in the old code
could carry over to the new code without too much difficulty... But
sure, I'll fix this.
This is a real flaw with the crypto API, IMHO. It seems like the key
should be part of the request rather than the tfm. I understand some
primitives can do per-key precomputations, but still, this is a real
wart.
Will be fixed in the next revision.
>
> memset(req_on_stack, 0, sizeof(req_on_stack));
> memzero_explicit(req_on_stack, sizeof(req_on_stack));
I didn't really want to do sizeof with a VLA, but actually, it looks
like gcc does the right thing, so I'll make that change.
> It isn't "aligned" anymore --- instead, it's leaving room for the authentiation
> tag. I don't think it even needs to be zeroed, does it? Can you update the
> code and comments below this?
Ack.
> I think these fixes are going to have to be done separately from the switch to
> get_random_bytes_wait(). Just make it use get_random_bytes() for now, and
> switch it to get_random_bytes_wait() later in a separate patch.
Blah, I really dislike doing that. Can I just roll it into the RNG
patchset, and then it'll go into Ted's tree with DavidH's sign-off?
> Please also add a brief comment that anyone who might try to add an update()
> method will find, e.g.:
>
> struct key_type key_type_big_key = {
> [...]
> .describe = big_key_describe,
> .read = big_key_read,
> /* no ->update() yet; don't add it without changing big_key_crypt() */
> };
>
> Otherwise someone will add one, and the crypto will be broken again.
Great idea, again.
Thanks a bunch for the review!
Jason
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