linux-kernel - Re: [GIT PULL] keys: Fix key->sem vs mmap

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Date:   Sat, 4 Apr 2020 23:10:57 -0400
From:   Waiman Long <longman@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        keyrings@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] keys: Fix key->sem vs mmap_sem issue when reading key

On 4/4/20 4:00 PM, Linus Torvalds wrote:
> On Mon, Mar 30, 2020 at 5:16 AM David Howells <dhowells@...hat.com> wrote:
>>  security/keys/internal.h                  |  12 ++++
> This isn't so much about this pull (which I have taken), as about the
> fact that this code re-inforces bad behavior we already in the slub
> layer, and now extends it further to kvfree.
>
> Doing this:
>
>
>    __kvzfree(const void *addr, size_t len)
>   ..
>                 memset((void *)addr, 0, len);
>                 kvfree(addr);
>
> is wrong to begin with. It's wrong because if the compiler ever knows
> that kvfree is a freeing function (with something like
> __attribute__((free)) - I don't think gcc is smart enough today), the
> compiler might throw the memset away.
>
> Yeah, so far we've only seen that for automatic stack clearing, but
> there are very much compilers that know that alloc/free are special
> (both for warning about use-after-free issues, and for "improving"
> code generation by blindly removing dead writes).
>
> We have a function for clearing sensitive information: it's called
> "memclear_explicit()", and it's about forced (explicit) clearing even
> if the data might look dead afterwards.
>
> The other problem with that function is the name: "__kvzfree()" is not
> a useful name for this function. We use the "__" format for internal
> low-level helpers, and it generally means that it does *less* than the
> full function. This does more, not less, and "__" is not following any
> sane naming model.
>
> So the name should probably be something like "kvfree_sensitive()" or
> similar. Or maybe it could go even further, and talk about _why_ it's
> sensitive, and call it "kvfree_cleartext()" or something like that.
>
> Because the clearing is really not what even matters. It might choose
> other patterns to overwrite things with, but it might do other things
> too, like putting special barriers for data leakage (or flags to tell
> return-to-user-mode to do so).
>
> And yes, kzfree() isn't a good name either, and had that same
> memset(), but at least it doesn't do the dual-underscore mistake.
>
> Including some kzfree()/crypto people explicitly - I hope we can get
> away from this incorrect and actively wrong pattern of thinking that
> "sensitive data should be memset(), and then we should add a random
> 'z' in the name somewhere to 'document' that".
>
>                Linus
>
Thanks for the suggestion, I will post a patch to rename the function to
kvzfree_explicit() and use memzero_explicit() for clearing memory.

Cheers,
Longman