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Date:   Wed, 27 May 2020 14:55:15 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     glider@google.com, adobriyan@gmail.com,
        linux-kernel@vger.kernel.org, sunhaoyl@outlook.com,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH] fs/binfmt_elf.c: allocate initialized memory in
 fill_thread_core_info()
Message-ID: <202005271450.7468AA95@keescook>
References: <20200419100848.63472-1-glider@google.com>
 <20200420153352.6682533e794f591dae7aafbc@linux-foundation.org>
 <202004201540.01C8F82B@keescook>
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On Mon, Apr 20, 2020 at 03:41:40PM -0700, Kees Cook wrote:
> On Mon, Apr 20, 2020 at 03:33:52PM -0700, Andrew Morton wrote:
> > On Sun, 19 Apr 2020 12:08:48 +0200 glider@google.com wrote:
> > 
> > > KMSAN reported uninitialized data being written to disk when dumping
> > > core. As a result, several kilobytes of kmalloc memory may be written to
> > > the core file and then read by a non-privileged user.
> 
> Ewww. That's been there for 12 years. Did something change in
> regset_size() or regset->get()? Do you know what leaves the hole?
> 
> > > 
> > > ...
> > >
> > > --- a/fs/binfmt_elf.c
> > > +++ b/fs/binfmt_elf.c
> > > @@ -1733,7 +1733,7 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
> > >  		    (!regset->active || regset->active(t->task, regset) > 0)) {
> > >  			int ret;
> > >  			size_t size = regset_size(t->task, regset);
> > > -			void *data = kmalloc(size, GFP_KERNEL);
> > > +			void *data = kzalloc(size, GFP_KERNEL);
> > >  			if (unlikely(!data))
> > >  				return 0;
> > >  			ret = regset->get(t->task, regset,
> > 
> > This seems to be a quite easy way of exposing quite a large amount of
> > kernel memory contents, so I think I'll add a cc:stable to this patch?
> 
> Yes please.
> 
> Acked-by: Kees Cook <keescook@chromium.org>

This has been in -next for a while, but we need to get this landed and
into -stable. Can you please send this to Linus for the final release? I
know Al is working on getting the complementary fixes landed too, but
this fix is also sufficient, trivial to backport, and provides some
future-proofing/defense in depth.

-Kees

-- 
Kees Cook