Re: [PATCH] Revert "ACPI: custom_method: fix memory leaks"

From: Kees Cook <keescook@chromium.org>
To: Mark Langsdorf <mlangsdo@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Wenwen Wang <wenwen@cs.uga.edu>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Len Brown <lenb@kernel.org>,
	linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org
Subject: Re: [PATCH] Revert "ACPI: custom_method: fix memory leaks"
Date: Mon, 3 May 2021 11:35:50 -0700	[thread overview]
Message-ID: <202105031134.A0E51D73@keescook> (raw)
In-Reply-To: <0fefece0-f8a1-6ee1-114f-0a2bb412b986@redhat.com>

On Mon, May 03, 2021 at 08:17:14AM -0500, Mark Langsdorf wrote:
> In 5/2/21 12:23 PM, Kees Cook wrote:
> > This reverts commit 03d1571d9513369c17e6848476763ebbd10ec2cb.
> > 
> > While /sys/kernel/debug/acpi/custom_method is already a privileged-only
> > API providing proxied arbitrary write access to kernel memory[1][2],
> > with existing race conditions[3] in buffer allocation and use that could
> > lead to memory leaks and use-after-free conditions, the above commit
> > appears to accidentally make the use-after-free conditions even easier
> > to accomplish. ("buf" is a global variable and prior kfree()s would set
> > buf back to NULL.)
> > 
> > This entire interface needs to be reworked (if not entirely removed).
> > 
> > [1] https://lore.kernel.org/lkml/20110222193250.GA23913@outflux.net/
> > [2] https://lore.kernel.org/lkml/201906221659.B618D83@keescook/
> > [3] https://lore.kernel.org/lkml/20170109231323.GA89642@beast/
> > 
> > Cc: Wenwen Wang <wenwen@cs.uga.edu>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> 
> I have two patches submitted to linux-acpi to fix the most obvious bugs in
> the current driver.  I don't think that just reverting this patch in its
> entirety is a good solution: it still leaves the buf allocated in -EINVAL,
> as well as the weird case where a not fully consumed buffer can be
> reallocated without being freed on a subsequent call.
> 
> https://lore.kernel.org/linux-acpi/20210427185434.34885-1-mlangsdo@redhat.com/
> 
> https://lore.kernel.org/linux-acpi/20210423152818.97077-1-mlangsdo@redhat.com/
> 
> I support rewriting this driver in its entirety, but reverting one bad patch
> to leave it in a different buggy state is less than ideal.

Thanks for working on that! It'd be nice if there was a lock held for
the duration of the "open", then all the concurrency races would go
away. But, I haven't spent a lot of time looking since it's root-only
and already blocked by lockdown, etc.

-- 
Kees Cook