linux-kernel - Re: [PATCH] Revert "ACPI: custom

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Date:   Mon, 3 May 2021 08:17:14 -0500
From:   Mark Langsdorf <mlangsdo@...hat.com>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Wenwen Wang <wenwen@...uga.edu>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org
Subject: Re: [PATCH] Revert "ACPI: custom_method: fix memory leaks"

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@...uga.edu>
> Signed-off-by: Kees Cook <keescook@...omium.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.

--Mark Langsdorf