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

[Rafael J. Wysocki]

On Mon, May 3, 2021 at 4:51 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxx/
> > > [2] https://lore.kernel.org/lkml/201906221659.B618D83@keescook/
> > > [3] https://lore.kernel.org/lkml/20170109231323.GA89642@beast/
> > >
> > > Cc: Wenwen Wang <wenwen@xxxxxxxxxx>
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> >
> > 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@xxxxxxxxxx/
> >
> > https://lore.kernel.org/linux-acpi/20210423152818.97077-1-mlangsdo@xxxxxxxxxx/
> >
> > 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.
>
> It's buggy now, and root-only, so it's a low bar at the moment :)

So dropping it completely may be a better choice.

IMO let's let the Mark's commits go in now and we'll see later.

Thanks!