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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, May 03, 2021 at 09:58:17AM -0500, Mark Langsdorf wrote:
> On 5/3/21 9:51 AM, Greg Kroah-Hartman 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@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.
> > It's buggy now, and root-only, so it's a low bar at the moment :)
> > 
> > Do those commits really fix the issues?  Is this debugfs code even
> > needed at all or can it just be dropped?
> 
> One of my commits removes the kfree(buf) at the end of the function, which
> is the code that causes the use after free for short writes.  The other adds
> a kfree(buf) before allocating the buffer, to make sure that the buffer is
> free before allocating it.
> 
> There are other bugs in the code that neither my patches nor the revert
> address, like the total lack of protection against concurrent writes.

Why would anyone care about concurrent writes for this debugfs file?
Is that a requirement here?

thanks,

greg k-h