linux-kernel - Re: usb/core: warning in usb_create_ep_devs/sysfs_create_dir

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Message-ID: <Pine.LNX.4.44L0.1612131034280.1533-100000@iolanthe.rowland.org>
Date:   Tue, 13 Dec 2016 10:52:05 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Dmitry Vyukov <dvyukov@...gle.com>
cc:     syzkaller <syzkaller@...glegroups.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kostya Serebryany <kcc@...gle.com>
Subject: Re: usb/core: warning in usb_create_ep_devs/sysfs_create_dir_ns

On Tue, 13 Dec 2016, Dmitry Vyukov wrote:

> >> >  If it is
> >> > not a bug in kernel source code, then it must not produce a WARNING.
> >
> > What about a memory allocation failure?  The memory management part of
> > the kernel produces a WARNING message if an allocation fails and the
> > caller did not specify __GFP_NOWARN.
> >
> > There is no way for a driver to guarantee that a memory allocation
> > request will succeed -- failure is always an option.  But obviously
> > memory allocation failures are not bugs in the kernel.
> >
> > Are you saying that mm/page_alloc.c:warn_alloc() should produce
> > something other than a WARNING?
> 
> 
> The main thing I am saying is that we absolutely need a way for a
> human or a computer program to be able to determine if there is
> anything wrong with kernel or not.

Okay, I agree with that.  I'm not at all sure that this decision should
be based on whether a message is a WARNING.

In my experience, there is no general consensus on what conditions
should qualify as a WARNING.  There aren't any hard-and-fast rules, so
people are forced to rely on their own judgment.  The general
interpretation seems to be that WARNING is appropriate for notifying
the user whenever something important has gone unexpectedly wrong.  
That covers a lot of ground, including things (like hardware failures)
that are not kernel bugs.

> Page_alloc produces a WARNING iff you ask for an amount of memory that
> can't possibly be allocated under any circumstances (order >=
> MAX_ORDER).

Doesn't it also produce a WARNING under other circumstances?

> That's not just an allocation failure. Kernel itself
> should not generally ask for such large amounts of memory. If the
> allocation size is dictated by user, then kernel code should either
> use __GFP_NOWARN, or impose own stricter limit dictated by context
> (e.g. if it's a text command of known format, then limit can be as
> small as say 128 bytes).

All right, yes, it makes sense to use __GFP_NOWARN when the allocation 
size is dictated by the user.

But still, even when a stricter limit has been imposed, a memory
allocation request may fail.  In fact, as far as I know, the kernel has
_no_ guarantee at all that any memory allocation request will succeed!
The best it offers is that sufficiently small requests may wait
indefinitely for memory to become available, which isn't much better
than failing.

> Re fake hardware. panic_on_warn will definitely cause problems. I
> don't know if it used in any paranoid production systems or not,
> though. But more generally, I don't see how it is different from
> incorrect syscall arguments or nonsensical network packets received
> from free internet. In ideal productions environments none of these
> incorrect inputs to kernel should happen. I don't see any single
> reason to not protect kernel from incorrect input in this case as
> well, as we do in all other cases.

I _do_ see a reason.  Real computers don't live in ideal production
environments.  Why go to extra trouble to add protection for some
particular incorrect input when the kernel is _already_ capable of
handling this input in a valid manner (even though this may include
logging a WARNING message)?

> In particular, it would resolve a
> very real and practical issue for us -- fuzzer will not reboot machine
> every minute, and we will not spend time looking at these WARNINGs,
> and we will not spend your time by reporting these WARNINGs.

Maybe you should decide that ERROR messages indicate a kernel bug, 
rather than WARNING messages.  Even that is questionable, but you will 
get far fewer false positives.

Even better, you should publicize this decision (in the kernel 
documentation, on various mailing lists, on LWN, and so forth), and 
enforce it by reducing existing ERROR severity levels to WARNINGs in 
places where they do not indicate a kernel bug.

Alan Stern