Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

[David Lang]

On Wed, 7 May 2014, Ingo Molnar wrote:

> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> > On Wed, May 07, 2014 at 02:24:44PM +0200, Ingo Molnar wrote:
> > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > >
> > > > > Ah this reminds me when we chased kprobes dangerous spots and we
> > > > > tried to declare __kprobes the functions which were too dangerous
> > > > > to hot patch.
> > > > >
> > > > > We eventually gave up because it was impossible to fix everything.
> > > > > And that was only for kprobes!
> > > > >
> > > > > So you can never tell if a given patch will impact a given
> > > > > kthread.
> > > >
> > > > If the user (or the person creating the patch for them) doesn't
> > > > understand all impacts of the patch, they have no business patching
> > > > their kernel with it.
> > >
> > > I think what is being somewhat lost is this discussion is the
> > > distinction between:
> > >
> > >  1) is the patch safe
> > >  2) is the _live patching_ safe
> > >
> > > It's really two different things. We should absolutely strive for live
> > > patching to be safe under all circumstances, as long as the patch
> > > being fed to it is safe in itself when building a new kernel the old
> > > fashioned way.
> > >
> > > I.e. it's natural that a kernel can be messed up via a patch, but this
> > > subsystem should absolutely make sure that it will safely reject
> > > totally fine patches that are unsafe to live patch.
> >
> > Thanks, that's a very succinct way to put it.  They are indeed two
> > different things, but at the same time they're interrelated: determining
> > whether a patch is safe requires making assumptions about how it will be
> > applied.
>
> No!
>
> A patch to the kernel source is 'safe' if it results in a correctly
> patched kernel source. Full stop!
>
> Live patching does not enter into this question, ever. The correctness
> of a patch to the source does not depend on 'live patching'
> considerations in any way, shape or form.
>
> Any mechanism that tries to blur these lines is broken by design.
>
> My claim is that if a patch is correct/safe in the old fashioned way,
> then a fundamental principle is that a live patching subsystem must
> either safely apply, or safely reject the live patching attempt,
> independently from any user input.
>
> It's similar to how kprobes (or ftrace) will safely reject or perform
> a live patching of the kernel.
>
> So for example, there's this recent upstream kernel fix:
>
>  3ca9e5d36afb agp: info leak in agpioc_info_wrap()
>
> which fixes an information leak. The 'patch' is Git commit
> 3ca9e5d36afb (i.e. it patches a very specific incoming kernel source
> tree that results in a specific outgoing source tree), and we know
> it's safe and correct.
>
> Any live patching subsystem must make sure that if this patch is
> live-patched, that this attempt is either rejected safely or performed
> safely.
>
> "We think/hope it won't blow up in most cases and we automated some
> checks halfways" or "the user must know what he is doing" is really
> not something that I think is a good concept for something as fragile
> as live patching.

In that case you will have to reject any kernel patch that changes any memory 
structure, because it's impossible as a general rule to say that changing memory 
structures is going to be safe (or even possible) to change.

that includes any access to memory that moves around a lock

David Lang
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