Re: Fix issue with alternatives/paravirt patches

[Jessica Yu]

+++ Christopher Arges [08/07/16 00:22 -0500]:
> On Tue, Jul 05, 2016 at 10:34:58PM -0400, Jessica Yu wrote:
> > Hi,
> >
> > A few months ago, Chris Arges reported a bug involving alternatives/paravirt
> > patching that was discussed here [1] and here [2]. To briefly summarize the
> > bug, patch modules that contained .altinstructions or .parainstructions
> > sections would break because these alternative/paravirt patches would be
> > applied first by the module loader (see x86 module_finalize()), then
> > livepatch would later clobber these patches when applying per-object
> > relocations. This lead to crashes and unpredictable behavior.
> >
> > One conclusion we reached from our last discussion was that we will
> > need to introduce some arch-specific code to address this problem.
> > This patchset presents a possible fix for the bug by adding a new
> > arch-specific arch_klp_init_object_loaded() function that by default
> > does nothing but can be overridden by different arches.
> >
> > To fix this issue for x86, since we can access a patch module's Elf
> > sections through mod->klp_info, we can simply delay the calls to
> > apply_paravirt() and apply_alternatives() to arch_klp_init_object_loaded(),
> > which is called after relocations have been written for an object.
> > In addition, for patch modules, .parainstructions and .altinstructions are
> > prefixed by ".klp.arch.${objname}" so that the module loader ignores them
> > and livepatch can apply them manually.
> >
> > Currently for kpatch, we don't support including jump table sections in
> > the patch module, and supporting .smp_locks is currently broken, so we
> > don't consider those sections (for now).
> >
> > I did some light testing with some patches to kvm and verified that the
> > original issue reported in [2] was fixed.
> >
> > Based on linux-next.
>
> Jessica,
>
> I was able to test these patches on top of linux-next. I took your kpatch
> branch and hacked it a bit to get it working and was able to
> apply a patch to 'kvm_arch_vm_ioctl' while running a VM workload.
>
> Great job!
>
> Tested-by: Chris J Arges <chris.j.arges@xxxxxxxxxxxxx>

Excellent, thanks for testing it out Chris!

Jessica