[PATCH v31 05/12] arm64: kdump: protect crash dump kernel memory

AKASHI Takahiro
2017-02-02T10:31:30Z

On Wed, Feb 01, 2017 at 06:00:08PM +0000, Mark Rutland wrote:
> On Wed, Feb 01, 2017 at 09:46:24PM +0900, AKASHI Takahiro wrote:
> > arch_kexec_protect_crashkres() and arch_kexec_unprotect_crashkres()
> > are meant to be called around kexec_load() in order to protect
> > the memory allocated for crash dump kernel once after it's loaded.
> >
> > The protection is implemented here by unmapping the region rather than
> > making it read-only.
> > To make the things work correctly, we also have to
> > - put the region in an isolated, page-level mapping initially, and
> > - move copying kexec's control_code_page to machine_kexec_prepare()
> >
> > Note that page-level mapping is also required to allow for shrinking
> > the size of memory, through /sys/kernel/kexec_crash_size, by any number
> > of multiple pages.
>
> Looking at kexec_crash_size_store(), I don't see where memory returned
> to the OS is mapped. AFAICT, if the region is protected when the user
> shrinks the region, the memory will not be mapped, yet handed over to
> the kernel for general allocation.

The region is protected only when the crash dump kernel is loaded,
and after that, we are no longer able to shrink the region.

> Surely we need an arch-specific callback to handle that? e.g.
>
> arch_crash_release_region(unsigned long base, unsigned long size)
> {
> /*
> * Ensure the region is part of the linear map before we return
> * it to the OS. We won't unmap this again, so we can use block
> * mappings.
> */
> create_pgd_mapping(&init_mm, start, __phys_to_virt(start),
> size, PAGE_KERNEL, false);
> }
>
> ... which we'd call from crash_shrink_memory() before we freed the
> reserved pages.

All the memory is mapped by my map_crashkernel() at boot time.

> [...]
>
> > +void arch_kexec_unprotect_crashkres(void)
> > +{
> > + /*
> > + * We don't have to make page-level mappings here because
> > + * the crash dump kernel memory is not allowed to be shrunk
> > + * once the kernel is loaded.
> > + */
> > + create_pgd_mapping(&init_mm, crashk_res.start,
> > + __phys_to_virt(crashk_res.start),
> > + resource_size(&crashk_res), PAGE_KERNEL,
> > + debug_pagealloc_enabled());
> > +
> > + flush_tlb_all();
> > +}
>
> We can lose the flush_tlb_all() here; TLBs aren't allowed to cache an
> invalid entry, so there's nothing to remove from the TLBs.

Ah, yes!

> [...]
>
> > @@ -538,6 +540,24 @@ static void __init map_mem(pgd_t *pgd)
> > if (memblock_is_nomap(reg))
> > continue;
> >
> > +#ifdef CONFIG_KEXEC_CORE
> > + /*
> > + * While crash dump kernel memory is contained in a single
> > + * memblock for now, it should appear in an isolated mapping
> > + * so that we can independently unmap the region later.
> > + */
> > + if (crashk_res.end &&
> > + (start <= crashk_res.start) &&
> > + ((crashk_res.end + 1) < end)) {
> > + if (crashk_res.start != start)
> > + __map_memblock(pgd, start, crashk_res.start);
> > +
> > + if ((crashk_res.end + 1) < end)
> > + __map_memblock(pgd, crashk_res.end + 1, end);
> > +
> > + continue;
> > + }
> > +#endif
>
> This wasn't quite what I had in mind. I had expected that here we would
> isolate the ranges we wanted to avoid mapping (with a comment as to why
> we couldn't move the memblock_isolate_range() calls earlier). In
> map_memblock(), we'd skip those ranges entirely.
>
> I believe the above isn't correct if we have a single memblock.memory
> region covering both the crashkernel and kernel regions. In that case,
> we'd erroneously map the portion which overlaps the kernel.
>
> It seems there are a number of subtle problems here. :/

I didn't see any problems, but I will go back with memblock_isolate_range()
here in map_mem().

Thanks,
-Takahiro AKASHI

> Thanks,
> Mark.