[PATCH v30 05/11] arm64: kdump: protect crash dump kernel memory

James,

On Sat, Jan 28, 2017 at 02:15:16AM +0900, AKASHI Takahiro wrote:

James,

On Fri, Jan 27, 2017 at 11:19:32AM +0000, James Morse wrote:

quoted

Hi Akashi,

On 26/01/17 11:28, AKASHI Takahiro wrote:

quoted

On Wed, Jan 25, 2017 at 05:37:38PM +0000, James Morse wrote:

quoted

On 24/01/17 08:49, AKASHI Takahiro wrote:

quoted

To protect the memory reserved for crash dump kernel once after loaded,
arch_kexec_protect_crashres/unprotect_crashres() are meant to deal with
permissions of the corresponding kernel mappings.

We also have to
- put the region in an isolated mapping, and
- move copying kexec's control_code_page to machine_kexec_prepare()
so that the region will be completely read-only after loading.

quoted

Note that the region must reside in linear mapping and have corresponding
page structures in order to be potentially freed by shrinking it through
/sys/kernel/kexec_crash_size.

Nasty! Presumably you have to build the crash region out of individual page
mappings,

This might be an alternative, but

quoted

so that they can be returned to the slab-allocator one page at a time,
and still be able to set/clear the valid bits on the remaining chunk.
(I don't see how that happens in this patch)

As far as shrinking feature is concerned, I believe, crash_shrink_memory(),
which eventually calls free_reserved_page(), will take care of all the things
to do. I can see increased number of "MemFree" in /proc/meminfo.

Except for arch specific stuff like reformatting the page tables. Maybe I've
overlooked the way out this. What happens with this scenario:

We boot with crashkernel=1G on the commandline.
Memblock_reserve allocates a naturally aligned 1GB block of memory for the crash
region.
Your code in __map_memblock() calls __create_pgd_mapping() ->
alloc_init_pud() which decides use_1G_block() looks like a good idea.

Some time later, the user decides to free half of this region,
free_reserved_page() does its thing and half of those struct page's now belong
to the memory allocator.

Now we load a kdump kernel, which causes arch_kexec_protect_crashkres() to be
called for the 512MB region that was left.

create_mapping_late() needs to split the 1GB mapping it originally made into a
smaller table, with the first half using PAGE_KERNEL_INVALID, and the second
half using PAGE_KERNEL. It can't do break-before-make because these pages may be
in-use by another CPU because we gave them back to the memory allocator. (in the
worst-possible world, that second half contains our stack!)

Yeah, this is a horrible case.
Now I understand why we should stick with page_mapping_only option.

quoted

Making this behave more like debug_pagealloc where the region is only built of
page-size mappings should avoid this. The smallest change to what you have is to
always pass page_mappings_only for the kdump region.

Ideally we just disable this resize feature for ARM64 and support it with some
later kernel version, but I can't see a way of doing this without adding Kconfig
symbols to other architectures.

quoted

(Please note that the region is memblock_reserve()'d at boot time.)

And free_reserved_page() does nothing to update memblock, so
memblock_is_reserved() says these pages are reserved, but in reality they
are in use by the memory allocator. This doesn't feel right.

Just FYI, no other architectures take care of this issue.

(and I don't know whether the memblock is reserved or not may have
any impact after booting.)

quoted

(Fortunately we can override crash_free_reserved_phys_range() so this can
 probably be fixed)

quoted

quoted

This secretly-unmapped is the sort of thing that breaks hibernate, it blindly
assumes pfn_valid() means it can access the page if it wants to. Setting
PG_Reserved is a quick way to trick it out of doing this, but that would leave
the crash kernel region un-initialised after resume, while kexec_crash_image
still has a value.

Ouch, I didn't notice this issue.

quoted

I think the best fix for this is to forbid hibernate if kexec_crash_loaded()
arguing these are mutually-exclusive features, and the protect crash-dump
feature exists to prevent things like hibernate corrupting the crash region.

This restriction is really painful.
Is there any hibernation hook that will be invoked before suspending and
after resuming? If so, arch_kexec_unprotect_crashkres()/protect_crashkres()
will be able to be called.

Those calls could go in swsusp_arch_suspend() in /arch/arm64/kernel/hibernate.c,

I will give it a try next week.

I took the following test scenario:
 - load the crash dump kernel
 - echo shutdown > /sys/power/disk
 - echo disk > /sys/power/state
 - power off and on the board
 - reboot with resume=...
 - after hibernate done, echo c > /proc/sysrq-trigger
 - after reboot, check /proc/vmcore

and everything looks to work fine.

If you think I'd better try more test cases, please let me know.

Thanks,
-Takahiro AKASHI
===8<===

diff --git a/arch/arm64/kernel/hibernate.c b/arch/arm64/kernel/hibernate.c
index fe301cbcb442..111a849333ee 100644
--- a/arch/arm64/kernel/hibernate.c
+++ b/arch/arm64/kernel/hibernate.c

@@ -16,6 +16,7 @@
  */
 #define pr_fmt(x) "hibernate: " x
 #include <linux/cpu.h>
+#include <linux/kexec.h>
 #include <linux/kvm_host.h>
 #include <linux/mm.h>
 #include <linux/pm.h>

@@ -289,6 +290,12 @@ int swsusp_arch_suspend(void)
 	local_dbg_save(flags);
 
 	if (__cpu_suspend_enter(&state)) {
+#ifdef CONFIG_KEXEC_CORE
+		/* make the crash dump kernel region mapped */
+		if (kexec_crash_image)
+			arch_kexec_unprotect_crashkres();
+#endif
+
 		sleep_cpu = smp_processor_id();
 		ret = swsusp_save();
 	} else {

@@ -300,6 +307,12 @@ int swsusp_arch_suspend(void)
 		if (el2_reset_needed())
 			dcache_clean_range(__hyp_idmap_text_start, __hyp_idmap_text_end);
 
+#ifdef CONFIG_KEXEC_CORE
+		/* make the crash dump kernel region unmapped */
+		if (kexec_crash_image)
+			arch_kexec_protect_crashkres();
+#endif
+
 		/*
 		 * Tell the hibernation core that we've just restored
 		 * the memory