Re: [PATCH 1/3 v4] x86/kdump: always reserve the low 1MiB when the crashkernel option is specified

From: lijiang
Date: Thu Oct 24 2019 - 07:25:13 EST

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å 2019å10æ24æ 16:13, d.hatayama@xxxxxxxxxxx åé:
> I don't find the corresponding patch in the v5 patchset, so I comment here.
>
Thanks for your comment.

>> -----Original Message-----
>> From: linux-kernel-owner@xxxxxxxxxxxxxxx
>> [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of lijiang
>> Sent: Wednesday, October 23, 2019 2:35 PM
>> To: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx;
>> hpa@xxxxxxxxx; x86@xxxxxxxxxx; bhe@xxxxxxxxxx; dyoung@xxxxxxxxxx;
>> jgross@xxxxxxxx; dhowells@xxxxxxxxxx; Thomas.Lendacky@xxxxxxx;
>> ebiederm@xxxxxxxxxxxx; vgoyal@xxxxxxxxxx; kexec@xxxxxxxxxxxxxxxxxxx
>> Subject: Re: [PATCH 1/3 v4] x86/kdump: always reserve the low 1MiB when the
>> crashkernel option is specified
>>
>> å 2019å10æ22æ 16:30, Borislav Petkov åé:
>>> This ifdeffery needs to be a function in kernel/kexec_core.c which is
>>> called by reserve_real_mode(), instead.
>>
>> Would you mind if i improve this patch as follow? Thanks.
>>
>> From 5804abec62279585f374d78ace1250505c44c6b7 Mon Sep 17 00:00:00 2001
>> From: Lianbo Jiang <lijiang@xxxxxxxxxx>
>> Date: Wed, 23 Oct 2019 11:27:04 +0800
>> Subject: [PATCH] x86/kdump: always reserve the low 1MiB when the crashkernel
>> option is specified
>>
>> Kdump kernel will reuse the first 640k region because the real mode
>> trampoline has to work in this area. When the vmcore is dumped, the
>> old memory in this area may be accessed, therefore, kernel has to
>> copy the contents of the first 640k area to a backup region so that
>> kdump kernel can read the old memory from the backup area of the
>> first 640k area, which is done in the purgatory().
>>
>> But, the current handling of copying the first 640k area runs into
>> problems when SME is enabled, kernel does not properly copy these
>> old memory to the backup area in the purgatory(), thereby, kdump
>> kernel reads out the encrypted contents, because the kdump kernel
>> must access the first kernel's memory with the encryption bit set
>> when SME is enabled in the first kernel. Please refer to this link:
>>
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=204793
>>
>> Finally, it causes the following errors, and the crash tool gets
>> invalid pointers when parsing the vmcore.
>>
>> crash> kmem -s|grep -i invalid
>> kmem: dma-kmalloc-512: slab:ffffd77680001c00 invalid
>> freepointer:a6086ac099f0c5a4
>> kmem: dma-kmalloc-512: slab:ffffd77680001c00 invalid
>> freepointer:a6086ac099f0c5a4
>> crash>
>>
>> To avoid the above errors, when the crashkernel option is specified,
>> lets reserve the remaining low 1MiB memory(after reserving real mode
>> memory) so that the allocated memory does not fall into the low 1MiB
>> area, which makes us not to copy the first 640k content to a backup
>> region in purgatory(). This indicates that it does not need to be
>> included in crash dumps or used for anything except the processor
>> trampolines that must live in the low 1MiB.
>>
>> Signed-off-by: Lianbo Jiang <lijiang@xxxxxxxxxx>
>> ---
>> BTW:I also tried to fix the above problem in purgatory(), but there
>> are too many restricts in purgatory() context, for example: i can't
>> allocate new memory to create the identity mapping page table for
>> SME situation.
>>
>> Currently, there are two places where the first 640k area is needed,
>> the first one is in the find_trampoline_placement(), another one is
>> in the reserve_real_mode(), and their content doesn't matter.
>>
>> In addition, also need to clean all the code related to the backup
>> region later.
>>
>> arch/x86/realmode/init.c | 2 ++
>> include/linux/kexec.h | 2 ++
>> kernel/kexec_core.c | 13 +++++++++++++
>> 3 files changed, 17 insertions(+)
>>
>> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
>> index 7dce39c8c034..064cc79a015d 100644
>> --- a/arch/x86/realmode/init.c
>> +++ b/arch/x86/realmode/init.c
>> @@ -3,6 +3,7 @@
>> #include <linux/slab.h>
>> #include <linux/memblock.h>
>> #include <linux/mem_encrypt.h>
>> +#include <linux/kexec.h>
>>
>> #include <asm/set_memory.h>
>> #include <asm/pgtable.h>
>> @@ -34,6 +35,7 @@ void __init reserve_real_mode(void)
>>
>> memblock_reserve(mem, size);
>> set_real_mode_mem(mem);
>> + kexec_reserve_low_1MiB();
>> }
>>
>> static void __init setup_real_mode(void)
>> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
>> index 1776eb2e43a4..30acf1d738bc 100644
>> --- a/include/linux/kexec.h
>> +++ b/include/linux/kexec.h
>> @@ -306,6 +306,7 @@ extern void __crash_kexec(struct pt_regs *);
>> extern void crash_kexec(struct pt_regs *);
>> int kexec_should_crash(struct task_struct *);
>> int kexec_crash_loaded(void);
>> +void kexec_reserve_low_1MiB(void);
>> void crash_save_cpu(struct pt_regs *regs, int cpu);
>> extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
>>
>> @@ -397,6 +398,7 @@ static inline void __crash_kexec(struct pt_regs *regs) { }
>> static inline void crash_kexec(struct pt_regs *regs) { }
>> static inline int kexec_should_crash(struct task_struct *p) { return 0; }
>> static inline int kexec_crash_loaded(void) { return 0; }
>> +static inline void kexec_reserve_low_1MiB(void) { }
>> #define kexec_in_progress false
>> #endif /* CONFIG_KEXEC_CORE */
>>
>> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
>> index 15d70a90b50d..5bd89f1fee42 100644
>> --- a/kernel/kexec_core.c
>> +++ b/kernel/kexec_core.c
>> @@ -37,6 +37,7 @@
>> #include <linux/compiler.h>
>> #include <linux/hugetlb.h>
>> #include <linux/frame.h>
>> +#include <linux/memblock.h>
>>
>> #include <asm/page.h>
>> #include <asm/sections.h>
>> @@ -70,6 +71,18 @@ struct resource crashk_low_res = {
>> .desc = IORES_DESC_CRASH_KERNEL
>> };
>>
>> +/*
>> + * When the crashkernel option is specified, only use the low
>> + * 1MiB for the real mode trampoline.
>> + */
>> +void kexec_reserve_low_1MiB(void)
>> +{
>> + if (strstr(boot_command_line, "crashkernel=")) {
>
> strstr() matches for example, ANYEXTRACHARACTERScrashkernel=ANYEXTRACHARACTERS.
>
> Is it enough to use cmdline_find_option_bool()?
>
The cmdline_find_option_bool() will find a boolean option, but the crashkernel option
is not a boolean option, maybe it looks odd. So, should we use the cmdline_find_option()
better?

+#include <asm/cmdline.h>

void __init kexec_reserve_low_1MiB(void)
{
- if (strstr(boot_command_line, "crashkernel=")) {
+ char buffer[4];
+
+ if (cmdline_find_option(boot_command_line, "crashkernel=",
+ buffer, sizeof(buffer))) {
memblock_reserve(0, 1<<20);
pr_info("Reserving the low 1MiB of memory for crashkernel\n");
}

And here, no need to parse the arguments of crashkernel(sometimes, which has a
complicated syntax), so the size of buffer should be enough. What's your opinion?

Thanks
Lianbo

>> + memblock_reserve(0, 1<<20);
>> + pr_info("Reserving the low 1MiB of memory for
>> crashkernel\n");
>> + }
>> +}
>> +
>> int kexec_should_crash(struct task_struct *p)
>> {
>> /*
>> --
>> 2.17.1
>

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In reply to: Borislav Petkov: "Re: [PATCH 1/3 v4] x86/kdump: always reserve the low 1MiB when the crashkernel option is specified"
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