Re: [PATCH 1/2 v7] x86/kdump: always reserve the low 1MiB when the crashkernel option is specified

[lijiang]

å 2019å10æ29æ 13:28, Baoquan He åé:
> On 10/29/19 at 10:10am, Lianbo Jiang wrote:
>> 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>
>> ---
>>  arch/x86/include/asm/crash.h |  6 ++++++
>>  arch/x86/kernel/crash.c      | 15 +++++++++++++++
>>  arch/x86/realmode/init.c     |  2 ++
>>  3 files changed, 23 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/crash.h b/arch/x86/include/asm/crash.h
>> index 0acf5ee45a21..3e966a3dc823 100644
>> --- a/arch/x86/include/asm/crash.h
>> +++ b/arch/x86/include/asm/crash.h
>> @@ -8,4 +8,10 @@ int crash_setup_memmap_entries(struct kimage *image,
>>  		struct boot_params *params);
>>  void crash_smp_send_stop(void);
>>  
>> +#ifdef CONFIG_KEXEC_CORE
>> +void __init kexec_reserve_low_1MiB(void);
>> +#else
>> +static inline void __init kexec_reserve_low_1MiB(void) { }
>> +#endif
>> +
>>  #endif /* _ASM_X86_CRASH_H */
>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>> index eb651fbde92a..144f519aef29 100644
>> --- a/arch/x86/kernel/crash.c
>> +++ b/arch/x86/kernel/crash.c
>> @@ -24,6 +24,7 @@
>>  #include <linux/export.h>
>>  #include <linux/slab.h>
>>  #include <linux/vmalloc.h>
>> +#include <linux/memblock.h>
>>  
>>  #include <asm/processor.h>
>>  #include <asm/hardirq.h>
>> @@ -39,6 +40,7 @@
>>  #include <asm/virtext.h>
>>  #include <asm/intel_pt.h>
>>  #include <asm/crash.h>
>> +#include <asm/cmdline.h>
>>  
>>  /* Used while preparing memory map entries for second kernel */
>>  struct crash_memmap_data {
>> @@ -68,6 +70,19 @@ static inline void cpu_crash_vmclear_loaded_vmcss(void)
>>  	rcu_read_unlock();
>>  }
>>  
>> +/*
>> + * When the crashkernel option is specified, only use the low
>> + * 1MiB for the real mode trampoline.
>> + */
>> +void __init kexec_reserve_low_1MiB(void)
> 
> Thanks for the effort, Lianbo. I believe everyone is confident with this
> solution and fix.
> 
> I have a tiny concern, why the function name is
> kexec_reserve_low_1MiB(), but not kexec_reserve_low_1M()?

Thanks for your comment, Baoquan.

It means that kernel will reserve 1M 'Byte' memory, the function name does not
have special meaning.

Would you mind if i change it to the crash_reserve_low_1M()?

void __init crash_reserve_low_1M(void)

Thanks.
Lianbo

> I searched in kernel code with below filter, didn't see MiB appearing in
> a function name. I am not sure about it either, just ask.
> 
> git grep "_[1-9]*M " arch/ kernel/ mm include/ drivers/ net/ init fs crypto/ certs/ ipc lib
> 
> Thanks
> Baoquan
> 
>> +{
>> +	if (cmdline_find_option(boot_command_line, "crashkernel",
>> +				NULL, 0) > 0) {
>> +		memblock_reserve(0, 1<<20);
>> +		pr_info("Reserving the low 1MiB of memory for crashkernel\n");
>> +	}
>> +}
>> +
>>  #if defined(CONFIG_SMP) && defined(CONFIG_X86_LOCAL_APIC)
>>  
>>  static void kdump_nmi_callback(int cpu, struct pt_regs *regs)
>> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
>> index 7dce39c8c034..b8bbd0017ca8 100644
>> --- a/arch/x86/realmode/init.c
>> +++ b/arch/x86/realmode/init.c
>> @@ -8,6 +8,7 @@
>>  #include <asm/pgtable.h>
>>  #include <asm/realmode.h>
>>  #include <asm/tlbflush.h>
>> +#include <asm/crash.h>
>>  
>>  struct real_mode_header *real_mode_header;
>>  u32 *trampoline_cr4_features;
>> @@ -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)
>> -- 
>> 2.17.1
>>