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Subject: Re: [PATCH 1/2 v5] x86/kdump: always reserve the low 1MiB when the
 crashkernel option is specified
To:     Simon Horman <horms@verge.net.au>
Cc:     linux-kernel@vger.kernel.org, jgross@suse.com,
        Thomas.Lendacky@amd.com, bhe@redhat.com, x86@kernel.org,
        kexec@lists.infradead.org, dhowells@redhat.com, mingo@redhat.com,
        bp@alien8.de, ebiederm@xmission.com, hpa@zytor.com,
        tglx@linutronix.de, dyoung@redhat.com, vgoyal@redhat.com,
        d.hatayama@fujitsu.com,
        "d.hatayama@fujitsu.com" <d.hatayama@fujitsu.com>
References: <20191023141912.29110-1-lijiang@redhat.com>
 <20191023141912.29110-2-lijiang@redhat.com>
 <20191024100719.GC11441@verge.net.au>
From:   lijiang <lijiang@redhat.com>
Message-ID: <4c1c4b78-23f0-a2b9-4be7-5bab0335f10a@redhat.com>
Date:   Thu, 24 Oct 2019 19:33:28 +0800
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=E5=9C=A8 2019=E5=B9=B410=E6=9C=8824=E6=97=A5 18:07, Simon Horman =E5=86=99=
=E9=81=93:
> Hi Linbo,
>=20
> thanks for your patch.
>=20
> On Wed, Oct 23, 2019 at 10:19:11PM +0800, 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=3D204793
>>
>> 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:a6086ac=
099f0c5a4
>> kmem: dma-kmalloc-512: slab:ffffd77680001c00 invalid freepointer:a6086ac=
099f0c5a4
>> 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@redhat.com>
>> ---
>> 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>
>> =20
>>  #include <asm/set_memory.h>
>>  #include <asm/pgtable.h>
>> @@ -34,6 +35,7 @@ void __init reserve_real_mode(void)
>> =20
>>  =09memblock_reserve(mem, size);
>>  =09set_real_mode_mem(mem);
>> +=09kexec_reserve_low_1MiB();
>>  }
>> =20
>>  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 __init kexec_reserve_low_1MiB(void);
>>  void crash_save_cpu(struct pt_regs *regs, int cpu);
>>  extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
>> =20
>> @@ -397,6 +398,7 @@ static inline void __crash_kexec(struct pt_regs *reg=
s) { }
>>  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 __init kexec_reserve_low_1MiB(void) { }
>>  #define kexec_in_progress false
>>  #endif /* CONFIG_KEXEC_CORE */
>> =20
>> 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>
>> =20
>>  #include <asm/page.h>
>>  #include <asm/sections.h>
>> @@ -70,6 +71,18 @@ struct resource crashk_low_res =3D {
>>  =09.desc  =3D IORES_DESC_CRASH_KERNEL
>>  };
>> =20
>> +/*
>> + * When the crashkernel option is specified, only use the low
>> + * 1MiB for the real mode trampoline.
>> + */
>> +void __init kexec_reserve_low_1MiB(void)
>> +{
>> +=09if (strstr(boot_command_line, "crashkernel=3D")) {
>=20
> Could you comment on the issue of using strstr which
> was raised by Hatayama-san in response to an earlier revision
> of this patch?
>=20

Thank you, Simon and Hatayama-san. Lets talk about it here.

> strstr() matches for example, ANYEXTRACHARACTERScrashkernel=3DANYEXTRACHA=
RACTERS.
>=20
> Is it enough to use cmdline_find_option_bool()?
>=20

The cmdline_find_option_bool() will find a boolean option, but the crashker=
nel 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=3D")) {
+       char buffer[4];
+
+       if (cmdline_find_option(boot_command_line, "crashkernel=3D",
+                               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 ha=
s a
complicated syntax), so the size of buffer should be enough. What's your op=
inion?

Thanks
Lianbo

> Thanks in advance!
>=20
>> +=09=09memblock_reserve(0, 1<<20);
>> +=09=09pr_info("Reserving the low 1MiB of memory for crashkernel\n");
>> +=09}
>> +}
>> +
>>  int kexec_should_crash(struct task_struct *p)
>>  {
>>  =09/*
>> --=20
>> 2.17.1
>>
>>
>> _______________________________________________
>> kexec mailing list
>> kexec@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/kexec
>>