linux-kernel - Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for kexec kernel

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Date:   Thu, 1 Jun 2023 16:13:01 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Tao Liu <ltao@...hat.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        ardb@...nel.org, linux-kernel@...r.kernel.org, dyoung@...hat.com,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for
 kexec kernel

On 06/01/23 at 03:20pm, Tao Liu wrote:
> A kexec kernel bootup hang is observed on Intel Atom cpu due to unmapped
> EFI config table.
> 
> Currently EFI system table is identity-mapped for the kexec kernel, but EFI
> config table is not mapped explicitly:
> 
>     commit 6bbeb276b71f ("x86/kexec: Add the EFI system tables and ACPI
>                           tables to the ident map")
> 
> Later in the following 2 commits, EFI config table will be accessed when
> enabling sev at kernel startup. This may result in a page fault due to EFI
> config table's unmapped address. Since the page fault occurs at an early
> stage, it is unrecoverable and kernel hangs.
> 
>     commit ec1c66af3a30 ("x86/compressed/64: Detect/setup SEV/SME features
>                           earlier during boot")
>     commit c01fce9cef84 ("x86/compressed: Add SEV-SNP feature
>                           detection/setup")
> 
> In addition, the issue doesn't appear on all systems, because the kexec
> kernel uses Page Size Extension (PSE) for identity mapping. In most cases,
> EFI config table can end up to be mapped into due to 1 GB page size.
> However if nogbpages is set, or cpu doesn't support pdpe1gb feature
> (e.g Intel Atom x6425RE cpu), EFI config table may not be mapped into
> due to 2 MB page size, thus a page fault hang is more likely to happen.
> 
> This patch will make sure the EFI config table is always mapped.
> 
> Signed-off-by: Tao Liu <ltao@...hat.com>
> ---
> Changes in v2:
> - Rephrase the change log based on Baoquan's suggestion.
> - Rename map_efi_sys_cfg_tab() to map_efi_tables().
> - Link to v1: https://lore.kernel.org/kexec/20230525094914.23420-1-ltao@redhat.com/
> ---
>  arch/x86/kernel/machine_kexec_64.c | 35 ++++++++++++++++++++++++++----
>  1 file changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 1a3e2c05a8a5..664aefa6e896 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -28,6 +28,7 @@
>  #include <asm/setup.h>
>  #include <asm/set_memory.h>
>  #include <asm/cpu.h>
> +#include <asm/efi.h>
>  
>  #ifdef CONFIG_ACPI
>  /*
> @@ -86,10 +87,12 @@ const struct kexec_file_ops * const kexec_file_loaders[] = {
>  #endif
>  
>  static int
> -map_efi_systab(struct x86_mapping_info *info, pgd_t *level4p)
> +map_efi_tables(struct x86_mapping_info *info, pgd_t *level4p)
>  {
>  #ifdef CONFIG_EFI
>  	unsigned long mstart, mend;
> +	void *kaddr;
> +	int ret;
>  
>  	if (!efi_enabled(EFI_BOOT))
>  		return 0;
> @@ -105,6 +108,30 @@ map_efi_systab(struct x86_mapping_info *info, pgd_t *level4p)
>  	if (!mstart)
>  		return 0;
>  
> +	ret = kernel_ident_mapping_init(info, level4p, mstart, mend);
> +	if (ret)
> +		return ret;
> +
> +	kaddr = memremap(mstart, mend - mstart, MEMREMAP_WB);
> +	if (!kaddr) {
> +		pr_err("Could not map UEFI system table\n");
> +		return -ENOMEM;
> +	}
> +
> +	mstart = efi_config_table;
> +
> +	if (efi_enabled(EFI_64BIT)) {
> +		efi_system_table_64_t *stbl = (efi_system_table_64_t *)kaddr;
> +
> +		mend = mstart + sizeof(efi_config_table_64_t) * stbl->nr_tables;
> +	} else {
> +		efi_system_table_32_t *stbl = (efi_system_table_32_t *)kaddr;
> +
> +		mend = mstart + sizeof(efi_config_table_32_t) * stbl->nr_tables;
> +	}
> +
> +	memunmap(kaddr);
> +
>  	return kernel_ident_mapping_init(info, level4p, mstart, mend);
>  #endif
>  	return 0;
> @@ -244,10 +271,10 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
>  	}
>  
>  	/*
> -	 * Prepare EFI systab and ACPI tables for kexec kernel since they are
> -	 * not covered by pfn_mapped.
> +	 * Prepare EFI systab, config table and ACPI tables for kexec kernel

The code comment need be updated too?

         * Prepare EFI tables and ACPI tables for kexec kernel since they are
         * not covered by pfn_mapped.

Other than this nit, this patch looks good to me, thanks.

Acked-by: Baoquan He <bhe@...hat.com>


> +	 * since they are not covered by pfn_mapped.
>  	 */
> -	result = map_efi_systab(&info, level4p);
> +	result = map_efi_tables(&info, level4p);
>  	if (result)
>  		return result;
>  
> -- 
> 2.33.1
>