Re: [PATCH v3] x86/efi: Free EFI memory map only when installing a new one.

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Ard,

On 6/12/2024 8:56 AM, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> The logic in __efi_memmap_init() is shared between two different
> execution flows:
> - mapping the EFI memory map early or late into the kernel VA space, so
>   that its entries can be accessed;
> - cloning the EFI memory map in order to insert new entries that are
>   created as a result of creating a memory reservation
>   (efi_arch_mem_reserve())
>
> In the former case, the underlying memory containing the kernel's view
> of the EFI memory map (which may be heavily modified by the kernel
> itself on x86) is not modified at all, and the only thing that changes
> is the virtual mapping of this memory, which is different between early
> and late boot.
>
> In the latter case, an entirely new allocation is created that carries a
> new, updated version of the kernel's view of the EFI memory map. When
> installing this new version, the old version will no longer be
> referenced, and if the memory was allocated by the kernel, it will leak
> unless it gets freed.
>
> The logic that implements this freeing currently lives on the code path
> that is shared between these two use cases, but it should only apply to
> the latter. So move it to the correct spot.
>
> While at it, move __efi_memmap_free() into its only caller, and drop the
> dummy definition for non-x86 architectures, as that is no longer needed.
>
> Cc: Ashish Kalra <Ashish.Kalra@amd.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Fixes: f0ef6523475f ("efi: Fix efi_memmap_alloc() leaks")
> Link: https://lore.kernel.org/all/36ad5079-4326-45ed-85f6-928ff76483d3@amd.com
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> v3:
> - don't move __efi_memmap_free() into what turned out not to be its only
>   caller
> - drop another CPP #define related to the dummy definition
>
> v2:
> - free old memory map only after installing the new one succeeded
> - move __efi_memmap_free() into its only caller
> - drop obsolete dummy declaration from generic code
>
>  arch/x86/include/asm/efi.h     |  1 -
>  arch/x86/platform/efi/memmap.c | 12 +++++++++++-
>  drivers/firmware/efi/memmap.c  |  9 ---------
>  3 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
> index 1dc600fa3ba5..481096177500 100644
> --- a/arch/x86/include/asm/efi.h
> +++ b/arch/x86/include/asm/efi.h
> @@ -401,7 +401,6 @@ extern int __init efi_memmap_alloc(unsigned int num_entries,
>  				   struct efi_memory_map_data *data);
>  extern void __efi_memmap_free(u64 phys, unsigned long size,
>  			      unsigned long flags);
> -#define __efi_memmap_free __efi_memmap_free
>  
>  extern int __init efi_memmap_install(struct efi_memory_map_data *data);
>  extern int __init efi_memmap_split_count(efi_memory_desc_t *md,
> diff --git a/arch/x86/platform/efi/memmap.c b/arch/x86/platform/efi/memmap.c
> index 4ef20b49eb5e..6ed1935504b9 100644
> --- a/arch/x86/platform/efi/memmap.c
> +++ b/arch/x86/platform/efi/memmap.c
> @@ -92,12 +92,22 @@ int __init efi_memmap_alloc(unsigned int num_entries,
>   */
>  int __init efi_memmap_install(struct efi_memory_map_data *data)
>  {
> +	unsigned long size = efi.memmap.desc_size * efi.memmap.nr_map;
> +	unsigned long flags = efi.memmap.flags;
> +	u64 phys = efi.memmap.phys_map;
> +	int ret;
> +
>  	efi_memmap_unmap();
>  
>  	if (efi_enabled(EFI_PARAVIRT))
>  		return 0;
>  
> -	return __efi_memmap_init(data);
> +	ret = __efi_memmap_init(data);
> +	if (ret)
> +		return ret;
> +
> +	__efi_memmap_free(phys, size, flags);
> +	return 0;
>  }
>  
>  /**
> diff --git a/drivers/firmware/efi/memmap.c b/drivers/firmware/efi/memmap.c
> index 3365944f7965..34109fd86c55 100644
> --- a/drivers/firmware/efi/memmap.c
> +++ b/drivers/firmware/efi/memmap.c
> @@ -15,10 +15,6 @@
>  #include <asm/early_ioremap.h>
>  #include <asm/efi.h>
>  
> -#ifndef __efi_memmap_free
> -#define __efi_memmap_free(phys, size, flags) do { } while (0)
> -#endif
> -
>  /**
>   * __efi_memmap_init - Common code for mapping the EFI memory map
>   * @data: EFI memory map data
> @@ -51,11 +47,6 @@ int __init __efi_memmap_init(struct efi_memory_map_data *data)
>  		return -ENOMEM;
>  	}
>  
> -	if (efi.memmap.flags & (EFI_MEMMAP_MEMBLOCK | EFI_MEMMAP_SLAB))
> -		__efi_memmap_free(efi.memmap.phys_map,
> -				  efi.memmap.desc_size * efi.memmap.nr_map,
> -				  efi.memmap.flags);
> -
>  	map.phys_map = data->phys_map;
>  	map.nr_map = data->size / data->desc_size;
>  	map.map_end = map.map + data->size;

Tested this patch with SNP guest kexec and i do not observe EFI memory map corruption with SNP guest kexec with this patch applied.

I have another question related to the discussion on the following email thread:

Re: [PATCH v7 1/3] efi/x86: Fix EFI memory map corruption with kexec - Dave Young (kernel.org) <https://lore.kernel.org/all/CALu+AoQ2jNjb+5MZfFtpT_Y=2gJRWwqEeWTpQkwNt0-U5fpO_w@mail.gmail.com/><https://lore.kernel.org/all/CALu+AoQ2jNjb+5MZfFtpT_Y=2gJRWwqEeWTpQkwNt0-U5fpO_w@mail.gmail.com/>

Do we still want to create another patch for skipping efi_arch_mem_reserve() when the EFI_MEMORY_RUNTIME bit was set already ?

Tested-by: Ashish Kalra <Ashish.Kalra@amd.com>