Message-ID: <db7edff7-8a87-41db-9e40-202a498c5e29@gmail.com>
Date: Mon, 13 Jan 2025 11:27:36 +0000
From: Usama Arif <usamaarif642@...il.com>
To: Dave Young <dyoung@...hat.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
 devel@...2.groups.io, kexec@...ts.infradead.org, hannes@...xchg.org,
 x86@...nel.org, linux-kernel@...r.kernel.org, leitao@...ian.org,
 gourry@...rry.net, kernel-team@...a.com
Subject: Re: [RFC 1/2] efi/memattr: Use desc_size instead of total size to
 check for corruption



On 13/01/2025 02:33, Dave Young wrote:
> On Fri, 10 Jan 2025 at 18:54, Usama Arif <usamaarif642@...il.com> wrote:
>>
>>
>>
>> On 10/01/2025 07:21, Ard Biesheuvel wrote:
>>> On Thu, 9 Jan 2025 at 17:36, Usama Arif <usamaarif642@...il.com> wrote:
>>>>
>>>>
>>>>
>>>> On 09/01/2025 15:45, Ard Biesheuvel wrote:
>>>>> On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@...il.com> wrote:
>>>>>>
>>>>>> The commit in [1] introduced a check to see if EFI memory attributes
>>>>>> table was corrupted. It assumed that efi.memmap.nr_map remains
>>>>>> constant, but it changes during late boot.
>>>>>> Hence, the check is valid during cold boot, but not in the subsequent
>>>>>> kexec boot.
>>>>>>
>>>>>> This is best explained with an exampled. At cold boot, for a test
>>>>>> machine:
>>>>>> efi.memmap.nr_map=91,
>>>>>> memory_attributes_table->num_entries=48,
>>>>>> desc_size = 48
>>>>>> Hence, the check introduced in [1] where 3x the size of the
>>>>>> entire EFI memory map is a reasonable upper bound for the size of this
>>>>>> table is valid.
>>>>>>
>>>>>> In late boot __efi_enter_virtual_mode calls 2 functions that updates
>>>>>> efi.memmap.nr_map:
>>>>>> - efi_map_regions which reduces the `count` of map entries
>>>>>>   (for e.g. if should_map_region returns false) and which is reflected
>>>>>>   in efi.memmap by __efi_memmap_init.
>>>>>>   At this point efi.memmap.nr_map becomes 46 in the test machine.
>>>>>> - efi_free_boot_services which also reduces the number of memory regions
>>>>>>   available (for e.g. if md->type or md->attribute is not the right value).
>>>>>>   At this point efi.memmap.nr_map becomes 9 in the test machine.
>>>>>> Hence when you kexec into a new kernel and pass efi.memmap, the
>>>>>> paramaters that are compared are:
>>>>>> efi.memmap.nr_map=9,
>>>>>> memory_attributes_table->num_entries=48,
>>>>>> desc_size = 48
>>>>>> where the check in [1] is no longer valid with such a low efi.memmap.nr_map
>>>>>> as it was reduced due to efi_map_regions and efi_free_boot_services.
>>>>>>
>>>>>> A more appropriate check is to see if the description size reported by
>>>>>> efi and memory attributes table is the same.
>>>>>>
>>>>>> [1] https://lore.kernel.org/all/20241031175822.2952471-2-ardb+git@google.com/
>>>>>>
>>>>>> Fixes: 8fbe4c49c0cc ("efi/memattr: Ignore table if the size is clearly bogus")
>>>>>> Reported-by: Breno Leitao <leitao@...ian.org>
>>>>>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
>>>>>> ---
>>>>>>  drivers/firmware/efi/memattr.c | 16 ++++++----------
>>>>>>  1 file changed, 6 insertions(+), 10 deletions(-)
>>>>>>
>>>>>
>>>>> The more I think about this, the more I feel that kexec on x86 should
>>>>> simply discard this table, and run with the firmware code RWX (which
>>>>> is not the end of the world).
>>>>
>>>>
>>>> By discard this table, do you mean kexec not use e820_table_firmware?
>>>
>>> No, I mean kexec ignores the memory attributes table.
>>>
>>>> Also a very basic question, what do you mean by run with the firmware RWX?
>>>>
>>>
>>> The memory attributes table is an overlay for the EFI memory map that
>>> describes which runtime code regions may be mapped with restricted
>>> permissions. Without this table, everything will be mapped writable as
>>> well as executable, but only in the EFI page tables, which are only
>>> active when an EFI call is in progress.
>>>
>>
>> Thanks for explaining!
>>
>> So basically get rid of memattr.c :)
>>
>> Do you mean get rid of it only for kexec, or not do it for any
>> boot (including cold boot)?
>> I do like this idea! I couldn't find this in the git history,
>> but do you know if this was added in the linux kernel just
>> because EFI spec added support for it, or if there was a
>> specific security problem?
>>
> 
> Usama, can you try the patch below?
> [ format is wrong due to webmail corruption.  But if it works I can
> send a formal patch later ]
> 
> $ git diff arch/x86
> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
> index 846bf49f2508..58dc77c5210e 100644
> --- a/arch/x86/platform/efi/quirks.c
> +++ b/arch/x86/platform/efi/quirks.c
> @@ -561,6 +561,11 @@ int __init efi_reuse_config(u64 tables, int nr_tables)
> 
>                 if (!efi_guidcmp(guid, SMBIOS_TABLE_GUID))
>                         ((efi_config_table_64_t *)p)->table = data->smbios;
> +
> +               /* Not bother to play with mem attr table across kexec */
> +               if (!efi_guidcmp(guid, EFI_MEMORY_ATTRIBUTES_TABLE_GUID))
> +                       ((efi_config_table_64_t *)p)->table =
> EFI_INVALID_TABLE_ADDR;
> +
>                 p += sz;
>         }
> 

This would work, I am guessing it will have a similar effect to what I sent
last week in 
https://lore.kernel.org/all/fd63613c-fd26-42de-b5ed-cc734f72eb36@gmail.com/

I think it needs to be wrapped in ifdef CONFIG_X86_64.

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