Message-ID: <138f28ec-341e-4c48-a14b-4371a8198de8@gmail.com>
Date: Mon, 13 Jan 2025 12:00:12 +0000
From: Usama Arif <usamaarif642@...il.com>
To: Dave Young <dyoung@...hat.com>, Ard Biesheuvel <ardb@...nel.org>
Cc: 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 11:27, Usama Arif wrote:
> 
> 
> 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.
> 

IMO we should consider the 2 patches in this series first before disabling it for
kexec. These patches actually fix the issue.

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