linux-kernel - Re: [RFC 1/2] efi/memattr: Use desc_size instead of total size to check for corruption

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Message-ID: <CALu+AoS8tb=HgaybDw5OG4A1QbOXHvuidpu0ynmz-nE1nBqzTA@mail.gmail.com>
Date: Mon, 13 Jan 2025 10:33:29 +0800
From: Dave Young <dyoung@...hat.com>
To: Usama Arif <usamaarif642@...il.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 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;
        }