On Fri, 10 Jan 2025 at 18:54, Usama Arif <usamaarif642@xxxxxxxxx> wrote: > > > > On 10/01/2025 07:21, Ard Biesheuvel wrote: > > On Thu, 9 Jan 2025 at 17:36, Usama Arif <usamaarif642@xxxxxxxxx> wrote: > >> > >> > >> > >> On 09/01/2025 15:45, Ard Biesheuvel wrote: > >>> On Wed, 8 Jan 2025 at 23:00, Usama Arif <usamaarif642@xxxxxxxxx> 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 where the check in [1] is no longer valid with such a low efi.memmap.nr_map 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@xxxxxxxxxx/ > >>>> > >>>> Fixes: 8fbe4c49c0cc ("efi/memattr: Ignore table if the size is clearly bogus") > >>>> Reported-by: Breno Leitao <leitao@xxxxxxxxxx> > >>>> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx> > >>>> --- > >>>> 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; }
