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 where the check in [1] is no longer valid with such a low efi.memmap.nr_map 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? Thanks!
