On 13/01/2025 02:33, Dave Young wrote: > 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 where the check in [1] is no longer valid with such a low efi.memmap.nr_map 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; > } > 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@xxxxxxxxx/ I think it needs to be wrapped in ifdef CONFIG_X86_64.
