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 where the check in [1] is no longer valid with such a low efi.memmap.nr_map 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? Also a very basic question, what do you mean by run with the firmware RWX? Sorry for the very basic questions above! > > The main reason is that the EFI memory map and the EFI memory > attributes table are supposed to be a matched pair, where each RTcode > entry in the former is broken down into multiple code and data > segments in the latter. The amount of mangling that the x86 arch code > does of the EFI memory map makes it intractable to ensure that they > remain in sync, and so it is better not to bother. > > >> diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c >> index c38b1a335590..d3bc161361fb 100644 >> --- a/drivers/firmware/efi/memattr.c >> +++ b/drivers/firmware/efi/memattr.c >> @@ -40,21 +40,17 @@ int __init efi_memattr_init(void) >> goto unmap; >> } >> >> - >> /* >> - * Sanity check: the Memory Attributes Table contains up to 3 entries >> - * for each entry of type EfiRuntimeServicesCode in the EFI memory map. >> - * So if the size of the table exceeds 3x the size of the entire EFI >> - * memory map, there is clearly something wrong, and the table should >> - * just be ignored altogether. >> + * Sanity check: the Memory Attributes Table desc_size and >> + * efi.memmap.desc_size should match. >> */ >> - size = tbl->num_entries * tbl->desc_size; >> - if (size > 3 * efi.memmap.nr_map * efi.memmap.desc_size) { >> - pr_warn(FW_BUG "Corrupted EFI Memory Attributes Table detected! (version == %u, desc_size == %u, num_entries == %u)\n", >> - tbl->version, tbl->desc_size, tbl->num_entries); >> + if (efi.memmap.desc_size != tbl->desc_size) { >> + pr_warn(FW_BUG "Corrupted EFI Memory Attributes Table detected! (version == %u, table desc_size == %u, efi.memmap.desc_size == %lu, table num_entries == %u)\n", >> + tbl->version, tbl->desc_size, efi.memmap.desc_size, tbl->num_entries); >> goto unmap; >> } >> >> + size = tbl->num_entries * tbl->desc_size; >> tbl_size = sizeof(*tbl) + size; >> memblock_reserve(efi_mem_attr_table, tbl_size); >> set_bit(EFI_MEM_ATTR, &efi.flags); >> -- >> 2.43.5 >>
