[PATCH] efi/memattr: Ignore table if the size is clearly bogus

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

There are reports [0] of cases where a corrupt EFI Memory Attributes
Table leads to out of memory issues at boot because the descriptor size
and entry count in the table header are still used to reserve the entire
table in memory, even though the resulting region is gigabytes in size.

Given that the EFI Memory Attributes Table is supposed to carry up to 3
entries for each EfiRuntimeServicesCode region in the EFI memory map,
and given that there is no reason for the descriptor size used in the
table to exceed the one used in the EFI memory map, 3x the size of the
entire EFI memory map is a reasonable upper bound for the size of this
table. This means that sizes exceeding that are highly likely to be
based on corrupted data, and the table should just be ignored instead.

[0] https://bugzilla.suse.com/show_bug.cgi?id=1231465

Cc: Gregory Price <gourry@gourry.net>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/all/20240912155159.1951792-2-ardb+git@google.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/memattr.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
index 164203429fa7..d85363d0422a 100644
--- a/drivers/firmware/efi/memattr.c
+++ b/drivers/firmware/efi/memattr.c
@@ -40,6 +40,20 @@ int __init efi_memattr_init(void)
 	}
 
 	tbl_size = sizeof(*tbl) + tbl->num_entries * tbl->desc_size;
+
+	/*
+	 * 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.
+	 */
+	if (tbl_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);
+		goto unmap;
+	}
+
 	memblock_reserve(efi_mem_attr_table, tbl_size);
 	set_bit(EFI_MEM_ATTR, &efi.flags);
 
-- 
2.47.0.163.g1226f6d8fa-goog