On 11/5/22 23:52, Ard Biesheuvel wrote:
The EFI spec requires that on arm64 systems, all runtime code and data
regions that share a 64k page can be mapped with the same memory type
attributes. Unfortunately, this does not take permission attributes into
account, and so the firmware is permitted to expose runtime code and
data regions that share 64k pages, and this may prevent the OS from
using restricted permissions in such cases, e.g., map data regions with
non-exec attributes.
This is the relevant paragraph in the UEFI specification:
<cite>
The ARM architecture allows mapping pages at a variety of granularities,
including 4KiB and 64KiB. If a 64KiB physical page contains any 4KiB
page with any of the following types listed below, then all 4KiB pages
in the 64KiB page must use identical ARM Memory Page Attributes (as
described in Map EFI Cacheability Attributes to AArch64 Memory Types):
- EfiRuntimeServicesCode
- EfiRuntimeServicesData
- EfiReserved
- EfiACPIMemoryNVS
Mixed attribute mappings within a larger page are not allowed.
</cite>
It remains unclear if only EFI Cacheability of also other page
attributes are meant. The UEFI specification should be clarified in this
respect.
We currently emit a warning when hitting this at boot, but the warning
is problematic for a number of reasons:
- it uses WARN() which spews a lot of irrelevant information into the
log about the execution context where the issue was detected;
- it only takes the start of the region into account and not the size
Is the occurrence of the warning specific to U-Boot or do you see the
warning with EDK II too?
Let's just drop the warning, as the condition does not strictly violate
the spec (although it only occurs with U-Boot), and fix the check to
take both the start and the end addresses into account.
Cc: Heinrich Schuchardt <heinrich.schuchardt@xxxxxxxxxxxxx>
Cc: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
arch/arm64/kernel/efi.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
index e1be6c429810d0d5..3dd6f0c66f8aeb78 100644
--- a/arch/arm64/kernel/efi.c
+++ b/arch/arm64/kernel/efi.c
@@ -25,8 +25,8 @@ static __init pteval_t create_mapping_protection(efi_memory_desc_t *md)
if (type == EFI_MEMORY_MAPPED_IO)
return PROT_DEVICE_nGnRE;
- if (WARN_ONCE(!PAGE_ALIGNED(md->phys_addr),
- "UEFI Runtime regions are not aligned to 64 KB -- buggy firmware?"))
+ if (!PAGE_ALIGNED(md->phys_addr) ||
+ !PAGE_ALIGNED(md->num_pages * EFI_PAGE_SIZE))
Enhancing the check is correct.
The warning tells that Linux cannot establish secure settings for some
pages. It would be preferable to keep it and fix the UEFI specification
and the firmware instead.
Best regards
Heinrich
/*
* If the region is not aligned to the page size of the OS, we
* can not use strict permissions, since that would also affect
