On 2022/3/1 15:22, Ard Biesheuvel wrote:
On Tue, 1 Mar 2022 at 07:50, Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
Hi Ard，Will and all maintainer，
As commit 3a262423755b ("efi/libstub: arm64: Relax 2M alignment again
for relocatable kernels") saids, a relocatable image does not need to
be copied to a 2M aligned offset if it was loaded on a 64k boundary
(for a 4 KB granule kernel) by EFI. But if there is some FIRMWARE BUG,
1) kernel image not aligned on 64k boundary
or
2) Image BSS overlaps adjacent EFI memory region
When kaslr is disabled(eg, EFI_RNG_PROTOCOL unavailable), it will leads
KPTI forced ON after kernel image relocated, message shown below,
CPU features: kernel page table isolation forced ON by KASLR
...
KASLR disabled due to lack of seed
The KASLR don't enabled actually, and KPTI is forced enabled which could
affect performance.
This message is misleading. If the alignment modulo 2M != 0, we still
have 5 bits of 'randomization', although these bits are probably
highly predictable on a given system.
Yes， this kernel boot message is misleading, I am confused and find
commit 3a262423755b ("efi/libstub: arm64: Relax 2M alignment again for
relocatable kernels") leads to different behavior about KPTI.
I found commit 7c116db24d94 ("efi/libstub/arm64: Retain 2MB kernel Image
alignment if !KASLR") in v5.8, should we retain 2M alignment if kernel image
is bad aligned at entry or BSS overlaps?
Personally, I think we're doing enough already to deal with Redhat's
broken out-of-tree GRUB/SHIM concoction, which is the primary reason
for these workarounds IIRC.
Not sure about this, what's your mean is that error message is enough and
no need to adjust the alignment when image with bad aligned at entry or
BSS overlaps?
You can already pass nokalsr on the kernel command line if you want to
avoid the downsides entirely, so as I understand it, this is mostly
about an unquantified performance gain on systems that use a broken
bootloader and lack the entropy source for a KASLR seed, but are not
able to put nokaslr on the command line?
nokaslr will use 2M alignment by default, but if some board with new
BIOS/GRUB
the kaslr won't enabled unless change the grub to drop it one by one, it
is not kind
for production deployment.
Do you think the following adjustment make sense or it is definitely wrong?
Any other option, thanks for your feedback.
diff --git a/drivers/firmware/efi/libstub/arm64-stub.c
b/drivers/firmware/efi/libstub/arm64-stub.c
index 9cc556013d08..47ecd9b80db3 100644
--- a/drivers/firmware/efi/libstub/arm64-stub.c
+++ b/drivers/firmware/efi/libstub/arm64-stub.c
@@ -87,6 +87,7 @@ efi_status_t handle_kernel_image(unsigned long
*image_addr,
{
efi_status_t status;
unsigned long kernel_size, kernel_memsize = 0;
+ bool need_2m_aligned = false;
u32 phys_seed = 0;
/*
@@ -119,9 +120,11 @@ efi_status_t handle_kernel_image(unsigned long
*image_addr,
if (image->image_base != _text)
efi_err("FIRMWARE BUG: efi_loaded_image_t::image_base
has bogus value\n");
- if (!IS_ALIGNED((u64)_text, SEGMENT_ALIGN))
+ if (!IS_ALIGNED((u64)_text, SEGMENT_ALIGN)) {
+ need_2m_aligned = true;
efi_err("FIRMWARE BUG: kernel image not aligned on %dk
boundary\n",
SEGMENT_ALIGN >> 10);
+ }
kernel_size = _edata - _text;
kernel_memsize = kernel_size + (_end - _edata);
@@ -142,6 +145,7 @@ efi_status_t handle_kernel_image(unsigned long
*image_addr,
if (status != EFI_SUCCESS) {
if (!check_image_region((u64)_text, kernel_memsize)) {
+ need_2m_aligned = true;
efi_err("FIRMWARE BUG: Image BSS overlaps
adjacent EFI memory region\n");
} else if (IS_ALIGNED((u64)_text, min_kimg_align)) {
/*
@@ -152,7 +156,8 @@ efi_status_t handle_kernel_image(unsigned long
*image_addr,
*reserve_size = 0;
return EFI_SUCCESS;
}
-
+ if (efi_nokaslr & need_2m_aligned)
+ min_kimg_align = MIN_KIMG_ALIGN;
status = efi_allocate_pages_aligned(*reserve_size,
reserve_addr,
ULONG_MAX,
min_kimg_align);
.
