On 2022/3/1 20:57, Ard Biesheuvel wrote:
On Tue, 1 Mar 2022 at 11:34, Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
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.
That commit log explains how we ended up forcing 2M alignment
inadvertently if the EFI_RNG_PROTOCOL was not available. If we don't
force 2M alignment, and the physical address of the kernel happens to
be misaligned modulo 2M, the resulting offset is reused for virtual
randomization as well. Hence the 5 additional bits.
Given the predictability of those bits when used on their own, we
could decide to disable KPTI in this case as well.
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?
I am having difficulty understanding which part of the current
behavior you think is causing a problem.
In a nutshell, what the current code aims to do is to only move the
image in memory if needed, and just boot if where it was loaded by EFI
otherwise.
Reasons for moving it could be any of:
- KASLR is enabled and we have a seed
- nokaslr was passed, and we are not aligned to 2M
- we are running a 64k pages + VMAP stack build, and the image is not
aligned to 128k
- the PE/COFF loader is broken, and ignored the minimum 64k alignment
in the PE/COFF header
- the PE/COFF loader is broken, and ignored the BSS size, resulting in
an overlap with a memory region that is already in use.
As you might have guessed, my grumbling about GRUB/SHIM was in
relation to the latter 2 points - upstream GRUB does not have its own
PE/COFF loader, but SHIM/GRUB implement their own, and don't follow
the PE/COFF spec too rigorously.
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?
I can only answer this if I understand which problem it solves. Why do
you need the 2M alignment in this case?
.
Sorry for the late response，my purpose is that we don't want to enable
KPTI if
KASLR is disabled. For now, if there are some firmware bug, the kernel
image is
relocated which lead to kaslr_requires_kpti() returen
ture(kaslr_offset() > 0).
the change to 2M alignment is a workaround and according to your
explanation,
I don't think the workaround is necessary. I want to make sure that the
above
scene is expected? thanks.
