[Question] Should retain 2M alignment if kernel image is bad aligned at entry or BSS overlaps?

[Kefeng Wang]

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.

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?


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);