Re: [PATCH 2/3] efi/loongarch: Randomize kernel preferred address for KASLR

[hev]

Hi Huacai,

On Mon, Apr 27, 2026 at 9:27 PM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote:
>
> Hi, Rui,
>
> On Mon, Apr 27, 2026 at 6:47 PM WANG Rui <r@xxxxxx> wrote:
> >
> > Introduce efi_get_kimg_kaslr_address() to compute the preferred
> > kernel image address dynamically when CONFIG_RANDOMIZE_BASE is
> > enabled. The function derives a random offset using EFI-provided
> > randomness combined with the timer value, and constrains it within
> > CONFIG_RANDOMIZE_BASE_MAX_OFFSET.
> >
> > Update EFI_KIMG_PREFERRED_ADDRESS to call this helper so that the
> > EFI stub can select a randomized load address when KASLR is active,
> > while preserving the original base address behavior when KASLR is
> > disabled or nokaslr is specified.
> >
> > Signed-off-by: WANG Rui <r@xxxxxx>
> > ---
> >  arch/loongarch/Kconfig                   |  2 +-
> >  arch/loongarch/include/asm/efi.h         |  4 +++-
> >  drivers/firmware/efi/libstub/loongarch.c | 16 ++++++++++++++++
> >  3 files changed, 20 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
> > index 3b042dbb2c41..a5afb70f73d9 100644
> > --- a/arch/loongarch/Kconfig
> > +++ b/arch/loongarch/Kconfig
> > @@ -730,7 +730,7 @@ config RANDOMIZE_BASE
> >  config RANDOMIZE_BASE_MAX_OFFSET
> >         hex "Maximum KASLR offset" if EXPERT
> >         depends on RANDOMIZE_BASE
> > -       range 0x0 0x10000000
> > +       range 0x20000 0x10000000
> Why modify this?

The reason for changeing the lower bound is to simplify the
random_offset calculation, making sure that
(CONFIG_RANDOMIZE_BASE_MAX_OFFSET - SZ_64K - 1) is always >= 0.

>
> >         default "0x01000000"
> >         help
> >           When KASLR is active, this provides the maximum offset that will
> > diff --git a/arch/loongarch/include/asm/efi.h b/arch/loongarch/include/asm/efi.h
> > index eddc8e79b3fa..f831320efd41 100644
> > --- a/arch/loongarch/include/asm/efi.h
> > +++ b/arch/loongarch/include/asm/efi.h
> > @@ -30,6 +30,8 @@ static inline unsigned long efi_get_kimg_min_align(void)
> >         return SZ_2M;
> >  }
> >
> > -#define EFI_KIMG_PREFERRED_ADDRESS     PHYSADDR(VMLINUX_LOAD_ADDRESS)
> > +unsigned long efi_get_kimg_kaslr_address(void);
> > +
> > +#define EFI_KIMG_PREFERRED_ADDRESS     efi_get_kimg_kaslr_address()
> Can we reuse something in drivers/firmware/efi/libstub/kaslr.c?

This aligns with the kernel's built-in KASLR, ensuring the random
range stays within CONFIG_RANDOMIZE_BASE_MAX_OFFSET. This is also why
we didn't reuse kaslr.c.

>
> >
> >  #endif /* _ASM_LOONGARCH_EFI_H */
> > diff --git a/drivers/firmware/efi/libstub/loongarch.c b/drivers/firmware/efi/libstub/loongarch.c
> > index 9825f5218137..df67ef8c68ab 100644
> > --- a/drivers/firmware/efi/libstub/loongarch.c
> > +++ b/drivers/firmware/efi/libstub/loongarch.c
> > @@ -38,6 +38,22 @@ static efi_status_t exit_boot_func(struct efi_boot_memmap *map, void *priv)
> >         return EFI_SUCCESS;
> >  }
> >
> > +unsigned long efi_get_kimg_kaslr_address(void)
> > +{
> > +       unsigned int random_offset = 0;
> > +
> > +#ifdef CONFIG_RANDOMIZE_BASE
> > +       if (!efi_nokaslr) {
> > +               efi_get_random_bytes(sizeof(random_offset), (u8 *)&random_offset);
> > +               random_offset ^= (rdtime_l() << 16);
> Use get_cycles() then the first patch can be dropped.

I'll use random_get_entropy() here.

Thanks,
Rui


>
> Huacai
>
> > +               random_offset &= (CONFIG_RANDOMIZE_BASE_MAX_OFFSET - SZ_64K - 1);
> > +               random_offset = ALIGN(random_offset, SZ_64K) + SZ_64K;
> > +       }
> > +#endif
> > +
> > +       return PHYSADDR(VMLINUX_LOAD_ADDRESS) + random_offset;
> > +}
> > +
> >  unsigned long __weak kernel_entry_address(unsigned long kernel_addr,
> >                 efi_loaded_image_t *image)
> >  {
> > --
> > 2.54.0
> >