KASLR: about the vmalloc area for randomization kernel image

KASLR: about the vmalloc area for randomization kernel image

[Zong Li]

Hi Ard,

I'm working on KASLR for RISC-V, and I'm interested in what is the
consideration of choosing the vmalloc area on arm64. I was wondering
if you could give me some advice.

There are other architectures had implemented KASLR, such as x86,
mips, s390 and ppc, they use the lowmem to randomize the address of
kernel image, it seems that only arm64 move the kernel image to
vmalloc area.

I tried to understand the history and reason for using vmalloc area
through some discussions at the end of mail, but I still couldn't know
what is the reason for using vmalloc area in the beginning.

For the randomization purpose, it seems to me that there is no
significant difference between using lowmem and vmalloc area. Would
you be willing to help to give some ideas about that? :)

Sorry if I missed anything. Thank you very much.

Regards,
Zong

RFC patch:
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/396427.html
sub-series patch:
https://www.spinics.net/lists/arm-kernel/msg483446.html
other discussion:
https://lore.kernel.org/linux-arm-kernel/CAKv+Gu_i4M=RJmofMRoq63rR6GBf42W36FtomhTC9-o9g-sAiw@mail.gmail.com/

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Re: KASLR: about the vmalloc area for randomization kernel image

[Ard Biesheuvel]

On Fri, 3 Jan 2020 at 07:52, Zong Li <zong.li@sifive.com> wrote:
>
> Hi Ard,
>
> I'm working on KASLR for RISC-V, and I'm interested in what is the
> consideration of choosing the vmalloc area on arm64. I was wondering
> if you could give me some advice.
>
> There are other architectures had implemented KASLR, such as x86,
> mips, s390 and ppc, they use the lowmem to randomize the address of
> kernel image, it seems that only arm64 move the kernel image to
> vmalloc area.
>
> I tried to understand the history and reason for using vmalloc area
> through some discussions at the end of mail, but I still couldn't know
> what is the reason for using vmalloc area in the beginning.
>
> For the randomization purpose, it seems to me that there is no
> significant difference between using lowmem and vmalloc area. Would
> you be willing to help to give some ideas about that? :)
>
> Sorry if I missed anything. Thank you very much.
>

Are we talking about 32-bit here? If not, what do you mean by lowmem?

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Re: KASLR: about the vmalloc area for randomization kernel image

[Zong Li]

On Fri, Jan 3, 2020 at 10:47 PM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
> On Fri, 3 Jan 2020 at 07:52, Zong Li <zong.li@sifive.com> wrote:
> >
> > Hi Ard,
> >
> > I'm working on KASLR for RISC-V, and I'm interested in what is the
> > consideration of choosing the vmalloc area on arm64. I was wondering
> > if you could give me some advice.
> >
> > There are other architectures had implemented KASLR, such as x86,
> > mips, s390 and ppc, they use the lowmem to randomize the address of
> > kernel image, it seems that only arm64 move the kernel image to
> > vmalloc area.
> >
> > I tried to understand the history and reason for using vmalloc area
> > through some discussions at the end of mail, but I still couldn't know
> > what is the reason for using vmalloc area in the beginning.
> >
> > For the randomization purpose, it seems to me that there is no
> > significant difference between using lowmem and vmalloc area. Would
> > you be willing to help to give some ideas about that? :)
> >
> > Sorry if I missed anything. Thank you very much.
> >
>
> Are we talking about 32-bit here? If not, what do you mean by lowmem?

Oops, sorry about that, I consider 32-bit and 64-bit for KASLR at the
same time, it should be linear mapping area here, not lowmem.

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