lkml.org 
[lkml]   [2018]   [Jul]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
SubjectRe: [PATCH v12 16/16] arm64: kexec_file: add kaslr support
From
Date
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<p>Hi Akashi,<br>
</p>
<br>
<div class="moz-cite-prefix">On 07/27/2018 09:31 AM, AKASHI Takahiro
wrote:<br>
</div>
<blockquote type="cite" cite="mid:20180727083104.GI11258@linaro.org">
<pre wrap="">On Thu, Jul 26, 2018 at 02:40:49PM +0100, James Morse wrote:
</pre>
<blockquote type="cite">
<pre wrap="">On 24/07/18 07:57, AKASHI Takahiro wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual
address randomization, at secondary kernel boot.
</pre>
</blockquote>
<pre wrap="">
Hmm, there are three things that get moved by CONFIG_RANDOMIZE_BASE. The kernel
physical placement when booted via the EFIstub, the kernel-text VAs and the
location of memory in the linear-map region. Adding the kaslr-seed only does the
last two.
</pre>
</blockquote>
<pre wrap="">
Yes, but I think that I and Mark has agreed that "kaslr" meant
"virtual" randomisation, not including "physical" randomisation.</pre>
</blockquote>
Okay, I'll update my terminology!<br>
<br>
<br>
<blockquote type="cite" cite="mid:20180727083104.GI11258@linaro.org">
<blockquote type="cite">
<pre wrap="">This means the physical placement of the new kernel is predictable from
/proc/iomem ... but this also tells you the physical placement of the current
kernel, so I don't think this is a problem.


</pre>
<blockquote type="cite">
<pre wrap="">We always do this as it will have no harm on kaslr-incapable kernel.
</pre>
</blockquote>
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap="">We don't have any "switch" to turn off this feature directly, but still
can suppress it by passing "nokaslr" as a kernel boot argument.
</pre>
</blockquote>
<pre wrap="">

</pre>
<blockquote type="cite">
<pre wrap="">diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
index 7356da5a53d5..47a4fbd0dc34 100644
--- a/arch/arm64/kernel/machine_kexec_file.c
+++ b/arch/arm64/kernel/machine_kexec_file.c
@@ -158,6 +160,12 @@ static int setup_dtb(struct kimage *image,
</pre>
</blockquote>
<pre wrap="">
Don't you need to reserve some space in the area you vmalloc()d for the DT?
</pre>
</blockquote>
<pre wrap="">
No, I don't think so.
All the data to be loaded are temporarily saved in kexec buffers,
which will eventually be copied to target locations in machine_kexec
(arm64_relocate_new_kernel, which, unlike its name, will handle
not only kernel but also other data as well).</pre>
</blockquote>
<br>
I think we're speaking at cross purposes. Don't you need:<br>
<pre wrap="">| buf_size += fdt_prop_len("kaslr<span class="ILfuVd yZ8quc">―</span>seed", sizeof(u64));</pre>
<br>
You can't assume the existing DTB had a kaslr-seed property, and the
difference may take us over a PAGE_SIZE boundary.<br>
<br>
<br>
<blockquote type="cite" cite="mid:20180727083104.GI11258@linaro.org"><br>
<blockquote type="cite"><br>
<blockquote type="cite">
<pre wrap="">+ /* add kaslr-seed */
+ get_random_bytes(&amp;value, sizeof(value));
</pre>
</blockquote>
<pre wrap="">
What happens if the crng isn't ready?
It looks like this will print a warning that these random-bytes aren't really up
to standard, but the new kernel doesn't know this happened.

crng_ready() isn't exposed, all we could do now is
wait_for_random_bytes(), but that may wait forever because we do this
unconditionally.

I'd prefer to leave this feature until we can check crng_ready(), and skip
adding a dodgy-seed if its not-ready. This avoids polluting the next-kernel's
entropy pool.
</pre>
</blockquote>
<pre wrap="">
OK. I would try to follow the same way as Bhupesh's userspace patch
does for kaslr-seed:
<a class="moz-txt-link-freetext" href="http://lists.infradead.org/pipermail/kexec/2018-April/020564.html">http://lists.infradead.org/pipermail/kexec/2018-April/020564.html</a></pre>
</blockquote>
<br>
(I really don't understand this 'copying code from user-space' that
happens with kexec_file_load)<br>
<br>
<br>
<blockquote type="cite" cite="mid:20180727083104.GI11258@linaro.org">
<pre wrap=""> if (not found kaslr-seed in 1st kernel's dtb)
don't care; go ahead</pre>
</blockquote>
<br>
Don' t bother. As you say in the commit-message its harmless if the
new kernel doesn't support it.<br>
Always having this would let you use kexec_file_load as a bootloader
that can get the crng to<br>
provide decent entropy even if the platform bootloader can't.<br>
<br>
<br>
<blockquote type="cite" cite="mid:20180727083104.GI11258@linaro.org">
<pre wrap=""> else
if (current kaslr-seed != 0)
error</pre>
</blockquote>
<br>
Don't bother. If this happens its a bug in another part of the
kernel that doesn't affect this one. We aren't second-guessing the
file-system when we read the kernel-fd, lets keep this simple.<br>
<br>
<blockquote type="cite" cite="mid:20180727083104.GI11258@linaro.org">
<pre wrap=""> if (crng_ready()) ; FIXME, it's a local macro
get_random_bytes(non-blocking)
set new kaslr-seed
else
error</pre>
</blockquote>
error? Something like pr_warn_once().<br>
<br>
I thought the kaslr-seed was added to the entropy pool, but now I
look again I see its a separate EFI table. So the new kernel will
add the same entropy ... that doesn't sound clever. (I can't see
where its zero'd or re-initialised)<br>
<br>
<br>
<br>
Thanks,<br>
<br>
James<br>
</body>
</html>
\
 
 \ /
  Last update: 2018-07-27 11:23    [W:0.184 / U:1.128 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site