Messages in this thread Patch in this message | | | Subject | Re: [PATCH v12 16/16] arm64: kexec_file: add kaslr support | From | James Morse <> | Date | Fri, 27 Jul 2018 10:22:31 +0100 |
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<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(&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> | |