Messages in this thread | | | Date | Tue, 10 Sep 2019 08:18:15 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86/boot/64: Make level2_kernel_pgt pages invalid outside kernel area. |
| |
* Kirill A. Shutemov <kirill@shutemov.name> wrote:
> On Fri, Sep 06, 2019 at 04:29:50PM -0500, Steve Wahl wrote: > > Our hardware (UV aka Superdome Flex) has address ranges marked > > reserved by the BIOS. These ranges can cause the system to halt if > > accessed. > > > > During kernel initialization, the processor was speculating into > > reserved memory causing system halts. The processor speculation is > > enabled because the reserved memory is being mapped by the kernel. > > > > The page table level2_kernel_pgt is 1 GiB in size, and had all pages > > initially marked as valid, and the kernel is placed anywhere in this > > range depending on the virtual address selected by KASLR. Later on in > > the boot process, the valid area gets trimmed back to the space > > occupied by the kernel. > > > > But during the interval of time when the full 1 GiB space was marked > > as valid, if the kernel physical address chosen by KASLR was close > > enough to our reserved memory regions, the valid pages outside the > > actual kernel space were allowing the processor to issue speculative > > accesses to the reserved space, causing the system to halt. > > > > This was encountered somewhat rarely on a normal system boot, and > > somewhat more often when starting the crash kernel if > > "crashkernel=512M,high" was specified on the command line (because > > this heavily restricts the physical address of the crash kernel, > > usually to within 1 GiB of our reserved space). > > > > The answer is to invalidate the pages of this table outside the > > address range occupied by the kernel before the page table is > > activated. This patch has been validated to fix this problem on our > > hardware. > > If the goal is to avoid *any* mapping of the reserved region to stop > speculation, I don't think this patch will do the job. We still (likely) > have the same memory mapped as part of the identity mapping. And it > happens at least in two places: here and before on decompression stage.
Yeah, this really needs a fix at the KASLR level: it should only ever map into regions that are fully RAM backed.
Is the problem that the 1 GiB mapping is a direct mapping, which can be speculated into? I presume KASLR won't accidentally map the kernel into the reserved region, right?
Thanks,
Ingo
| |