Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 13 Nov 2017 18:27:54 -0800 | Subject | Re: [RFC 6/7] x86/asm: Remap the TSS into the cpu entry area |
| |
On Mon, Nov 13, 2017 at 11:22 AM, Dave Hansen <dave.hansen@intel.com> wrote: > On 11/10/2017 08:05 PM, Andy Lutomirski wrote: >> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h >> index fbc9b7f4e35e..8a9ba5553cab 100644 >> --- a/arch/x86/include/asm/fixmap.h >> +++ b/arch/x86/include/asm/fixmap.h >> @@ -52,6 +52,13 @@ extern unsigned long __FIXADDR_TOP; >> struct cpu_entry_area >> { >> char gdt[PAGE_SIZE]; >> + >> + /* >> + * The gdt is just below cpu_tss and thus serves (on x86_64) as a >> + * a read-only guard page for the SYSENTER stack at the bottom >> + * of the TSS region. >> + */ >> + struct tss_struct tss; >> }; >> > > Aha, and here's the place that you need sizeof(tss_struct) to be nice > and page-aligned. > > But why don't we just do: > > char tss_space[PAGE_SIZE*something];
The idea is to save some space. The TSS plus IO bitmap is slightly over a page, so, if we're giving it a dedicated block of pages, we have almost a page of unused space. I want to use some of that space for the SYSENTER stack. To reliably detect overflow, that space should be at the beginning.
It turns out that using almost a page is way too *big*: it masks bugs. I want anything nontrivial that accidentally runs on the SYSENTER stack to overflow and crash very quickly rather than having a decent chance of working or of causing nasty corruption with a crash down the road. So I'm going to make it much smaller and instead just add a build-time assertion that we don't cross a page boundary.
| |