Messages in this thread |  | | | Date | Mon, 8 Sep 2008 12:35:47 +0100 (BST) | | From | Hugh Dickins <> | | Subject | Re: [PATCH RFC] x86: check for and defend against BIOS memory corruption |
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On Fri, 29 Aug 2008, Jeremy Fitzhardinge wrote: > Hugh Dickins wrote: > > > Is this the right moment for me to mention again that I'm not sure > > your reuse of existing pagetables was quite right anyway: NX being > > excluded from level2_ident_pgt, but wanted in the direct map? > > We could add NX. What's the behaviour of setting NX in a > non-NX-supporting CPU? I don't think it would trigger a "reserved bit" > exception (the other high pte flags don't). Or failing that, we could > mask out NX once we've worked out the CPU doesn't support it (at the > same time it relocates the pagetables to the kernel's load-time address).
I've no experience of what happens if NX is set to a non-NX-supporting CPU, so can't advise on that at all. But I think you're looking at it the wrong way round - or else I am.
Here's the declaration and comment on level2_ident_pgt in head_64.S:
NEXT_PAGE(level2_ident_pgt) /* Since I easily can, map the first 1G. * Don't set NX because code runs from these pages. */ PMDS(0, __PAGE_KERNEL_LARGE_EXEC, PTRS_PER_PMD) (The "Since I easily can" comment is there because it used to map only a subset needed for early kernel startup, not the whole 1G.)
So it's very intentionally leaving NX out there. I believe the level2_ident_pgt page appears twice or more in the pagetable layout, used to map two or more areas of virtual address space - once to provide the direct map at ffff880000000000 and once to provide the kernel image virtual mapping at ffffffff80200000. I think we don't want NX on Linux kernel text ;-?
Before your 2.6.27-rc changes, init_memory_mapping subsequently replaced the direct map usage by a separately constructed pagetable, similar to it but with NX set throughout. After your 2.6.27-rc changes, level2_ident_pgt is found there already so left untouched - leaving NX out of that first 1G of the direct map forever (when CPA splits it up, the smaller pages inherit the lack of NX too).
I noticed when changing /proc/meminfo to show DirectMap in kB, and tried to fix it, but only gave myself a non-booting system (I probably shrank my direct map to 0 while implicitly using it). And at that stage I didn't realize at all that it was a recent regression arising from your mods.
I'm reluctant to delve in there again at present, and unsure what the right fix should be: perhaps the code which checks if an entry is already there, should check if it has the desired flags?
Hugh
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