linux/arch
Andi Kleen 6b28baca9b x86/speculation/l1tf: Protect PROT_NONE PTEs against speculation
When PTEs are set to PROT_NONE the kernel just clears the Present bit and
preserves the PFN, which creates attack surface for L1TF speculation
speculation attacks.

This is important inside guests, because L1TF speculation bypasses physical
page remapping. While the host has its own migitations preventing leaking
data from other VMs into the guest, this would still risk leaking the wrong
page inside the current guest.

This uses the same technique as Linus' swap entry patch: while an entry is
is in PROTNONE state invert the complete PFN part part of it. This ensures
that the the highest bit will point to non existing memory.

The invert is done by pte/pmd_modify and pfn/pmd/pud_pte for PROTNONE and
pte/pmd/pud_pfn undo it.

This assume that no code path touches the PFN part of a PTE directly
without using these primitives.

This doesn't handle the case that MMIO is on the top of the CPU physical
memory. If such an MMIO region was exposed by an unpriviledged driver for
mmap it would be possible to attack some real memory.  However this
situation is all rather unlikely.

For 32bit non PAE the inversion is not done because there are really not
enough bits to protect anything.

Q: Why does the guest need to be protected when the HyperVisor already has
   L1TF mitigations?

A: Here's an example:

   Physical pages 1 2 get mapped into a guest as
   GPA 1 -> PA 2
   GPA 2 -> PA 1
   through EPT.

   The L1TF speculation ignores the EPT remapping.

   Now the guest kernel maps GPA 1 to process A and GPA 2 to process B, and
   they belong to different users and should be isolated.

   A sets the GPA 1 PA 2 PTE to PROT_NONE to bypass the EPT remapping and
   gets read access to the underlying physical page. Which in this case
   points to PA 2, so it can read process B's data, if it happened to be in
   L1, so isolation inside the guest is broken.

   There's nothing the hypervisor can do about this. This mitigation has to
   be done in the guest itself.

[ tglx: Massaged changelog ]

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Dave Hansen <dave.hansen@intel.com>
2018-06-20 19:10:00 +02:00
..
alpha Kbuild updates for v4.18 2018-06-06 11:00:15 -07:00
arc mm: introduce ARCH_HAS_PTE_SPECIAL 2018-06-07 17:34:35 -07:00
arm docs: Fix some broken references 2018-06-15 18:10:01 -03:00
arm64 docs: Fix some broken references 2018-06-15 18:10:01 -03:00
c6x Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
h8300 Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
hexagon hexagon: drop the unused variable zero_page_mask 2018-06-15 07:55:25 +09:00
ia64 - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
m68k Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gerg/m68knommu 2018-06-05 10:51:30 -07:00
microblaze docs: Fix some broken references 2018-06-15 18:10:01 -03:00
mips Merge branch 'akpm' (patches from Andrew) 2018-06-15 08:51:42 +09:00
nds32 Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
nios2 Merge branch 'timers-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-06-04 20:27:54 -07:00
openrisc Kbuild updates for v4.18 2018-06-06 11:00:15 -07:00
parisc arch/*: Kconfig: fix documentation for NMI watchdog 2018-06-15 12:48:59 -03:00
powerpc Mostly the PPC part of the release, but also switching to Arnd's fix 2018-06-16 06:37:04 +09:00
riscv RISC-V Updates for the 4.18 Merge Window 2018-06-16 06:42:43 +09:00
s390 - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
sh arch/*: Kconfig: fix documentation for NMI watchdog 2018-06-15 12:48:59 -03:00
sparc arch/*: Kconfig: fix documentation for NMI watchdog 2018-06-15 12:48:59 -03:00
um Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
unicore32 docs: Fix some broken references 2018-06-15 18:10:01 -03:00
x86 x86/speculation/l1tf: Protect PROT_NONE PTEs against speculation 2018-06-20 19:10:00 +02:00
xtensa docs: Fix some broken references 2018-06-15 18:10:01 -03:00
.gitignore
Kconfig docs: Fix some broken references 2018-06-15 18:10:01 -03:00