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Subject[PATCH RFC V3 0/9] PKS: Add Protection Keys Supervisor (PKS) support RFC v3
From: Ira Weiny <>

This RFC series has been reviewed by Dave Hansen.

Introduce a new page protection mechanism for supervisor pages, Protection Key
Supervisor (PKS).

2 use cases for PKS are being developed, trusted keys and PMEM. Trusted keys
is a newer use case which is still being explored. PMEM was submitted as part
of the RFC (v2) series[1]. However, since then it was found that some callers
of kmap() require a global implementation of PKS. Specifically some users of
kmap() expect mappings to be available to all kernel threads. While global use
of PKS is rare it needs to be included for correctness. Unfortunately the
kmap() updates required a large patch series to make the needed changes at the
various kmap() call sites so that patch set has been split out. Because the
global PKS feature is only required for that use case it will be deferred to
that set as well.[2] This patch set is being submitted as a precursor to both
of the use cases.

For an overview of the entire PKS ecosystem, a git tree including this series
and the 2 use cases can be found here:

PKS enables protections on 'domains' of supervisor pages to limit supervisor
mode access to those pages beyond the normal paging protections. PKS works in
a similar fashion to user space pkeys, PKU. As with PKU, supervisor pkeys are
checked in addition to normal paging protections and Access or Writes can be
disabled via a MSR update without TLB flushes when permissions change. Also
like PKU, a page mapping is assigned to a domain by setting pkey bits in the
page table entry for that mapping.

Access is controlled through a PKRS register which is updated via WRMSR/RDMSR.

XSAVE is not supported for the PKRS MSR. Therefore the implementation
saves/restores the MSR across context switches and during exceptions. Nested
exceptions are supported by each exception getting a new PKS state.

For consistent behavior with current paging protections, pkey 0 is reserved and
configured to allow full access via the pkey mechanism, thus preserving the
default paging protections on mappings with the default pkey value of 0.

Other keys, (1-15) are allocated by an allocator which prepares us for key
contention from day one. Kernel users should be prepared for the allocator to
fail either because of key exhaustion or due to PKS not being supported on the
arch and/or CPU instance.

The following are key attributes of PKS.

1) Fast switching of permissions
1a) Prevents access without page table manipulations
1b) No TLB flushes required
2) Works on a per thread basis

PKS is available with 4 and 5 level paging. Like PKRU it consumes 4 bits from
the PTE to store the pkey within the entry.

and a testing patch

Fenghua Yu (3):
x86/fpu: Refactor arch_set_user_pkey_access() for PKS support
x86/pks: Enable Protection Keys Supervisor (PKS)
x86/pks: Add PKS kernel API

Ira Weiny (6):
x86/pkeys: Create pkeys_common.h
x86/pks: Preserve the PKRS MSR on context switch
x86/entry: Pass irqentry_state_t by reference
x86/entry: Preserve PKRS MSR across exceptions
x86/fault: Report the PKRS state on fault
x86/pks: Add PKS test code

Documentation/core-api/protection-keys.rst | 102 ++-
arch/x86/Kconfig | 1 +
arch/x86/entry/common.c | 57 +-
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/idtentry.h | 29 +-
arch/x86/include/asm/msr-index.h | 1 +
arch/x86/include/asm/pgtable.h | 13 +-
arch/x86/include/asm/pgtable_types.h | 12 +
arch/x86/include/asm/pkeys.h | 15 +
arch/x86/include/asm/pkeys_common.h | 36 +
arch/x86/include/asm/processor.h | 13 +
arch/x86/include/uapi/asm/processor-flags.h | 2 +
arch/x86/kernel/cpu/common.c | 17 +
arch/x86/kernel/cpu/mce/core.c | 4 +
arch/x86/kernel/fpu/xstate.c | 22 +-
arch/x86/kernel/kvm.c | 4 +-
arch/x86/kernel/nmi.c | 7 +-
arch/x86/kernel/process.c | 21 +
arch/x86/kernel/traps.c | 21 +-
arch/x86/mm/fault.c | 86 ++-
arch/x86/mm/pkeys.c | 188 +++++-
include/linux/entry-common.h | 19 +-
include/linux/pgtable.h | 4 +
include/linux/pkeys.h | 23 +-
kernel/entry/common.c | 28 +-
lib/Kconfig.debug | 12 +
lib/Makefile | 3 +
lib/pks/Makefile | 3 +
lib/pks/pks_test.c | 690 ++++++++++++++++++++
mm/Kconfig | 2 +
tools/testing/selftests/x86/Makefile | 3 +-
tools/testing/selftests/x86/test_pks.c | 65 ++
32 files changed, 1376 insertions(+), 128 deletions(-)
create mode 100644 arch/x86/include/asm/pkeys_common.h
create mode 100644 lib/pks/Makefile
create mode 100644 lib/pks/pks_test.c
create mode 100644 tools/testing/selftests/x86/test_pks.c


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