[v3,0/5] KVM: rseq: Fix and a test for a KVM+rseq bug

Message

Sean Christopherson Sept. 1, 2021, 8:30 p.m. UTC

Patch 1 fixes a KVM+rseq bug where KVM's handling of TIF_NOTIFY_RESUME,
e.g. for task migration, clears the flag without informing rseq and leads
to stale data in userspace's rseq struct.

Patch 2 is a cleanup to try and make future bugs less likely.  It's also
a baby step towards moving and renaming tracehook_notify_resume() since
it has nothing to do with tracing.

Patch 3 is a fix/cleanup to stop overriding x86's unistd_{32,64}.h when
the include path (intentionally) omits tools' uapi headers.  KVM's
selftests do exactly that so that they can pick up the uapi headers from
the installed kernel headers, and still use various tools/ headers that
mirror kernel code, e.g. linux/types.h.  This allows the new test in
patch 4 to reference __NR_rseq without having to manually define it.

Patch 4 is a regression test for the KVM+rseq bug.

Patch 5 is a cleanup made possible by patch 3.

Based on commit 835d31d319d9 ("Merge tag 'media/v5.15-1' of ...").

v3:
  - Collect Ack/Review. [Mathieu, Ben]
  - Add explicit smp_wmb() instead of relying on atomic_inc() to do a full
    barrier. [Mathieu]
  - Add lots and lots of comments in the selftest, especially around why
    the migration thread needs a udelay(). [Mathieu]
  - Delay between 1us and 10us to reduce the odds of having a hard
    dependency on arch/kernel behavior.  [Mathieu]
  - Dropped an s390 change in patch 2 after a rebase to upstream master.

v2:
  - https://lkml.kernel.org/r/[e-mail]
  - Don't touch rseq_cs when handling KVM case so that rseq_syscall() will
    still detect a naughty userspace. [Mathieu]
  - Use a sequence counter + retry in the test to ensure the process isn't
    migrated between sched_getcpu() and reading rseq.cpu_id, i.e. to
    avoid a flaky test. [Mathieu]
  - Add Mathieu's ack for patch 2.
  - Add more comments in the test.

v1: https://lkml.kernel.org/r/[e-mail]

Sean Christopherson (5):
  KVM: rseq: Update rseq when processing NOTIFY_RESUME on xfer to KVM
    guest
  entry: rseq: Call rseq_handle_notify_resume() in
    tracehook_notify_resume()
  tools: Move x86 syscall number fallbacks to .../uapi/
  KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration
    bugs
  KVM: selftests: Remove __NR_userfaultfd syscall fallback

 arch/arm/kernel/signal.c                      |   1 -
 arch/arm64/kernel/signal.c                    |   1 -
 arch/csky/kernel/signal.c                     |   4 +-
 arch/mips/kernel/signal.c                     |   4 +-
 arch/powerpc/kernel/signal.c                  |   4 +-
 include/linux/tracehook.h                     |   2 +
 kernel/entry/common.c                         |   4 +-
 kernel/rseq.c                                 |  14 +-
 .../x86/include/{ => uapi}/asm/unistd_32.h    |   0
 .../x86/include/{ => uapi}/asm/unistd_64.h    |   3 -
 tools/testing/selftests/kvm/.gitignore        |   1 +
 tools/testing/selftests/kvm/Makefile          |   3 +
 tools/testing/selftests/kvm/rseq_test.c       | 236 ++++++++++++++++++
 13 files changed, 257 insertions(+), 20 deletions(-)
 rename tools/arch/x86/include/{ => uapi}/asm/unistd_32.h (100%)
 rename tools/arch/x86/include/{ => uapi}/asm/unistd_64.h (83%)
 create mode 100644 tools/testing/selftests/kvm/rseq_test.c