Usermode Indirect Branch Tracking

Adds basic support for x86 userspace IBT.

IBT is part of Intel CET. It requires indirect call and jump targets
to start with an endbr{32,64} instruction, otherwise throwing #CP.

In summary, this patch does 3 things:
- Config wiring ensuring supervisor XSAVE contains IBT state
- Allow userspace to enable IBT via prctl(PR_CFI_*) for an entire thread
- Enable IBT support (ENDBR instructions) in VDSO

Unlike the arm64 BTI API:
- does not support mixed usermode (all or nothing)
- does not touch page table code
- not enabled automatically (no ELF GNU note parsing)
- temporarily disables IBT enforcement when handling signals
These can all be cleanly added later.

The main question is whether glibc is happy with this prctl syscall API.

Changes from v1:
- Removed all signal handler changes
- Removed all uapi changes
  (v1 saved state via a redundant uc_flags bit, which was a complete
  mess, and not compatible with sigframe_ia32)
- Don't allow unknown PR_CFI_* syscall flags
  see RISC-V patch: https://lore.kernel.org/lkml/20260518183918.322545-1-ripa...
- Added 32-bit support

The original usermode IBT patches were written by Yu Cheng years ago,
but are quite different (no syscall API to enable IBT, no FRED support)
https://lore.kernel.org/all/20210830182221.3535-1-yu-chen...

There is one notable gap in this patch series, to do with signals:

  000a: mov rax, 0x100a
  000f: jmp rax
  *** signal occurs ***
  *** signal handler runs, does sigreturn ***
  100a: nop

The above sequence does not crash.

With IBT, it should crash at the nop (because an endr64 is expected there).
The IBT state (WAIT_FOR_ENDBR in IA32_U_CET MSR) is not backed up to the
signal frame though.  So, when userland does a sigreturn, the CPU has
forgotten that it was doing an indirect branch before the signal.
(This specifically only occurs with signal handlers that sigreturn.)

This is because IA32_U_CET is part of XSAVE 'supervisor' state, so
regular XSAVE/XRSTOR can't access it.  Doing a manual backup is tricky.

A related problem is that the signal handler routine is not checked for
endbr preamble.

Basic IBT is better than no IBT, though.

Richard Patel (5):
  x86: add userspace IBT config option
  x86: shstk: don't clobber IBT bits in U_CET MSR
  x86: expose user IBT via PR_CFI_BRANCH_LANDING_PADS
  x86/entry/vdso: build with IBT support
  selftests/x86: test usermode IBT

 arch/x86/Kconfig                            |  18 ++
 arch/x86/entry/vdso/common/Makefile.include |   3 +-
 arch/x86/include/asm/cpufeatures.h          |   1 +
 arch/x86/include/asm/ibt.h                  |  14 ++
 arch/x86/include/asm/processor.h            |   5 +
 arch/x86/kernel/Makefile                    |   1 +
 arch/x86/kernel/cet.c                       |   3 +-
 arch/x86/kernel/cpu/common.c                |  14 +-
 arch/x86/kernel/ibt.c                       |  98 ++++++++
 arch/x86/kernel/process_64.c                |   2 +
 arch/x86/kernel/shstk.c                     |  12 +-
 tools/arch/x86/include/asm/cpufeatures.h    |   1 +
 tools/testing/selftests/x86/Makefile        |   5 +-
 tools/testing/selftests/x86/user_ibt.c      | 247 ++++++++++++++++++++
 14 files changed, 416 insertions(+), 8 deletions(-)
 create mode 100644 arch/x86/kernel/ibt.c
 create mode 100644 tools/testing/selftests/x86/user_ibt.c

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2.47.3