[GIT PULL] x86/urgent for v6.5-rc2

[Peter Zijlstra]

Hi Linus,

please pull a bunch of CFI fixes for 6.5-rc2.

Thanks!

(also yeah, sometimes I can't type -- I only now noticed :/)

---

The following changes since commit 06c2afb862f9da8dc5efa4b6076a0e48c3fbaaa5:

  Linux 6.5-rc1 (2023-07-09 13:53:13 -0700)

are available in the Git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git tags/x86_urgent_for_6.5_rc2

for you to fetch changes up to 535d0ae39185a266536a1e97ff9a8956d7fbb9df:

  x86/cfi: Only define poison_cfi() if CONFIG_X86_KERNEL_IBT=y (2023-07-11 10:17:55 +0200)

----------------------------------------------------------------
Fix kCFI/FineIBT weaknesses

The primary bug Alyssa noticed was that with FineIBT enabled function
prologues have a spurious ENDBR instruction:

  __cfi_foo:
	endbr64
	subl	$hash, %r10d
	jz	1f
	ud2
	nop
  1:
  foo:
	endbr64 <--- *sadface*

This means that any indirect call that fails to target the __cfi symbol
and instead targets (the regular old) foo+0, will succeed due to that
second ENDBR.

Fixing this lead to the discovery of a single indirect call that was
still doing this: ret_from_fork(), since that's an assembly stub the
compmiler would not generate the proper kCFI indirect call magic and it
would not get patched.

Brian came up with the most comprehensive fix -- convert the thing to C
with only a very thin asm wrapper. This ensures the kernel thread
boostrap is a proper kCFI call.

While discussing all this, Kees noted that kCFI hashes could/should be
poisoned to seal all functions whose address is never taken, further
limiting the valid kCFI targets -- much like we already do for IBT.

So what was a 'simple' observation and fix cascaded into a bunch of
inter-related CFI infrastructure fixes.

----------------------------------------------------------------
Brian Gerst (2):
      x86/32: Remove schedule_tail_wrapper()
      x86: Rewrite ret_from_fork() in C

Ingo Molnar (1):
      x86/cfi: Only define poison_cfi() if CONFIG_X86_KERNEL_IBT=y

Peter Zijlstra (4):
      x86/cfi: Extend {JMP,CAKK}_NOSPEC comment
      x86/alternative: Rename apply_ibt_endbr()
      x86/cfi: Extend ENDBR sealing to kCFI
      x86/fineibt: Poison ENDBR at +0

 arch/um/kernel/um_arch.c             |  2 +-
 arch/x86/entry/entry_32.S            | 53 +++++++--------------------
 arch/x86/entry/entry_64.S            | 33 ++++-------------
 arch/x86/include/asm/alternative.h   |  2 +-
 arch/x86/include/asm/ibt.h           |  2 +-
 arch/x86/include/asm/nospec-branch.h |  4 ++
 arch/x86/include/asm/switch_to.h     |  4 +-
 arch/x86/kernel/alternative.c        | 71 ++++++++++++++++++++++++++++++++++--
 arch/x86/kernel/module.c             |  2 +-
 arch/x86/kernel/process.c            | 22 ++++++++++-
 10 files changed, 120 insertions(+), 75 deletions(-)