x86/fineibt: Poison ENDBR at +0

Alyssa noticed that when building the kernel with CFI_CLANG+IBT and
booting on IBT enabled hardware to obtain FineIBT, the indirect
functions look like:

  __cfi_foo:
	endbr64
	subl	$hash, %r10d
	jz	1f
	ud2
	nop
  1:
  foo:
	endbr64

This is because the compiler generates code for kCFI+IBT. In that case
the caller does the hash check and will jump to +0, so there must be
an ENDBR there. The compiler doesn't know about FineIBT at all; also
it is possible to actually use kCFI+IBT when booting with 'cfi=kcfi'
on IBT enabled hardware.

Having this second ENDBR however makes it possible to elide the CFI
check. Therefore, we should poison this second ENDBR when switching to
FineIBT mode.

Fixes: 931ab63664f0 ("x86/ibt: Implement FineIBT")
Reported-by: "Milburn, Alyssa" <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Sami Tolvanen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

0 files changed, 0 insertions, 0 deletions