x86/entry: Fix ORC unwinder for PUSH_REGS with save_ret=1

PUSH_REGS with save_ret=1 is used by interrupt entry helper functions that
initially start with a UNWIND_HINT_FUNC ORC state.

However, save_ret=1 means that we clobber the helper function's return
address (and then later restore the return address further down on the
stack); after that point, the only thing on the stack we can unwind through
is the IRET frame, so use UNWIND_HINT_IRET_REGS until we have a full
pt_regs frame.

( An alternate approach would be to move the pt_regs->di overwrite down
  such that it is the final step of pt_regs setup; but I don't want to
  rearrange entry code just to make unwinding a tiny bit more elegant. )

Fixes: 9e809d1 ("x86/entry: Reduce the code footprint of the 'idtentry' macro")
Signed-off-by: Jann Horn <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: thejh

arch/x86/entry/calling.h

@@ -70,6 +70,8 @@ For 32-bit we have the following conventions - kernel is built with
 	pushq	%rsi		/* pt_regs->si */
 	movq	8(%rsp), %rsi	/* temporarily store the return address in %rsi */
 	movq	%rdi, 8(%rsp)	/* pt_regs->di (overwriting original return address) */
+	/* We just clobbered the return address - use the IRET frame for unwinding: */
+	UNWIND_HINT_IRET_REGS offset=3*8
 	.else
 	pushq   %rdi		/* pt_regs->di */
 	pushq   %rsi		/* pt_regs->si */