objtool: Add entry UNRET validation

commit a09a6e2399ba0595c3042b3164f3ca68a3cff33e upstream.

Since entry asm is tricky, add a validation pass that ensures the
retbleed mitigation has been done before the first actual RET
instruction.

Entry points are those that either have UNWIND_HINT_ENTRY, which acts
as UNWIND_HINT_EMPTY but marks the instruction as an entry point, or
those that have UWIND_HINT_IRET_REGS at +0.

This is basically a variant of validate_branch() that is
intra-function and it will simply follow all branches from marked
entry points and ensures that all paths lead to ANNOTATE_UNRET_END.

If a path hits RET or an indirection the path is a fail and will be
reported.

There are 3 ANNOTATE_UNRET_END instances:

 - UNTRAIN_RET itself
 - exception from-kernel; this path doesn't need UNTRAIN_RET
 - all early exceptions; these also don't need UNTRAIN_RET

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
[cascardo: arch/x86/entry/entry_64.S no pt_regs return at .Lerror_entry_done_lfence]
[cascardo: tools/objtool/builtin-check.c no link option validation]
[cascardo: tools/objtool/check.c opts.ibt is ibt]
[cascardo: tools/objtool/include/objtool/builtin.h leave unret option as bool, no struct opts]
[cascardo: objtool is still called from scripts/link-vmlinux.sh]
[cascardo: no IBT support]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
[bwh: Backported to 5.10:
 - In scripts/link-vmlinux.sh, use "test -n" instead of is_enabled
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

scripts/link-vmlinux.sh

@@ -65,6 +65,9 @@ objtool_link()
 
 	if [ -n "${CONFIG_VMLINUX_VALIDATION}" ]; then
 		objtoolopt="check"
+		if [ -n "${CONFIG_RETPOLINE}" ]; then
+			objtoolopt="${objtoolopt} --unret"
+		fi
 		if [ -z "${CONFIG_FRAME_POINTER}" ]; then
 			objtoolopt="${objtoolopt} --no-fp"
 		fi