x86: Add magic AMD return-thunk

commit a149180fbcf336e97ce4eb2cdc13672727feb94d upstream.

Note: needs to be in a section distinct from Retpolines such that the
Retpoline RET substitution cannot possibly use immediate jumps.

ORC unwinding for zen_untrain_ret() and __x86_return_thunk() is a
little tricky but works due to the fact that zen_untrain_ret() doesn't
have any stack ops and as such will emit a single ORC entry at the
start (+0x3f).

Meanwhile, unwinding an IP, including the __x86_return_thunk() one
(+0x40) will search for the largest ORC entry smaller or equal to the
IP, these will find the one ORC entry (+0x3f) and all works.

  [ Alexandre: SVM part. ]
  [ bp: Build fix, massages. ]

Suggested-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
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: conflicts at arch/x86/entry/entry_64_compat.S]
[cascardo: there is no ANNOTATE_NOENDBR]
[cascardo: objtool commit 34c861e806478ac2ea4032721defbf1d6967df08 missing]
[cascardo: conflict fixup]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 17 insertions, 3 deletions

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index a955cf4cf932..a90f0d657716 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1145,7 +1145,7 @@ static void add_retpoline_call(struct objtool_file *file, struct instruction *in
 	annotate_call_site(file, insn, false);
 }
 
-static void add_return_call(struct objtool_file *file, struct instruction *insn)
+static void add_return_call(struct objtool_file *file, struct instruction *insn, bool add)
 {
 	/*
 	 * Return thunk tail calls are really just returns in disguise,
@@ -1155,7 +1155,7 @@ static void add_return_call(struct objtool_file *file, struct instruction *insn)
 	insn->retpoline_safe = true;
 
 	/* Skip the non-text sections, specially .discard ones */
-	if (insn->sec->text)
+	if (add && insn->sec->text)
 		list_add_tail(&insn->call_node, &file->return_thunk_list);
 }
 
@@ -1184,7 +1184,7 @@ static int add_jump_destinations(struct objtool_file *file)
 			add_retpoline_call(file, insn);
 			continue;
 		} else if (reloc->sym->return_thunk) {
-			add_return_call(file, insn);
+			add_return_call(file, insn, true);
 			continue;
 		} else if (insn->func) {
 			/* internal or external sibling call (with reloc) */
@@ -1201,6 +1201,7 @@ static int add_jump_destinations(struct objtool_file *file)
 
 		insn->jump_dest = find_insn(file, dest_sec, dest_off);
 		if (!insn->jump_dest) {
+			struct symbol *sym = find_symbol_by_offset(dest_sec, dest_off);
 
 			/*
 			 * This is a special case where an alt instruction
@@ -1210,6 +1211,19 @@ static int add_jump_destinations(struct objtool_file *file)
 			if (!strcmp(insn->sec->name, ".altinstr_replacement"))
 				continue;
 
+			/*
+			 * This is a special case for zen_untrain_ret().
+			 * It jumps to __x86_return_thunk(), but objtool
+			 * can't find the thunk's starting RET
+			 * instruction, because the RET is also in the
+			 * middle of another instruction.  Objtool only
+			 * knows about the outer instruction.
+			 */
+			if (sym && sym->return_thunk) {
+				add_return_call(file, insn, false);
+				continue;
+			}
+
 			WARN_FUNC("can't find jump dest instruction at %s+0x%lx",
 				  insn->sec, insn->offset, dest_sec->name,
 				  dest_off);