[tip: x86/bugs] objtool: Fix return thunk patching in retpolines

[tip-bot2 for Josh Poimboeuf]

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     3ab1bb69862d4477e2ffa556075a251bd7328910
Gitweb:        https://git.kernel.org/tip/3ab1bb69862d4477e2ffa556075a251bd7328910
Author:        Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate:    Wed, 11 Oct 2023 19:47:37 -07:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 12 Oct 2023 08:21:29 +02:00

objtool: Fix return thunk patching in retpolines

With CONFIG_RETHUNK enabled, the compiler replaces every RET with a tail
call to a return thunk ('JMP __x86_return_thunk').  Objtool annotates
all such return sites so they can be patched during boot by
apply_returns().

The implementation of __x86_return_thunk() is just a bare RET.  It's
only meant to be used temporarily until apply_returns() patches all
return sites with either a JMP to another return thunk or an actual RET.

The following commit:

  e92626af3234 ("x86/retpoline: Remove .text..__x86.return_thunk section")

broke objtool's detection of return sites in retpolines.  Since
retpolines and return thunks are now in the same section, the compiler
no longer uses relocations for the intra-section jumps between the
retpolines and the return thunk, causing objtool to overlook them.

As a result, none of the retpolines' return sites get patched.  Each one
stays at 'JMP __x86_return_thunk', effectively a bare RET.

Fix it by teaching objtool to detect when a non-relocated jump target is
a return thunk (or retpoline).

Fixes: e92626af3234 ("x86/retpoline: Remove .text..__x86.return_thunk section")
Reported-by: David Kaplan <david.kaplan@xxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231012024737.eg5phclogp67ik6x@treble
---
 tools/objtool/check.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e308d1b..e94756e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1611,6 +1611,22 @@ static int add_jump_destinations(struct objtool_file *file)
 		}
 
 		/*
+		 * An intra-TU jump in retpoline.o might not have a relocation
+		 * for its jump dest, in which case the above
+		 * add_{retpoline,return}_call() didn't happen.
+		 */
+		if (jump_dest->sym && jump_dest->offset == jump_dest->sym->offset) {
+			if (jump_dest->sym->retpoline_thunk) {
+				add_retpoline_call(file, insn);
+				continue;
+			}
+			if (jump_dest->sym->return_thunk) {
+				add_return_call(file, insn, true);
+				continue;
+			}
+		}
+
+		/*
 		 * Cross-function jump.
 		 */
 		if (insn_func(insn) && insn_func(jump_dest) &&