[tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't used at runtime

[tip-bot2 for David Kaplan]

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

Commit-ID:     91174087dcc7565d8bf0d576544e42d5b1de6f39
Gitweb:        https://git.kernel.org/tip/91174087dcc7565d8bf0d576544e42d5b1de6f39
Author:        David Kaplan <david.kaplan@xxxxxxx>
AuthorDate:    Thu, 12 Oct 2023 16:10:31 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 12 Oct 2023 19:44:34 +02:00

x86/retpoline: Ensure default return thunk isn't used at runtime

All CPU bugs that require a return thunk define a special return thunk
to use (e.g., srso_return_thunk).  The default thunk,
__x86_return_thunk, should never be used after apply_returns()
completes.  Otherwise this could lead to potential speculation holes.

Enforce this by replacing this thunk with a ud2 when alternatives are
applied.  Alternative instructions are applied after apply_returns().

The default thunk is only used during kernel boot, it is not used during
module init since that occurs after apply_returns().

Signed-off-by: David Kaplan <david.kaplan@xxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20231012141031.GHZSf+V1NjjUJTc9a9@fat_crate.local
---
 arch/x86/lib/retpoline.S | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 6376d01..fe05c13 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -356,15 +356,17 @@ SYM_FUNC_END(call_depth_return_thunk)
  * This function name is magical and is used by -mfunction-return=thunk-extern
  * for the compiler to generate JMPs to it.
  *
- * This code is only used during kernel boot or module init.  All
+ * This code is only used during kernel boot.  All
  * 'JMP __x86_return_thunk' sites are changed to something else by
  * apply_returns().
+ *
+ * This thunk is turned into a ud2 to ensure it is never used at runtime.
+ * Alternative instructions are applied after apply_returns().
  */
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
-	ANNOTATE_UNRET_SAFE
-	ret
+	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE;ret),"ud2", X86_FEATURE_ALWAYS
 	int3
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)