From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
Date: Thu, 28 Mar 2024 13:59:05 +0100
Commit 4535e1a4174c4111d92c5a9a21e542d232e0fcaa upstream.
The original version of the mitigation would patch in the calls to the
untraining routines directly. That is, the alternative() in UNTRAIN_RET
will patch in the CALL to srso_alias_untrain_ret() directly.
However, even if commit e7c25c441e9e ("x86/cpu: Cleanup the untrain
mess") meant well in trying to clean up the situation, due to micro-
architectural reasons, the untraining routine srso_alias_untrain_ret()
must be the target of a CALL instruction and not of a JMP instruction as
it is done now.
Reshuffle the alternative macros to accomplish that.
Fixes: e7c25c441e9e ("x86/cpu: Cleanup the untrain mess")
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/asm-prototypes.h | 1 +
arch/x86/include/asm/nospec-branch.h | 20 ++++++++++++++------
arch/x86/lib/retpoline.S | 4 +---
3 files changed, 16 insertions(+), 9 deletions(-)
diff --git a/arch/x86/include/asm/asm-prototypes.h b/arch/x86/include/asm/asm-prototypes.h
index 8f80de627c60..5cdccea45554 100644
--- a/arch/x86/include/asm/asm-prototypes.h
+++ b/arch/x86/include/asm/asm-prototypes.h
@@ -12,6 +12,7 @@
#include <asm/special_insns.h>
#include <asm/preempt.h>
#include <asm/asm.h>
+#include <asm/nospec-branch.h>
#ifndef CONFIG_X86_CMPXCHG64
extern void cmpxchg8b_emu(void);
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 7711ba5342a1..6be8e76ac696 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -155,11 +155,20 @@
.Lskip_rsb_\@:
.endm
+/*
+ * The CALL to srso_alias_untrain_ret() must be patched in directly at
+ * the spot where untraining must be done, ie., srso_alias_untrain_ret()
+ * must be the target of a CALL instruction instead of indirectly
+ * jumping to a wrapper which then calls it. Therefore, this macro is
+ * called outside of __UNTRAIN_RET below, for the time being, before the
+ * kernel can support nested alternatives with arbitrary nesting.
+ */
+.macro CALL_UNTRAIN_RET
#ifdef CONFIG_CPU_UNRET_ENTRY
-#define CALL_UNTRAIN_RET "call entry_untrain_ret"
-#else
-#define CALL_UNTRAIN_RET ""
+ ALTERNATIVE_2 "", "call entry_untrain_ret", X86_FEATURE_UNRET, \
+ "call srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
#endif
+.endm
/*
* Mitigate RETBleed for AMD/Hygon Zen uarch. Requires KERNEL CR3 because the
@@ -176,9 +185,8 @@
#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
defined(CONFIG_CPU_SRSO)
ANNOTATE_UNRET_END
- ALTERNATIVE_2 "", \
- CALL_UNTRAIN_RET, X86_FEATURE_UNRET, \
- "call entry_ibpb", X86_FEATURE_ENTRY_IBPB
+ CALL_UNTRAIN_RET
+ ALTERNATIVE "", "call entry_ibpb", X86_FEATURE_ENTRY_IBPB
#endif
.endm
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 6f5321b36dbb..8acafe60220a 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -249,9 +249,7 @@ SYM_CODE_START(srso_return_thunk)
SYM_CODE_END(srso_return_thunk)
SYM_FUNC_START(entry_untrain_ret)
- ALTERNATIVE_2 "jmp retbleed_untrain_ret", \
- "jmp srso_untrain_ret", X86_FEATURE_SRSO, \
- "jmp srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
+ ALTERNATIVE "jmp retbleed_untrain_ret", "jmp srso_untrain_ret", X86_FEATURE_SRSO
SYM_FUNC_END(entry_untrain_ret)
__EXPORT_THUNK(entry_untrain_ret)
--
2.43.0
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
