x86/its: Add support for ITS-safe return thunk

commit a75bf27fe41abe658c53276a0c486c4bf9adecfc upstream.

RETs in the lower half of cacheline may be affected by ITS bug,
specifically when the RSB-underflows. Use ITS-safe return thunk for such
RETs.

RETs that are not patched:

- RET in retpoline sequence does not need to be patched, because the
  sequence itself fills an RSB before RET.
- RET in Call Depth Tracking (CDT) thunks __x86_indirect_{call|jump}_thunk
  and call_depth_return_thunk are not patched because CDT by design
  prevents RSB-underflow.
- RETs in .init section are not reachable after init.
- RETs that are explicitly marked safe with ANNOTATE_UNRET_SAFE.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 9542c582d546b2ae270f3568794c57de4478d0b4..2112939460856456314bee31f55b56aa91f5b4aa 100644 (file)
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -81,6 +81,20 @@ extern void apply_ibt_endbr(s32 *start, s32 *end);
 
 struct module;
 
+#if defined(CONFIG_RETHUNK) && defined(CONFIG_OBJTOOL)
+extern bool cpu_wants_rethunk(void);
+extern bool cpu_wants_rethunk_at(void *addr);
+#else
+static __always_inline bool cpu_wants_rethunk(void)
+{
+       return false;
+}
+static __always_inline bool cpu_wants_rethunk_at(void *addr)
+{
+       return false;
+}
+#endif
+
 #ifdef CONFIG_SMP
 extern void alternatives_smp_module_add(struct module *mod, char *name,
                                        void *locks, void *locks_end,

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index b5ad7f5f731883f9710f329cfd2e40302274c995..cccd5072e09f3d13536de03fda2d10b5e137d55c 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -257,6 +257,12 @@ extern void __x86_return_thunk(void);
 static inline void __x86_return_thunk(void) {}
 #endif
 
+#ifdef CONFIG_MITIGATION_ITS
+extern void its_return_thunk(void);
+#else
+static inline void its_return_thunk(void) {}
+#endif
+
 extern void retbleed_return_thunk(void);
 extern void srso_return_thunk(void);
 extern void srso_alias_return_thunk(void);

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 0cb30cefb8edbc680506a088c1ce274ad4d0a13c..d70129eea9237350dc66dcc099ca39700361d713 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -614,6 +614,21 @@ void __init_or_module noinline apply_retpolines(s32 *start, s32 *end)
 
 #ifdef CONFIG_RETHUNK
 
+bool cpu_wants_rethunk(void)
+{
+       return cpu_feature_enabled(X86_FEATURE_RETHUNK);
+}
+
+bool cpu_wants_rethunk_at(void *addr)
+{
+       if (!cpu_feature_enabled(X86_FEATURE_RETHUNK))
+               return false;
+       if (x86_return_thunk != its_return_thunk)
+               return true;
+
+       return !((unsigned long)addr & 0x20);
+}
+
 /*
  * Rewrite the compiler generated return thunk tail-calls.
  *
@@ -629,7 +644,7 @@ static int patch_return(void *addr, struct insn *insn, u8 *bytes)
 {
        int i = 0;
 
-       if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
+       if (cpu_wants_rethunk_at(addr)) {
                if (x86_return_thunk == __x86_return_thunk)
                        return -1;
 

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index ec51ce713dea45deb89ef67ca85b50aceebd2370..e737437015303a5bc4d467820e4e1b266fcbc04c 100644 (file)
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -360,7 +360,7 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
                goto fail;
 
        ip = trampoline + size;
-       if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
+       if (cpu_wants_rethunk_at(ip))
                __text_gen_insn(ip, JMP32_INSN_OPCODE, ip, x86_return_thunk, JMP32_INSN_SIZE);
        else
                memcpy(ip, retq, sizeof(retq));

diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index ca8dfc0c9edba767e8427900e658d7a6ace4be41..93809a4dc79364c3322dc751cf35e82221ff8f6d 100644 (file)
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -79,7 +79,7 @@ static void __ref __static_call_transform(void *insn, enum insn_type type,
                break;
 
        case RET:
-               if (cpu_feature_enabled(X86_FEATURE_RETHUNK))
+               if (cpu_wants_rethunk_at(insn))
                        code = text_gen_insn(JMP32_INSN_OPCODE, insn, x86_return_thunk);
                else
                        code = &retinsn;

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 778ea89b737c4d182369f948edb98a1059a1889e..35aeec9d226718e624795735f0b2f14746be842b 100644 (file)
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -534,6 +534,10 @@ INIT_PER_CPU(irq_stack_backing_store);
 . = ASSERT(__x86_indirect_its_thunk_array == __x86_indirect_its_thunk_rax, "Gap in ITS thunk array");
 #endif
 
+#if defined(CONFIG_MITIGATION_ITS) && !defined(CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B)
+. = ASSERT(its_return_thunk & 0x20, "its_return_thunk not in second half of cacheline");
+#endif
+
 #endif /* CONFIG_X86_64 */
 
 #ifdef CONFIG_KEXEC_CORE

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index be948f44fc2416cad4c2eec6eae01177a32a159d..e426615000948b1e9f9f8be60ddc7728d3df2b1d 100644 (file)
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -284,7 +284,18 @@ SYM_CODE_START(__x86_indirect_its_thunk_array)
        .align 64, 0xcc
 SYM_CODE_END(__x86_indirect_its_thunk_array)
 
-#endif
+.align 64, 0xcc
+.skip 32, 0xcc
+SYM_CODE_START(its_return_thunk)
+       UNWIND_HINT_FUNC
+       ANNOTATE_NOENDBR
+       ANNOTATE_UNRET_SAFE
+       ret
+       int3
+SYM_CODE_END(its_return_thunk)
+EXPORT_SYMBOL(its_return_thunk)
+
+#endif /* CONFIG_MITIGATION_ITS */
 
 SYM_CODE_START(__x86_return_thunk)
        UNWIND_HINT_FUNC

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index c73d85255bbca1fcebe9f291994036c432caf8ad..b5fd521a7fa05ed61fae0c0bd00db02b3a2aeca6 100644 (file)
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -487,7 +487,7 @@ static void emit_return(u8 **pprog, u8 *ip)
 {
        u8 *prog = *pprog;
 
-       if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
+       if (cpu_wants_rethunk()) {
                emit_jump(&prog, x86_return_thunk, ip);
        } else {
                EMIT1(0xC3);            /* ret */