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.
- 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 a364971967c40e2673624917f0ddea2a8ed6f95e..4038b893449a7d38f4079e213a924493e67f4231 100644 (file)
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -80,6 +80,20 @@ extern void apply_returns(s32 *start, s32 *end);
 
 struct module;
 
+#ifdef CONFIG_RETHUNK
+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 37b66b45337102e44826d76e20798958d6cbe5ff..17156b61fcc32d503bb52ad4703c4c121d5ab3cc 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -250,6 +250,12 @@ 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 5951c77723787401f8ddb470fdcda76a488ca524..c3df557be55e37e256d05a83f55e4ebfdee9d451 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -605,6 +605,21 @@ 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.
  *
@@ -621,7 +636,7 @@ static int patch_return(void *addr, struct insn *insn, u8 *bytes)
        int i = 0;
 
        /* Patch the custom return thunks... */
-       if (cpu_feature_enabled(X86_FEATURE_RETHUNK)) {
+       if (cpu_wants_rethunk_at(addr)) {
                i = JMP32_INSN_SIZE;
                __text_gen_insn(bytes, JMP32_INSN_OPCODE, addr, x86_return_thunk, i);
        } else {

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 85c09843df1b9e373039572a9e1441e5d20e33da..c15e3bdc61e300a8cecef46db842afb44da7c011 100644 (file)
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -367,7 +367,7 @@ 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 a5dd11c92d055c1db17664f63bf012a83895d3c4..74eb1d6c7bb0d0aad778a89b98da5a3d28c0e57c 100644 (file)
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -81,7 +81,7 @@ 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 f85810b435b9bf4faca58e0293af877d31d98662..c570da8be0307744de7c986fa7f1bae9462f6f37 100644 (file)
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -538,6 +538,10 @@ 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 4a037315e9e9f942809be7fb8fd9f0b91430e50e..ae0151c6caba57b885e191f8ce93a4c47e535948 100644 (file)
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -280,7 +280,18 @@ 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 a3e974a956f141863bb2295fa9917b6d486a342a..8be74c2aa7b895604855b1d8e340eddc004694e3 100644 (file)
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -466,7 +466,7 @@ 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 */