x86/its: FineIBT-paranoid vs ITS

commit e52c1dc7455d32c8a55f9949d300e5e87d011fa6 upstream.

FineIBT-paranoid was using the retpoline bytes for the paranoid check,
disabling retpolines, because all parts that have IBT also have eIBRS
and thus don't need no stinking retpolines.

Except... ITS needs the retpolines for indirect calls must not be in
the first half of a cacheline :-/

So what was the paranoid call sequence:

  <fineibt_paranoid_start>:
   0:   41 ba 78 56 34 12       mov    $0x12345678, %r10d
   6:   45 3b 53 f7             cmp    -0x9(%r11), %r10d
   a:   4d 8d 5b <f0>           lea    -0x10(%r11), %r11
   e:   75 fd                   jne    d <fineibt_paranoid_start+0xd>
  10:   41 ff d3                call   *%r11
  13:   90                      nop

Now becomes:

  <fineibt_paranoid_start>:
   0:   41 ba 78 56 34 12       mov    $0x12345678, %r10d
   6:   45 3b 53 f7             cmp    -0x9(%r11), %r10d
   a:   4d 8d 5b f0             lea    -0x10(%r11), %r11
   e:   2e e8 XX XX XX XX	cs call __x86_indirect_paranoid_thunk_r11

  Where the paranoid_thunk looks like:

   1d:  <ea>                    (bad)
   __x86_indirect_paranoid_thunk_r11:
   1e:  75 fd                   jne 1d
   __x86_indirect_its_thunk_r11:
   20:  41 ff eb                jmp *%r11
   23:  cc                      int3

[ dhansen: remove initialization to false ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
[ Just a portion of the original commit, in order to fix a build issue
  in stable kernels due to backports ]
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Link: https://lore.kernel.org/r/20250514113952.GB16434@noisy.programming.kicks-ass.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 16 insertions, 1 deletions

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 7a27a9e1df59..6740b839153a 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -5,6 +5,7 @@
 #include <linux/types.h>
 #include <linux/stringify.h>
 #include <asm/asm.h>
+#include <asm/bug.h>
 
 #define ALT_FLAGS_SHIFT		16
 
@@ -134,10 +135,17 @@ static __always_inline int x86_call_depth_emit_accounting(u8 **pprog,
 extern void its_init_mod(struct module *mod);
 extern void its_fini_mod(struct module *mod);
 extern void its_free_mod(struct module *mod);
+extern u8 *its_static_thunk(int reg);
 #else /* CONFIG_MITIGATION_ITS */
 static inline void its_init_mod(struct module *mod) { }
 static inline void its_fini_mod(struct module *mod) { }
 static inline void its_free_mod(struct module *mod) { }
+static inline u8 *its_static_thunk(int reg)
+{
+	WARN_ONCE(1, "ITS not compiled in");
+
+	return NULL;
+}
 #endif
 
 #if defined(CONFIG_RETHUNK) && defined(CONFIG_OBJTOOL)
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index c6d9a3882ec8..4817e424d696 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1449,6 +1449,13 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
 static void poison_cfi(void *addr) { }
 #endif
 
+u8 *its_static_thunk(int reg)
+{
+	u8 *thunk = __x86_indirect_its_thunk_array[reg];
+
+	return thunk;
+}
+
 #endif
 
 void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 9b7df9d7b22b..07592eef253c 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -475,7 +475,7 @@ static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
 	if (IS_ENABLED(CONFIG_MITIGATION_ITS) &&
 	    cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) {
 		OPTIMIZER_HIDE_VAR(reg);
-		emit_jump(&prog, &__x86_indirect_its_thunk_array[reg], ip);
+		emit_jump(&prog, its_static_thunk(reg), ip);
 	} else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) {
 		EMIT_LFENCE();
 		EMIT2(0xFF, 0xE0 + reg);