Add a macro which prevents a function from getting sealed if there are
no compile-time references to it.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
v1.1:
- add empty IBT_NOSEAL for CONFIG_X86_KERNEL_IBT=n

 arch/x86/include/asm/ibt.h | 11 +++++++++++
 tools/objtool/check.c      |  3 ++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
index 689880eca9ba..9b08082a5d9f 100644
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -31,6 +31,16 @@
 
 #define __noendbr	__attribute__((nocf_check))
 
+/*
+ * Create a dummy function pointer reference to prevent objtool from marking
+ * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
+ * apply_ibt_endbr()).
+ */
+#define IBT_NOSEAL(fname)				\
+	".pushsection .discard.ibt_endbr_noseal\n\t"	\
+	_ASM_PTR fname "\n\t"				\
+	".popsection\n\t"
+
 static inline __attribute_const__ u32 gen_endbr(void)
 {
 	u32 endbr;
@@ -84,6 +94,7 @@ extern __noendbr void ibt_restore(u64 save);
 #ifndef __ASSEMBLY__
 
 #define ASM_ENDBR
+#define IBT_NOSEAL(name)
 
 #define __noendbr
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0cec74da7ffe..91678252a9b6 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4096,7 +4096,8 @@ static int validate_ibt(struct objtool_file *file)
 		 * These sections can reference text addresses, but not with
 		 * the intent to indirect branch to them.
 		 */
-		if (!strncmp(sec->name, ".discard", 8)			||
+		if ((!strncmp(sec->name, ".discard", 8) &&
+		     strcmp(sec->name, ".discard.ibt_endbr_noseal"))	||
 		    !strncmp(sec->name, ".debug", 6)			||
 		    !strcmp(sec->name, ".altinstructions")		||
 		    !strcmp(sec->name, ".ibt_endbr_seal")		||
-- 
2.37.2

Hi Poimboeuf,
  I installed your patches based on v5.19 kernel.

  And ran syzkaller test on TGL-H and ADL-P for more than
  4 hours with above kernel, this issue could not be reproduced.
  This issue should be fixed.

  Thanks!
  BR.

On 2022-08-18 at 14:39:27 -0700, Josh Poimboeuf wrote:
> Add a macro which prevents a function from getting sealed if there are
> no compile-time references to it.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
> v1.1:
> - add empty IBT_NOSEAL for CONFIG_X86_KERNEL_IBT=n
> 
>  arch/x86/include/asm/ibt.h | 11 +++++++++++
>  tools/objtool/check.c      |  3 ++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
> index 689880eca9ba..9b08082a5d9f 100644
> --- a/arch/x86/include/asm/ibt.h
> +++ b/arch/x86/include/asm/ibt.h
> @@ -31,6 +31,16 @@
>  
>  #define __noendbr	__attribute__((nocf_check))
>  
> +/*
> + * Create a dummy function pointer reference to prevent objtool from marking
> + * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
> + * apply_ibt_endbr()).
> + */
> +#define IBT_NOSEAL(fname)				\
> +	".pushsection .discard.ibt_endbr_noseal\n\t"	\
> +	_ASM_PTR fname "\n\t"				\
> +	".popsection\n\t"
> +
>  static inline __attribute_const__ u32 gen_endbr(void)
>  {
>  	u32 endbr;
> @@ -84,6 +94,7 @@ extern __noendbr void ibt_restore(u64 save);
>  #ifndef __ASSEMBLY__
>  
>  #define ASM_ENDBR
> +#define IBT_NOSEAL(name)
>  
>  #define __noendbr
>  
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 0cec74da7ffe..91678252a9b6 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -4096,7 +4096,8 @@ static int validate_ibt(struct objtool_file *file)
>  		 * These sections can reference text addresses, but not with
>  		 * the intent to indirect branch to them.
>  		 */
> -		if (!strncmp(sec->name, ".discard", 8)			||
> +		if ((!strncmp(sec->name, ".discard", 8) &&
> +		     strcmp(sec->name, ".discard.ibt_endbr_noseal"))	||
>  		    !strncmp(sec->name, ".debug", 6)			||
>  		    !strcmp(sec->name, ".altinstructions")		||
>  		    !strcmp(sec->name, ".ibt_endbr_seal")		||
> -- 
> 2.37.2
>