[tip: objtool/urgent] objtool, lkdtm: Obfuscate the do_nothing() pointer

["tip-bot2 for Josh Poimboeuf" <tip-bot2@linutronix.de>]

The following commit has been merged into the objtool/urgent branch of tip:

Commit-ID:     42e5650944b8cd4361385cc9414f03a6d6748989
Gitweb:        https://git.kernel.org/tip/42e5650944b8cd4361385cc9414f03a6d6748989
Author:        Josh Poimboeuf <jpoimboe@kernel.org>
AuthorDate:    Mon, 24 Mar 2025 14:56:12 -07:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 27 Mar 2025 11:59:11 +01:00

objtool, lkdtm: Obfuscate the do_nothing() pointer

If execute_location()'s memcpy of do_nothing() gets inlined and unrolled
by the compiler, it copies one word at a time:

    mov    0x0(%rip),%rax    R_X86_64_PC32    .text+0x1374
    mov    %rax,0x38(%rbx)
    mov    0x0(%rip),%rax    R_X86_64_PC32    .text+0x136c
    mov    %rax,0x30(%rbx)
    ...

Those .text references point to the middle of the function, causing
objtool to complain about their lack of ENDBR.

Prevent that by resolving the function pointer at runtime rather than
build time.  This fixes the following warning:

  drivers/misc/lkdtm/lkdtm.o: warning: objtool: execute_location+0x23: relocation to !ENDBR: .text+0x1378

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/30b9abffbddeb43c4f6320b1270fa9b4d74c54ed.1742852847.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/202503191453.uFfxQy5R-lkp@intel.com/
---
 drivers/misc/lkdtm/perms.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 5b861db..6c24426 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -29,6 +29,13 @@ static const unsigned long rodata = 0xAA55AA55;
 static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
 
 /*
+ * This is a pointer to do_nothing() which is initialized at runtime rather
+ * than build time to avoid objtool IBT validation warnings caused by an
+ * inlined unrolled memcpy() in execute_location().
+ */
+static void __ro_after_init *do_nothing_ptr;
+
+/*
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
  */
@@ -65,13 +72,12 @@ static noinline __nocfi void execute_location(void *dst, bool write)
 {
 	void (*func)(void);
 	func_desc_t fdesc;
-	void *do_nothing_text = dereference_function_descriptor(do_nothing);
 
-	pr_info("attempting ok execution at %px\n", do_nothing_text);
+	pr_info("attempting ok execution at %px\n", do_nothing_ptr);
 	do_nothing();
 
 	if (write == CODE_WRITE) {
-		memcpy(dst, do_nothing_text, EXEC_SIZE);
+		memcpy(dst, do_nothing_ptr, EXEC_SIZE);
 		flush_icache_range((unsigned long)dst,
 				   (unsigned long)dst + EXEC_SIZE);
 	}
@@ -267,6 +273,8 @@ static void lkdtm_ACCESS_NULL(void)
 
 void __init lkdtm_perms_init(void)
 {
+	do_nothing_ptr = dereference_function_descriptor(do_nothing);
+
 	/* Make sure we can write to __ro_after_init values during __init */
 	ro_after_init |= 0xAA;
 }