binfmt_elf_fdpic: fix AUXV size calculation when ELF_HWCAP2 is defined

create_elf_fdpic_tables() does not correctly account the space for the
AUX vector when an architecture has ELF_HWCAP2 defined. Prior to the
commit 10e29251 ("binfmt_elf_fdpic: fix /proc/<pid>/auxv") it
resulted in the last entry of the AUX vector being set to zero, but with
that change it results in a kernel BUG.

Fix that by adding one to the number of AUXV entries (nitems) when
ELF_HWCAP2 is defined.

Fixes: 10e29251 ("binfmt_elf_fdpic: fix /proc/<pid>/auxv")
Cc: stable@vger.kernel.org
Reported-by: Greg Ungerer <gerg@kernel.org>
Closes: https://lore.kernel.org/lkml/5b51975f-6d0b-413c-8b38-39a6a45e8821@westnet.com.au/Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Tested-by: Greg Ungerer <gerg@kernel.org>
Link: https://lore.kernel.org/r/20240826032745.3423812-1-jcmvbkbc@gmail.comSigned-off-by: Kees Cook <kees@kernel.org>

fs/binfmt_elf_fdpic.c

@@ -589,6 +589,9 @@ static int create_elf_fdpic_tables(struct linux_binprm *bprm,
 
 	if (bprm->have_execfd)
 		nitems++;
+#ifdef ELF_HWCAP2
+	nitems++;
+#endif
 
 	csp = sp;
 	sp -= nitems * 2 * sizeof(unsigned long);