exec: Fix NOMMU linux_binprm::exec in transfer_args_to_stack()

In NOMMU kernel the value of linux_binprm::p is the offset inside the
temporary program arguments array maintained in separate pages in the
linux_binprm::page. linux_binprm::exec being a copy of linux_binprm::p
thus must be adjusted when that array is copied to the user stack.
Without that adjustment the value passed by the NOMMU kernel to the ELF
program in the AT_EXECFN entry of the aux array doesn't make any sense
and it may break programs that try to access memory pointed to by that
entry.

Adjust linux_binprm::exec before the successful return from the
transfer_args_to_stack().

Cc: <stable@vger.kernel.org>
Fixes: b6a2fea3 ("mm: variable length argument support")
Fixes: 5edc2a51 ("binfmt_elf_fdpic: wire up AT_EXECFD, AT_EXECFN, AT_SECURE")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/r/20240320182607.1472887-1-jcmvbkbc@gmail.comSigned-off-by: Kees Cook <keescook@chromium.org>

fs/exec.c

@@ -895,6 +895,7 @@ int transfer_args_to_stack(struct linux_binprm *bprm,
 			goto out;
 	}
 
+	bprm->exec += *sp_location - MAX_ARG_PAGES * PAGE_SIZE;
 	*sp_location = sp;
 
 out: