[PATCH 2/3] exec: Introduce finalize_exec() before start_thread()

[Kees Cook]

Provide a final call back into fs/exec.c before start_thread() takes
over, to handle any last-minute changes, like the coming restoration of
the stack limit.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
As an alternative, fs/exec.c could provide a wrapper for start_thread()...
---
 fs/binfmt_aout.c        | 1 +
 fs/binfmt_elf.c         | 1 +
 fs/binfmt_elf_fdpic.c   | 1 +
 fs/binfmt_flat.c        | 1 +
 fs/exec.c               | 6 ++++++
 include/linux/binfmts.h | 1 +
 6 files changed, 11 insertions(+)

diff --git a/fs/binfmt_aout.c b/fs/binfmt_aout.c
index ce1824f47ba6..c3deb2e35f20 100644
--- a/fs/binfmt_aout.c
+++ b/fs/binfmt_aout.c
@@ -330,6 +330,7 @@ static int load_aout_binary(struct linux_binprm * bprm)
 #ifdef __alpha__
 	regs->gp = ex.a_gpvalue;
 #endif
+	finalize_exec(bprm);
 	start_thread(regs, ex.a_entry, current->mm->start_stack);
 	return 0;
 }
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index bdb201230bae..3edca6cb9a33 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1155,6 +1155,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	ELF_PLAT_INIT(regs, reloc_func_desc);
 #endif
 
+	finalize_exec(bprm);
 	start_thread(regs, elf_entry, bprm->p);
 	retval = 0;
 out:
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 429326b6e2e7..d90993adeffa 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -463,6 +463,7 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 			    dynaddr);
 #endif
 
+	finalize_exec(bprm);
 	/* everything is now ready... get the userspace context ready to roll */
 	entryaddr = interp_params.entry_addr ?: exec_params.entry_addr;
 	start_thread(regs, entryaddr, current->mm->start_stack);
diff --git a/fs/binfmt_flat.c b/fs/binfmt_flat.c
index 5d6b94475f27..82a48e830018 100644
--- a/fs/binfmt_flat.c
+++ b/fs/binfmt_flat.c
@@ -994,6 +994,7 @@ static int load_flat_binary(struct linux_binprm *bprm)
 	FLAT_PLAT_INIT(regs);
 #endif
 
+	finalize_exec(bprm);
 	pr_debug("start_thread(regs=0x%p, entry=0x%lx, start_stack=0x%lx)\n",
 		 regs, start_addr, current->mm->start_stack);
 	start_thread(regs, start_addr, current->mm->start_stack);
diff --git a/fs/exec.c b/fs/exec.c
index 7074913ad2e7..e4ae20ff6278 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1384,6 +1384,12 @@ void setup_new_exec(struct linux_binprm * bprm)
 }
 EXPORT_SYMBOL(setup_new_exec);
 
+/* Runs immediately before start_thread() takes over. */
+void finalize_exec(struct linux_binprm *bprm)
+{
+}
+EXPORT_SYMBOL(finalize_exec);
+
 /*
  * Prepare credentials and lock ->cred_guard_mutex.
  * install_exec_creds() commits the new creds and drops the lock.
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index b0abe21d6cc9..40e52afbb2b0 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -118,6 +118,7 @@ extern int __must_check remove_arg_zero(struct linux_binprm *);
 extern int search_binary_handler(struct linux_binprm *);
 extern int flush_old_exec(struct linux_binprm * bprm);
 extern void setup_new_exec(struct linux_binprm * bprm);
+extern void finalize_exec(struct linux_binprm *bprm);
 extern void would_dump(struct linux_binprm *, struct file *);
 
 extern int suid_dumpable;
-- 
2.7.4