exec: Remove recursion from search_binary_handler

Recursion in kernel code is generally a bad idea as it can overflow
the kernel stack.  Recursion in exec also hides that the code is
looping and that the loop changes bprm->file.

Instead of recursing in search_binary_handler have the methods that
would recurse set bprm->interpreter and return 0.  Modify exec_binprm
to loop when bprm->interpreter is set.  Consolidate all of the
reassignments of bprm->file in that loop to make it clear what is
going on.

The structure of the new loop in exec_binprm is that all errors return
immediately, while successful completion (ret == 0 &&
!bprm->interpreter) just breaks out of the loop and runs what
exec_bprm has always run upon successful completion.

Fail if the an interpreter is being call after execfd has been set.
The code has never properly handled an interpreter being called with
execfd being set and with reassignments of bprm->file and the
assignment of bprm->executable in generic code it has finally become
possible to test and fail when if this problematic condition happens.

With the reassignments of bprm->file and the assignment of
bprm->executable moved into the generic code add a test to see if
bprm->executable is being reassigned.

In search_binary_handler remove the test for !bprm->file.  With all
reassignments of bprm->file moved to exec_binprm bprm->file can never
be NULL in search_binary_handler.

Link: https://lkml.kernel.org/r/87sgfwyd84.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 653508b25815..7fc05929c967 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -50,8 +50,8 @@ struct linux_binprm {
 #ifdef __alpha__
 	unsigned int taso:1;
 #endif
-	unsigned int recursion_depth; /* only for search_binary_handler() */
 	struct file * executable; /* Executable to pass to the interpreter */
+	struct file * interpreter;
 	struct file * file;
 	struct cred *cred;	/* new credentials */
 	int unsafe;		/* how unsafe this exec is (mask of LSM_UNSAFE_*) */
@@ -117,7 +117,6 @@ static inline void insert_binfmt(struct linux_binfmt *fmt)
 extern void unregister_binfmt(struct linux_binfmt *);
 
 extern int __must_check remove_arg_zero(struct linux_binprm *);
-extern int search_binary_handler(struct linux_binprm *);
 extern int begin_new_exec(struct linux_binprm * bprm);
 extern void setup_new_exec(struct linux_binprm * bprm);
 extern void finalize_exec(struct linux_binprm *bprm);