Re: [PATCH 6/6] exec: Set the point of no return sooner

[Kees Cook]

On Fri, May 08, 2020 at 01:48:13PM -0500, Eric W. Biederman wrote:
> 
> Make the code more robust by marking the point of no return sooner.
> This ensures that future code changes don't need to worry about how
> they return errors if they are past this point.
> 
> This results in no actual change in behavior as __do_execve_file does
> not force SIGSEGV when there is a pending fatal signal pending past
> the point of no return.  Further the only error returns from de_thread
> and exec_mmap that can occur result in fatal signals being pending.
> 
> Signed-off-by: "Eric W. Biederman" 

Yes, thank you. I'm a fan; this makes the comment above the function a
bit easier to understand, since the very first thing is to set the
point_of_no_return. :)

Reviewed-by: Kees Cook 

-- 
Kees Cook

[PATCH 6/6] exec: Set the point of no return sooner

[Eric W. Biederman]

Make the code more robust by marking the point of no return sooner.
This ensures that future code changes don't need to worry about how
they return errors if they are past this point.

This results in no actual change in behavior as __do_execve_file does
not force SIGSEGV when there is a pending fatal signal pending past
the point of no return.  Further the only error returns from de_thread
and exec_mmap that can occur result in fatal signals being pending.

Signed-off-by: "Eric W. Biederman" 
---
 fs/exec.c | 12 +---
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 443eb960f9a0..b0620d5ebc66 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1304,6 +1304,11 @@ int begin_new_exec(struct linux_binprm * bprm)
struct task_struct *me = current;
int retval;
 
+   /*
+* Ensure all future errors are fatal.
+*/
+   bprm->point_of_no_return = true;
+
/*
 * Make this the only thread in the thread group.
 */
@@ -1326,13 +1331,6 @@ int begin_new_exec(struct linux_binprm * bprm)
if (retval)
goto out;
 
-   /*
-* With the new mm installed it is completely impossible to
-* fail and return to the original process.  If anything from
-* here on returns an error, the check in __do_execve_file()
-* will SEGV current.
-*/
-   bprm->point_of_no_return = true;
bprm->mm = NULL;
 
 #ifdef CONFIG_POSIX_TIMERS
-- 
2.20.1