Re: [PATCH -next] exec: Remove redundant check in do_open_execat/uselib

[Kees Cook <keescook@chromium.org>]

On Wed, May 18, 2022 at 10:46:01AM -0700, Andrew Morton wrote:
> On Wed, 18 May 2022 16:12:27 +0800 Zhihao Cheng <chengzhihao1@huawei.com> wrote:
> 
> > There is a false positive WARNON happening in execve(2)/uselib(2)
> > syscalls with concurrent noexec-remount.
> > 
> >        execveat                           remount
> > do_open_execat(path/bin)
> >   do_filp_open
> >     path_openat
> >       do_open
> >         may_open
> >           path_noexec() // PASS
> > 	                            remount(path->mnt, MS_NOEXEC)
> > WARNON(path_noexec(&file->f_path)) // path_noexec() checks fail

Did you encounter this in the real world?

> 
> You're saying this is a race condition?  A concurrent remount causes
> this warning?

It seems not an unreasonable thing to warn about. Perhaps since it's
technically reachable from userspace, it could be downgraded to
pr_warn(), but I certainly don't want to remove the checks.

> 
> > Since may_open() has already checked the same conditions, fix it by
> > removing 'S_ISREG' and 'path_noexec' check in do_open_execat()/uselib(2).
> > 
> > ...
> >
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -141,16 +141,6 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
> >  	if (IS_ERR(file))
> >  		goto out;
> >  
> > -	/*
> > -	 * may_open() has already checked for this, so it should be
> > -	 * impossible to trip now. But we need to be extra cautious
> > -	 * and check again at the very end too.
> > -	 */
> > -	error = -EACCES;
> > -	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> > -			 path_noexec(&file->f_path)))
> > -		goto exit;
> > -
> 
> Maybe we should retain the `goto exit'.  The remount has now occurred,
> so the execution attempt should be denied.  If so, the comment should
> be updated to better explain what's happening.
> 
> I guess we'd still be racy against `mount -o exec', but accidentally
> denying something seems less serious than accidentally permitting it.

I'd like to leave this as-is, since we _do_ want to find the cases where
we're about to allow an exec and a very important security check was NOT
handled.

-- 
Kees Cook