Re: [PATCH 4/7] exec: Move bprm_mm_init into alloc_bprm

[Kees Cook]

On Tue, Jul 14, 2020 at 08:30:02AM -0500, Eric W. Biederman wrote:
> 
> Currently it is necessary for the usermode helper code and the code that
> launches init to use set_fs so that pages coming from the kernel look like
> they are coming from userspace.
> 
> To allow that usage of set_fs to be removed cleanly the argument copying
> from userspace needs to happen earlier.  Move the allocation and
> initialization of bprm->mm into alloc_bprm so that the bprm->mm is
> available early to store the new user stack into.  This is a prerequisite
> for copying argv and envp into the new user stack early before ther rest of
> exec.
> 
> To keep the things consistent the cleanup of bprm->mm is moved into
> free_bprm.  So that bprm->mm will be cleaned up whenever bprm->mm is
> allocated and free_bprm are called.
> 
> Moving bprm_mm_init earlier is safe as it does not depend on any files,
> current->in_execve, current->fs->in_exec, bprm->unsafe, or the if the file
> table is shared. (AKA bprm_mm_init does not depend on any of the code that
> happens between alloc_bprm and where it was previously called.)
> 
> This moves bprm->mm cleanup after current->fs->in_exec is set to 0.  This
> is safe because current->fs->in_exec is only used to preventy taking an
> additional reference on the fs_struct.
> 
> This moves bprm->mm cleanup after current->in_execve is set to 0.  This is
> safe because current->in_execve is only used by the lsms (apparmor and
> tomoyou) and always for LSM specific functions, never for anything to do
> with the mm.
> 
> This adds bprm->mm cleanup into the successful return path.  This is safe
> because being on the successful return path implies that begin_new_exec
> succeeded and set brpm->mm to NULL.  As bprm->mm is NULL bprm cleanup I am
> moving into free_bprm will do nothing.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

This looks correct, and is required before moving the arg pages stuff,
so good. :)

-- 
Kees Cook