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SubjectRe: [PATCH 4/7] exec: Move bprm_mm_init into alloc_bprm
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@xmission.com>

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

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