Re: [PATCH v2 18/18] uaccess: drop maining CONFIG_SET_FS users

Eric W. Biederman
2022-02-17T22:54:19Z

Arnd Bergmann <arnd@kernel.org> writes:

> From: Arnd Bergmann <arnd@arndb.de>
>
> There are no remaining callers of set_fs(), so CONFIG_SET_FS
> can be removed globally, along with the thread_info field and
> any references to it.
>
> This turns access_ok() into a cheaper check against TASK_SIZE_MAX.
>
> With CONFIG_SET_FS gone, so drop all remaining references to
> set_fs()/get_fs(), mm_segment_t and uaccess_kernel().

For the bits I have looked at recently, and think I understand.

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> fs/exec.c | 6 --
> kernel/exit.c | 14 -----
> kernel/kthread.c | 5 --
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 79f2c9483302..bc68a0c089ac 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1303,12 +1303,6 @@ int begin_new_exec(struct linux_binprm * bprm)
> if (retval)
> goto out_unlock;
>
> - /*
> - * Ensure that the uaccess routines can actually operate on userspace
> - * pointers:
> - */
> - force_uaccess_begin();
> -
> if (me->flags & PF_KTHREAD)
> free_kthread_struct(me);
> me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
> diff --git a/kernel/exit.c b/kernel/exit.c
> index b00a25bb4ab9..0884a75bc2f8 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -737,20 +737,6 @@ void __noreturn do_exit(long code)
>
> WARN_ON(blk_needs_flush_plug(tsk));
>
> - /*
> - * If do_dead is called because this processes oopsed, it's possible
> - * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
> - * continuing. Amongst other possible reasons, this is to prevent
> - * mm_release()->clear_child_tid() from writing to a user-controlled
> - * kernel address.
> - *
> - * On uptodate architectures force_uaccess_begin is a noop. On
> - * architectures that still have set_fs/get_fs in addition to handling
> - * oopses handles kernel threads that run as set_fs(KERNEL_DS) by
> - * default.
> - */
> - force_uaccess_begin();
> -
> kcov_task_exit(tsk);
>
> coredump_task_exit(tsk);
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 38c6dd822da8..16c2275d4b50 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -55,7 +55,6 @@ struct kthread {
> int result;
> int (*threadfn)(void *);
> void *data;
> - mm_segment_t oldfs;
> struct completion parked;
> struct completion exited;
> #ifdef CONFIG_BLK_CGROUP
> @@ -1441,8 +1440,6 @@ void kthread_use_mm(struct mm_struct *mm)
> mmdrop(active_mm);
> else
> smp_mb();
> -
> - to_kthread(tsk)->oldfs = force_uaccess_begin();
> }
> EXPORT_SYMBOL_GPL(kthread_use_mm);
>
> @@ -1457,8 +1454,6 @@ void kthread_unuse_mm(struct mm_struct *mm)
> WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
> WARN_ON_ONCE(!tsk->mm);
>
> - force_uaccess_end(to_kthread(tsk)->oldfs);
> -
> task_lock(tsk);
> /*
> * When a kthread stops operating on an address space, the loop