Re: [PATCH] fs: Prevent panic from NULL dereference in alloc_fs_context() during do_exit()

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, May 06, 2025 at 05:38:02AM +0900, Yunseong Kim wrote:
> The function alloc_fs_context() assumes that current->nsproxy and its
> net_ns field are valid. However, this assumption can be violated in
> cases such as task teardown during do_exit(), where current->nsproxy can
> be NULL or already cleared.
> 
> This issue was triggered during stress-ng's kernel-coverage.sh testing,
> Since alloc_fs_context() can be invoked in various contexts — including
> from asynchronous or teardown paths like do_exit() — it's difficult to
> guarantee that its input arguments are always valid.
> 
> A follow-up patch will improve the granularity of this fix by moving the
> check closer to the actual mount trigger(e.g., in efivarfs_pm_notify()).

UGH.

> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 582d33e81117..529de43b8b5e 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -282,6 +282,9 @@ static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
>  	struct fs_context *fc;
>  	int ret = -ENOMEM;
>  
> +	if (!current->nsproxy || !current->nsproxy->net_ns)
> +		return ERR_PTR(-EINVAL);
> +
>  	fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL_ACCOUNT);
>  	if (!fc)
>  		return ERR_PTR(-ENOMEM);

That might paper over the oops, but I very much doubt that this will be
a correct fix...  Note that in efivarfs_pm_notify() we have other
fun issues when run from such context - have task_work_add() fail in
fput() and if delayed_fput() runs right afterwards and
        efivar_init(efivarfs_check_missing, sfi->sb, false);
in there might end up with UAF...