linux-kernel - Re: [PATCH v2] fs,security: Fix file_set

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Message-ID: <o6ptrfa7gjdukphqtp6dakq3ykndrjusuhi4fyvpc5ne6amcig@lqbdb2dg7yzv>
Date: Mon, 12 Aug 2024 23:45:29 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Christian Brauner <brauner@...nel.org>, 
	Paul Moore <paul@...l-moore.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, selinux@...r.kernel.org, Jan Kara <jack@...e.cz>, 
	Tahera Fahimi <fahimitahera@...il.com>, Al Viro <viro@...iv.linux.org.uk>, 
	Casey Schaufler <casey@...aufler-ca.com>, James Morris <jmorris@...ei.org>, Jann Horn <jannh@...gle.com>, 
	Ondrej Mosnacek <omosnace@...hat.com>, "Serge E . Hallyn" <serge@...lyn.com>, 
	Stephen Smalley <stephen.smalley.work@...il.com>
Subject: Re: [PATCH v2] fs,security: Fix file_set_fowner LSM hook
 inconsistencies

On Mon, Aug 12, 2024 at 07:44:17PM +0200, Mickaël Salaün wrote:

No opinion about the core idea, I'll note though that this conflicts
with a patch to move f_owner out of the struct:
https://lore.kernel.org/linux-fsdevel/20240809-koriander-biobauer-6237cbc106f3@brauner/

Presumably nothing which can't get sorted out with some shoveling.

I do have actionable remark concerning creds though: both get_cred and
put_cred are slow. Sorting that out is on my todo list.

In the meantime adding more calls can be avoided:

> diff --git a/fs/file_table.c b/fs/file_table.c
> index 4f03beed4737..d28b76aef4f3 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -66,6 +66,7 @@ static inline void file_free(struct file *f)
>  	if (likely(!(f->f_mode & FMODE_NOACCOUNT)))
>  		percpu_counter_dec(&nr_files);
>  	put_cred(f->f_cred);
> +	put_cred(f->f_owner.cred);

	if (likely(f->f_cred == f->f_owner.cred)) {
		put_cred_many(f->f_cred, 2);
	} else {
		put_cred(f->f_cred);
		put_cred(f->f_owner.cred);
	}

>  	if (unlikely(f->f_mode & FMODE_BACKING)) {
>  		path_put(backing_file_user_path(f));
>  		kfree(backing_file(f));
> @@ -149,9 +150,11 @@ static int init_file(struct file *f, int flags, const struct cred *cred)
>  	int error;
>  
>  	f->f_cred = get_cred(cred);
> +	f->f_owner.cred = get_cred(cred);

	f->f_cred = f->f_owner.cred = get_cred_many(cred, 2);

>  	error = security_file_alloc(f);
>  	if (unlikely(error)) {
>  		put_cred(f->f_cred);
> +		put_cred(f->f_owner.cred);

		put_cred_many(cred, 2);

>  		return error;
>  	}