linux-kernel - Re: [PATCH for v4.9 LTS 72/87] libfs: Modify mount_pseudo_xattr to be clear it is not a userspace mount

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Date:   Sat, 15 Jul 2017 02:50:30 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Levin\, Alexander \(Sasha Levin\)" <alexander.levin@...izon.com>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH for v4.9 LTS 72/87] libfs: Modify mount_pseudo_xattr to be clear it is not a userspace mount


*Scratches my head*

Is there code in v4.9 where this matters?  At the time I merged this to
my knowledge there were no in kernel users that cared.  Which is why I
did not cc stable in the first place.

Eric


"Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com> writes:

> From: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> [ Upstream commit 75422726b0f717d67db3283c2eb5bc14fa2619c5 ]
>
> Add MS_KERNMOUNT to the flags that are passed.
> Use sget_userns and force &init_user_ns instead of calling sget so that
> even if called from a weird context the internal filesystem will be
> considered to be in the intial user namespace.
>
> Luis Ressel reported that the the failure to pass MS_KERNMOUNT into
> mount_pseudo broke his in development graphics driver that uses the
> generic drm infrastructure.  I am not certain the deriver was bug
> free in it's usage of that infrastructure but since
> mount_pseudo_xattr can never be triggered by userspace it is clearer
> and less error prone, and less problematic for the code to be explicit.
>
> Reported-by: Luis Ressel <aranea@...ah.de>
> Tested-by: Luis Ressel <aranea@...ah.de>
> Acked-by: Al Viro <viro@...IV.linux.org.uk>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
> ---
>  fs/libfs.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 48826d4da189..9588780ad43e 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -245,7 +245,8 @@ struct dentry *mount_pseudo_xattr(struct file_system_type *fs_type, char *name,
>  	struct inode *root;
>  	struct qstr d_name = QSTR_INIT(name, strlen(name));
>  
> -	s = sget(fs_type, NULL, set_anon_super, MS_NOUSER, NULL);
> +	s = sget_userns(fs_type, NULL, set_anon_super, MS_KERNMOUNT|MS_NOUSER,
> +			&init_user_ns, NULL);
>  	if (IS_ERR(s))
>  		return ERR_CAST(s);