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Date:   Wed, 13 Nov 2019 00:22:40 +0100
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Topi Miettinen <toiwoton@gmail.com>
Cc:     Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
        <linux-fsdevel@vger.kernel.org>, linux-api@vger.kernel.org
Subject: Re: [PATCH] Allow restricting permissions in /proc/sys
Message-ID: <20191112232239.yevpeemgxz4wy32b@wittgenstein>
References: <74a91362-247c-c749-5200-7bdce704ed9e@gmail.com>
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[Cc+ linux-api@vger.kernel.org]

since that's potentially relevant to quite a few people.

On Sun, Nov 03, 2019 at 04:55:48PM +0200, Topi Miettinen wrote:
> Several items in /proc/sys need not be accessible to unprivileged
> tasks. Let the system administrator change the permissions, but only
> to more restrictive modes than what the sysctl tables allow.
> 
> Signed-off-by: Topi Miettinen <toiwoton@gmail.com>
> ---
>  fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
>  1 file changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index d80989b6c344..88c4ca7d2782 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int
> mask)
>         if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
>                 return -EACCES;
> 
> +       error = generic_permission(inode, mask);
> +       if (error)
> +               return error;
> +
>         head = grab_header(inode);
>         if (IS_ERR(head))
>                 return PTR_ERR(head);
> @@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry,
> struct iattr *attr)
>         struct inode *inode = d_inode(dentry);
>         int error;
> 
> -       if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
> +       if (attr->ia_valid & (ATTR_UID | ATTR_GID))
>                 return -EPERM;
> 
> +       if (attr->ia_valid & ATTR_MODE) {
> +               struct ctl_table_header *head = grab_header(inode);
> +               struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> +               umode_t max_mode = 0777; /* Only these bits may change */
> +
> +               if (IS_ERR(head))
> +                       return PTR_ERR(head);
> +
> +               if (!table) /* global root - r-xr-xr-x */
> +                       max_mode &= ~0222;
> +               else /*
> +                     * Don't allow permissions to become less
> +                     * restrictive than the sysctl table entry
> +                     */
> +                       max_mode &= table->mode;
> +
> +               sysctl_head_finish(head);
> +
> +               /* Execute bits only allowed for directories */
> +               if (!S_ISDIR(inode->i_mode))
> +                       max_mode &= ~0111;
> +
> +               if (attr->ia_mode & ~S_IFMT & ~max_mode)
> +                       return -EPERM;
> +       }
> +
>         error = setattr_prepare(dentry, attr);
>         if (error)
>                 return error;
> @@ -853,17 +883,8 @@ static int proc_sys_getattr(const struct path *path,
> struct kstat *stat,
>                             u32 request_mask, unsigned int query_flags)
>  {
>         struct inode *inode = d_inode(path->dentry);
> -       struct ctl_table_header *head = grab_header(inode);
> -       struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> -
> -       if (IS_ERR(head))
> -               return PTR_ERR(head);
> 
>         generic_fillattr(inode, stat);
> -       if (table)
> -               stat->mode = (stat->mode & S_IFMT) | table->mode;
> -
> -       sysctl_head_finish(head);
>         return 0;
>  }
> 
> -- 
> 2.24.0.rc1
>