linux-kernel - Re: [kernel-hardening] [PATCH] fs: Use CAP_DAC

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Date:   Sat, 21 Oct 2017 07:08:31 -0700
From:   Nick Kralevich <nnk@...gle.com>
To:     Nicolas Belouin <nicolas@...ouin.fr>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [kernel-hardening] [PATCH] fs: Use CAP_DAC_OVERRIDE to allow for
 file dedupe

On Sat, Oct 21, 2017 at 6:28 AM, Nicolas Belouin <nicolas@...ouin.fr> wrote:
> In its current implementation the check is against CAP_SYS_ADMIN,
> however this capability is bloated and inapropriate for this use.
> Indeed the check aims to avoid dedupe against non writable files,
> falling directly in the use case of CAP_DAC_OVERRIDE.
>
> Signed-off-by: Nicolas Belouin <nicolas@...ouin.fr>
> ---
>  fs/read_write.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/read_write.c b/fs/read_write.c
> index f0d4b16873e8..43cc7e84e29e 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1965,7 +1965,7 @@ int vfs_dedupe_file_range(struct file *file, struct file_dedupe_range *same)
>         u64 len;
>         int i;
>         int ret;
> -       bool is_admin = capable(CAP_SYS_ADMIN);
> +       bool is_admin = capable(CAP_SYS_ADMIN) || capable(CAP_DAC_OVERRIDE);

Can you please reverse the order of the checks? In particular, on an
SELinux based system, a capable() call generates an SELinux denial,
and people often instinctively allow the first operation performed.
Reordering the elements will ensure that the CAP_DAC_OVERRIDE denial
(least permissive) is generated first.

>         u16 count = same->dest_count;
>         struct file *dst_file;
>         loff_t dst_off;
> --
> 2.14.2
>



-- 
Nick Kralevich | Android Security | nnk@...gle.com | 650.214.4037