linux-kernel - Re: [PATCH] Honor mmap_min_addr with the actual minimum

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Date:	Wed, 6 Apr 2016 15:40:14 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Hector Marco-Gisbert <hecmargi@....es>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	James Morris <james.l.morris@...cle.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Ismael Ripoll <iripoll@....es>, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH] Honor mmap_min_addr with the actual minimum

On Wed, Apr 6, 2016 at 12:07 PM, Hector Marco-Gisbert <hecmargi@....es> wrote:
> The minimum address that a process is allowed to mmap when LSM is
> enabled is 0x10000 (65536). This value is tunable and exported via
> /proc/sys/vm/mmap_min_addr but it is not honored with the actual
> minimum value.

I think this is working as intended already, based on the commit log
for 788084aba2ab7348257597496befcbccabdc98a3

See cap_mmap_addr (which uses dac_mmap_min_addr) vs SELinux's hook
(which uses CONFIG_LSM_MMAP_MIN_ADDR), and everything else (that uses
mmap_min_addr).

Without CONFIG_LSM_MMAP_MIN_ADDR, dac_mmap_min_addr always == mmap_min_addr.

With CONFIG_LSM_MMAP_MIN_ADDR, dac_mmap_min_addr can be less than
mmap_min_addr, but mmap_min_addr will always be at least
CONFIG_LSM_MMAP_MIN_ADDR.

Eric may be able to shed more light on this...

-Kees

>
> It can be easily checked in a system typing:
>
> $ cat /proc/sys/vm/mmap_min_addr
> 4096    # <= Incorrect, it should be 65536
>
> $ echo 1024 > /proc/sys/vm/mmap_min_addr
> $ cat /proc/sys/vm/mmap_min_addr
> 1024    # <= Incorrect, it should be 65536
>
> After applying the patch:
>
> $ cat /proc/sys/vm/mmap_min_addr
> 65536    # <= It is correct
>
> $ echo 1024 > /proc/sys/vm/mmap_min_addr
> $ cat /proc/sys/vm/mmap_min_addr
> 65536    # <= It is correct
>
>
>
> Signed-off-by: Hector Marco-Gisbert <hecmargi@....es>
> Acked-by: Ismael Ripoll Ripoll <iripoll@....es>
> ---
>  security/min_addr.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/security/min_addr.c b/security/min_addr.c
> index f728728..96d1811 100644
> --- a/security/min_addr.c
> +++ b/security/min_addr.c
> @@ -15,10 +15,12 @@ unsigned long dac_mmap_min_addr = CONFIG_DEFAULT_MMAP_MIN_ADDR;
>  static void update_mmap_min_addr(void)
>  {
>  #ifdef CONFIG_LSM_MMAP_MIN_ADDR
> -       if (dac_mmap_min_addr > CONFIG_LSM_MMAP_MIN_ADDR)
> +       if (dac_mmap_min_addr > CONFIG_LSM_MMAP_MIN_ADDR) {
>                 mmap_min_addr = dac_mmap_min_addr;
> -       else
> +       } else {
>                 mmap_min_addr = CONFIG_LSM_MMAP_MIN_ADDR;
> +               dac_mmap_min_addr = CONFIG_LSM_MMAP_MIN_ADDR;
> +       }
>  #else
>         mmap_min_addr = dac_mmap_min_addr;
>  #endif
> --
> 1.9.1
>



-- 
Kees Cook
Chrome OS & Brillo Security