Re: [PATCH v3] sysctl: control functionality of /proc/pid/mem

[Andrew Morton]

On Mon, 23 Jan 2012 13:21:15 -0800
Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> 
> Add the "proc_pid_mem" sysctl to control whether or not /proc/pid/mem is
> allowed to work: 0: disabled, 1: read only, 2: read/write (default).
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> v3:
>  - document the default, thanks to Randy Dunlap.
>  - remove needless CONFIG_PROC_FS checks, thanks to Eric W. Biederman.

I was wondering about that.  Is CONFIG_PROC_FS=n, CONFIG_SYSCTL=y an
impossible combination?  If so, why?

> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -56,6 +56,7 @@ show up in /proc/sys/kernel:
>  - printk_delay
>  - printk_ratelimit
>  - printk_ratelimit_burst
> +- proc_pid_mem
>  - randomize_va_space
>  - real-root-dev               ==> Documentation/initrd.txt
>  - reboot-cmd                  [ SPARC only ]
> @@ -477,6 +478,20 @@ send before ratelimiting kicks in.
>  
>  ==============================================================
>  
> +proc_pid_mem:
> +
> +This option can be used to select the level of access given to potential
> +ptracers when using the per-process "mem" file in /proc/pid/mem.
> +
> +0 - Disable entirely.
> +
> +1 - Allow potential ptracers read access to process memory, but not writes.
> +
> +2 - Allow potential ptracers read and write access to process memory. This
> +    is the default.
> +
> +==============================================================

I agree with Colin on this (he stole my line!).



Overall, the patch looks really hacky and random.  I felt the same way
as Vasily: it's easy to see how a significant number of similar (and
hacky and random) patches could be added, resulting in a regrettable
mess.

Is there some better designed, more organized way of approaching all of
this?  Random ideas:

- A parallel /procfs-perms filesystem.  You write a number into
  /procfs-perms/stat to affect access to /proc/stat (although why the
  heck not just run `chmod /proc/stat'?) It's unclear how to handle
  /proc/pid/.  Perhaps literally have a /procfs-perms/pid/ directory.

- Make tasks inherit their /proc/pid/* permissions across fork, do a
  chmod /proc/1/whatever in initscripts.

- Other and better things ;)   This particular approach makes my toes
  curl.



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