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

[ebiederm@xmission.com (Eric W. Biederman)]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Mon, 23 Jan 2012 13:21:15 -0800
> Kees Cook <keescook@chromium.org> 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@chromium.org>
>> ---
>> 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?

Fundamentally because the only way sysctls get to usespace is threw
/proc/sys/.  The binary sysctl emulation layer reads /proc/sys through
the internal kernel mount.

As I recall the symbol define tree:
CONFIG_PROC_SYSCTL select CONFIG_SYSCTL
CONFIG_PROC_SYSCTL depends on CONFIG_PROC_FS.
And the only way CONFIG_SYSCTL gets set is if it is select.

If anyone cares enough we can probably cleanup the Kconfig bits to have
fewer symbols.

At a practical level I think the real reason I objected is that it is
ugly to just dump things into kernel/sysctl.c with #defines everywhere.

Eric


>> --- 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.