Re: [PATCH v2 3/4] sysctl: allow for strict write position handling

[Kees Cook]

On Mon, Apr 21, 2014 at 3:45 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 17 Apr 2014 17:16:22 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> When writing to a sysctl string, each write, regardless of VFS position,
>> begins writing the string from the start. This means the contents of
>> the last write to the sysctl controls the string contents instead of
>> the first:
>>
>> open("/proc/sys/kernel/modprobe", O_WRONLY)   = 1
>> write(1, "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"..., 4096) = 4096
>> write(1, "/bin/true", 9)                = 9
>> close(1)                                = 0
>>
>> $ cat /proc/sys/kernel/modprobe
>> /bin/true
>>
>> Expected behaviour would be to have the sysctl be "AAAA..." capped at
>> maxlen (in this case KMOD_PATH_LEN: 256), instead of truncating to the
>> contents of the second write. Similarly, multiple short writes would not
>> append to the sysctl.
>>
>> This provides CONFIG_PROC_SYSCTL_STRICT_WRITES as a way to make this
>> behavior act in a less surprising manner for strings, and disallows
>> non-zero file position when writing numeric sysctls (similar to what is
>> already done when reading from non-zero file positions).
>
> Adding a Kconfig knob to alter the behavior of procfs writes creeps me
> out.  I wonder why.
>
> - I doubt if many people have a sufficient amount of control over
>   their entire systems to be able to confidently set
>   CONFIG_PROC_SYSCTL_STRICT_WRITES.
>
> - Software will be shipped which runs OK with one setting but breaks
>   with the other setting.
>
> So what to do?
>
> I think we can *detect* this situation easily enough.  So some options are
>
> a) change the behaviour and add code which detects when userspace is
>    doing a write whose behaviour is now altered.  Print a warning.   Or
>
> b) leave the behaviour as-is.  Add a detector which tells people
>    "hey, your userspace is probably broken - please fix".  Wait N
>    years.  Then alter the behaviour as in a).
>
> In either case the detector should display current->comm, the procfs
> pathname and the contents of the write, to aid people in hunting down
> and fixing their userspace.

How about a tri-state sysctl (har har control sysctl behavior with a
sysctl) that defaults ("1") to existing behavior (to not break
anything) with a warning. Mode "2" uses new behavior, and mode "0"
uses existing behavior without a warning? Then we can wait N years and
switch the default to "2"?

-Kees

-- 
Kees Cook
Chrome OS Security
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