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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 17 Apr 2014 17:16:22 -0700 Kees Cook <keescook@chromium.org> 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.