On Thu, 17 Apr 2014 17:16:22 -0700 Kees Cook <keesc...@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.
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