Messages in this thread | | | Date | Mon, 21 Apr 2014 15:45:05 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v2 3/4] sysctl: allow for strict write position handling |
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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.
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