Re: sysctl_writes_strict documentation + an oddity?

From: Kees Cook
Date: Tue Jun 16 2015 - 12:32:23 EST

On Tue, Jun 16, 2015 at 3:03 AM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> On 06/04/2015 09:36 PM, Kees Cook wrote:
>> On Sat, May 9, 2015 at 1:54 AM, Michael Kerrisk (man-pages)
>> <mtk.manpages@xxxxxxxxx> wrote:
>>> ===== 2) Behavior puzzle (a) =====
>>> The last sentence quoted from the man page was based on your sentence
>>> Writes to numeric sysctl entries must always be at file position 0
>>> and the value must be fully contained in the buffer sent in the write
>>> syscall.
>>> So, I had interpreted /proc/sys/kernel/sysctl_writes_strict==1 to
>>> mean that if one writes into a numeric /proc/sys file at an offset
>>> other than zero, the write() will fail with some kind of error.
>> Reporting back an error wasn't something I'd tested before. Looking at
>> the code again now, it should be possible make this change.
>> Regardless, in the case of the numeric value error condition, it's the
>> same as the "past the end" string error condition: "Anything written
>> beyond the maximum length of the value buffer will be ignored." i.e.
>> anything other than file offset 0 is considered "past the end of the
>> buffer" for a numeric value and is ignored.
>>> But this seems not to be the case. Instead, the write() succeeds,
>>> but the file is left unmodified. That's surprising, I find. So, I'm
>>> wondering whether the implementation deviates from your intention.
>>> There's a test program below, which takes arguments as follows
>>> ./a.out pathname offset string
>> I have tests in tools/testing/selftests/sysctl for checking the
>> various behaviors too. They don't actually examine any error
>> conditions from the sysctl writing itself. It should be simple to make
>> sysctl_writes_strict failures return an error, though.
> So, what do you think: is it *desirable* to make sysctl_writes_strict
> failures return an error?

I think it would be desirable, yes. I want to improve the tests to add
error checking first, so I can make sure the change doesn't introduce
anything unexpected. The fix is simple, but since the code is a little
twisty, I want to be careful.

>>> ===== 2) Behavior puzzle (b) =====
>>> In commit f88083005ab319abba5d0b2e4e997558245493c8, there is this note:
>>> This adds the sysctl kernel.sysctl_writes_strict to control the write
>>> behavior. The default (0) reports when VFS position is non-0 on a
>>> write, but retains legacy behavior, -1 disables the warning, and 1
>>> enables the position-respecting behavior.
>>> The long-term plan here is to wait for userspace to be fixed in response
>>> to the new warning and to then switch the default kernel behavior to the
>>> new position-respecting behavior.
>>> (That last para was added to the commit message by AKPM, I see.)
>>> But, I wonder here whether /proc/sys/kernel/sysctl_writes_strict==0
>>> is going to help with the long-term plan. The problem is that in
>>> warn_sysctl_write(), pr_warn_once() is used. This means that only
>>> the first offending user-space application that writes to *any*
>>> /proc/sys file will generate the printk warning. If that application
>>> isn't fixed, then none of the other "broken" applications will be
>>> discovered. It therefore seems possible that it could be a very long
>>> time before we could "switch the default kernel behavior to the
>>> new position-respecting behavior".
>>> Looking over old mails
>>> (,
>>> I see that you're aware of the problem, but it seems to me that
>>> the switch to pr_warn_once() (for fear of spamming the log) likely
>>> dooms the long-term plan to failure. Your thoughts?
>> In actual regular use, the situation that triggers the warning should
>> be vanishingly rare, but the condition can be trivially met by someone
>> intending to hit it for the purposes of filling log files. As such, it
>> makes sense to me to use _once to avoid spamming, but still catch a
>> rare usage under normal conditions.
> So, I'm not clear whether you think I'm wrong or not ;-).
> Do you disagree with my point that this approach may doom
> the long-term project to failure? (That was my main point.)

Sorry! No, I don't think using pr_warn_once() will doom the
transition. I think that if we see the warning, we need to investigate
what's using sysctl that way. If we never see it, then we can switch
the default. (Using _once protects against log spamming.) I would
basically expect to never see this warning, but akpm wanted to be very
cautious, which I can't argue with. :)


Kees Cook
Chrome OS Security
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