Re: [PATCH] exec: Make suid_dumpable apply to SUID/SGID binaries irrespective of invoking users

[Willy Tarreau <w@1wt.eu>]

On Tue, Dec 21, 2021 at 02:27:47PM -0500, Waiman Long wrote:
> On 12/21/21 13:19, Linus Torvalds wrote:
> > On Tue, Dec 21, 2021 at 10:01 AM Waiman Long <longman@redhat.com> wrote:
> > > Default RLIMIT_CORE to 0 will likely mitigate this vulnerability.
> > > However, there are still some userspace impacts as existing behavior
> > > will be modified. For instance, we may need to modify su to restore a
> > > proper value for RLIMIT_CORE after successful authentication.
> > We had a "clever" idea for this that I thought people were ok with.
> > 
> > It's been some time since this came up, but iirc the notion was to
> > instead of setting the rlimit to zero (which makes it really hard to
> > restore afterwards, because you don't know what the restored value
> > would be, so you are dependent on user space doing it), we just never
> > reset set_dumpable() when we execve.
> > 
> > So any suid exec will do set_dumpable() to suid_dumpable, and exec'ing
> > something else does nothing at all - it stays non-dumpable (obviously
> > "non-dumpable" here depends on the actual value for "suid_dumpable" -
> > you can enable suid dump debugging manually).
> > 
> > And instead, we say that operations like "setsid()" that start a new
> > session - *those* are the ones that enable core dumping again. Or
> > doing things like a "ulimit(RLIMIT_CORE)" (which clearly implies "I
> > want core-dumps").
> > 
> > Those will all very naturally make "login" and friends work correctly,
> > while keeping core-dumps disabled for some suid situation that doesn't
> > explicitly set up a new context.
> > 
> > I think the basic problem with the traditional UNIX model of "suid
> > exec doesn't core dump" is that the "enter non-core-dump" is a nice
> > clear "your privileges changed".
> > 
> > But then the "exit non-core-dump" thing is an exec that *doesn't*
> > change privileges. That's the odd and crazy part: you just disabled
> > core-dumps because there was a privilege level change, and then you
> > enable core-dumps again because there *wasn't* a privilege change -
> > even if you're still at those elevated privileges.
> > 
> > Now, this is clearly not a Linux issue - we're just doing what others
> > have been doing too. But I think we should just admit that "what
> > others have been doing" is simply broken.
> > 
> > And yes, some odd situation migth be broken by this kind of change,
> > but I think this kind of "the old model was broken" may simply require
> > that. I suspect we can find a solution that fixes all the regular
> > cases.
> > 
> > Hmm?
> 
> I think this is a pretty clever idea. At least it is better than resetting
> RLIMIT_CORE to 0.

Another problem that was raised when discussing RLIMIT_CORE to zero was
the loss of the old value.

> As it is all done within the kernel, there is no need to
> change any userspace code. We may need to add a flag bit in the task
> structure to indicate using the suid_dumpable setting so that it can be
> inherited across fork/exec.

Depending on what we change there can be some subtly visible changes.
In one of my servers I explicitly re-enable dumpable before setsid()
when a core dump is desired for debugging. But other deamons could do
the exact opposite. If setsid() systematically restores suid_dumpable,
a process that explicitly disables it before calling setsid() would
see it come back. But if we have a special "suid_in_progress" flag
to mask suid_dumpable and that's reset by setsid() and possibly
prctl(PR_SET_DUMPABLE) then I think it could even cover that unlikely
case.

Just my two cents,
Willy