Re: [PATCH] signal: restore the override_rlimit logic

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, Nov 01, 2024 at 03:58:07PM -0500, Eric W. Biederman wrote:
> Roman Gushchin <roman.gushchin@linux.dev> writes:
> 
> > On Fri, Nov 01, 2024 at 02:51:00PM -0500, Eric W. Biederman wrote:
> >> Roman Gushchin <roman.gushchin@linux.dev> writes:
> >> 
> >> > Prior to commit d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of
> >> > ucounts") UCOUNT_RLIMIT_SIGPENDING rlimit was not enforced for a class
> >> > of signals. However now it's enforced unconditionally, even if
> >> > override_rlimit is set.
> >> 
> >> Not true.
> >> 
> >> It added a limit on the number of siginfo structures that
> >> a container may allocate.  Have you tried not limiting your
> >> container?
> >> 
> >> >This behavior change caused production issues.
> >> 
> >> > For example, if the limit is reached and a process receives a SIGSEGV
> >> > signal, sigqueue_alloc fails to allocate the necessary resources for the
> >> > signal delivery, preventing the signal from being delivered with
> >> > siginfo. This prevents the process from correctly identifying the fault
> >> > address and handling the error. From the user-space perspective,
> >> > applications are unaware that the limit has been reached and that the
> >> > siginfo is effectively 'corrupted'. This can lead to unpredictable
> >> > behavior and crashes, as we observed with java applications.
> >> 
> >> Note.  There are always conditions when the allocation may fail.
> >> The structure is allocated with __GFP_ATOMIC so it is much more likely
> >> to fail than a typical kernel memory allocation.
> >> 
> >> But I agree it does look like there is a quality of implementation issue
> >> here.
> >> 
> >> > Fix this by passing override_rlimit into inc_rlimit_get_ucounts() and
> >> > skip the comparison to max there if override_rlimit is set. This
> >> > effectively restores the old behavior.
> >> 
> >> Instead please just give the container and unlimited number of siginfo
> >> structures it can play with.
> >
> > Well, personally I'd not use this limit too, but I don't think
> > "it's broken, userspace shouldn't use it" argument is valid.
> 
> I said if you don't want the limit don't use it.
> 
> A version of "Doctor it hurts when I do this". To which the doctor
> replies "Don't do that then".
> 
> I was also asking that you test with the limit disabled (at user
> namespace creation time) so that you can verify that is problem.
> 
> >> The maximum for rlimit(RLIM_SIGPENDING) is the rlimit(RLIM_SIGPENDING)
> >> value when the user namespace is created.
> >> 
> >> Given that it took 3 and half years to report this.  I am going to
> >> say this really looks like a userspace bug.
> >
> > The trick here is another bug fixed by https://lkml.org/lkml/2024/10/31/185.
> > Basically it's a leak of the rlimit value.
> > If a limit is set and reached in the reality, all following signals
> > will not have a siginfo attached, causing applications which depend on
> > handling SIGSEGV to crash.
> 
> I will take a deeper look at the patch you are referring to.
> 
> >> Beyond that your patch is actually buggy, and should not be applied.
> >> 
> >> If we want to change the semantics and ignore the maximum number of
> >> pending signals in a container (when override_rlimit is set) then
> >> the code should change the computation of the max value (pegging it at
> >> LONG_MAX) and not ignore it.
> >
> > Hm, isn't the unconditional (new < 0) enough to capture the overflow?
> > Actually I'm not sure I understand how "long new" can be "> LONG_MAX"
> > anyway.
> 
> Agreed "new < 0" should catch that, but still splitting the logic
> between the calculation of max and the test of max is quite confusing.
> It makes much more sense to put the logic into the calculate of max.

You mean something like this?

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 046b3d57ebb4..49fcec41e5b4 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -317,11 +317,12 @@ long inc_rlimit_get_ucounts(struct ucounts *ucounts, enum rlimit_type type,

        for (iter = ucounts; iter; iter = iter->ns->ucounts) {
                long new = atomic_long_add_return(1, &iter->rlimit[type]);
-               if (new < 0 || (!override_rlimit && (new > max)))
+               if (new < 0 || new > max)
                        goto unwind;
                if (iter == ucounts)
                        ret = new;
-               max = get_userns_rlimit_max(iter->ns, type);
+               if (!override_rlimit)
+                       max = get_userns_rlimit_max(iter->ns, type);
                /*
                 * Grab an extra ucount reference for the caller when
                 * the rlimit count was previously 0.

--

If you strongly prefer this version, I can send a v2. I like my original
version slightly better, but not a strong preference. Please, let me know.

Thanks!