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From: Roman Gushchin To: "Eric W. Biederman" Cc: linux-kernel@vger.kernel.org, Andrei Vagin , Kees Cook , Alexey Gladkov , stable@vger.kernel.org Subject: Re: [PATCH] signal: restore the override_rlimit logic Message-ID: References: <20241031200438.2951287-1-roman.gushchin@linux.dev> <87zfmi3f8b.fsf@email.froward.int.ebiederm.org> <87o72y3c4g.fsf@email.froward.int.ebiederm.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87o72y3c4g.fsf@email.froward.int.ebiederm.org> X-Migadu-Flow: FLOW_OUT On Fri, Nov 01, 2024 at 03:58:07PM -0500, Eric W. Biederman wrote: > Roman Gushchin writes: > > > On Fri, Nov 01, 2024 at 02:51:00PM -0500, Eric W. Biederman wrote: > >> Roman Gushchin 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!