From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 765493DEACF for ; Fri, 10 Apr 2026 18:21:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775845303; cv=none; b=c6XWSPWh4Z5rAS2q2HgOC+2knryQOuz8EPv7ibkHe9MLO5coTgWyrOoe28QfPKhxzQ39HZOSuzY2trhXOqeW4g/tW0TljG1tcF4UAWtn43w2SWsobrs2/JmjVN6/Zp145jceJ00xkfOcnb4ii9PC1qW/oSf6dwjrmSgTQhejnHk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775845303; c=relaxed/simple; bh=oFRwkRX6bHFTBZv16pKYlFfMBaDt6ha55/7Cyw0xWQ4=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=FO9qcdYwOq0TZLCGlzaNpWaVCCNrfYyio2T1WivTK4z29OGBrvrzk4ewJKsFFU26iEheUx3SWx3Fq4wV37yjrp516KrvL3egIwoZtq6Kj8SfBePR/iHrYell/ipEAE2b3yIwnxc/5mMv/emHkEqZc29YIJgScQCMdgLiGufFVh8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nZYe/mZd; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="nZYe/mZd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A7D9DC19421; Fri, 10 Apr 2026 18:21:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775845303; bh=oFRwkRX6bHFTBZv16pKYlFfMBaDt6ha55/7Cyw0xWQ4=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=nZYe/mZdd7gOx4uhdwlq0w4Yt5Wh7GHpmkJGl9JwbdRcGrH1gEBso4QwIqiALHrmF Zukseja+XOHo4v+CmsVxOM3FXr8PqzFDlBLm354yTZlWNcltPMFnrNIqTyHbhMCYsF kuUgvb46x9oixRyS38LebDp3PEYUx83yZXdDs+27HT6PfQzqc6WveapVtBfVFnYfLi KdSX3j0XcsFbNF+ePcLMRbQg3mg5pTV5ixJa8xigdV7QBn8kcBmGOwY7iE7jbnCR3H NYClEvy+8bh/lrZo2VZzY3IxKNjbfy0t4eQt1hneXyH8FPw4OQRFN5ZTOTizDY7f99 zn7VjZIJxldoA== Message-ID: <4305138de599923591df7403aefc4d663f50324a.camel@kernel.org> Subject: Re: [RFC][PATCH] make sure that lock_for_kill() callers drop the locks in safe order From: Jeff Layton To: Linus Torvalds , Boqun Feng Cc: Al Viro , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , linux-fsdevel@vger.kernel.org, Christian Brauner , Jan Kara , Nikolay Borisov , Max Kellermann , Eric Sandeen , Paulo Alcantara Date: Fri, 10 Apr 2026 14:21:39 -0400 In-Reply-To: References: <20260122202025.GG3183987@ZenIV> <20260404080751.2526990-1-viro@zeniv.linux.org.uk> <53b4795292d1df2fe1569fc724325ab52fcab322.camel@kernel.org> <20260409190213.GQ3836593@ZenIV> <41cfd0f95b7fde411c0d59463dce979be89cb8ef.camel@kernel.org> <20260409215733.GS3836593@ZenIV> <20260410084839.GA1310153@ZenIV> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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at 09:27, Boqun Feng wrote: > >=20 > > Notice in rcu_read_unlock_special(), atomicity/preemption count is > > checked in two ways: >=20 > Yeah, I didn't go that deep ... That code is inscrutable. >=20 > Raising a softirq looks pretty strangein there, since there's > absolutely no reason to believe that the preemption-off region has > ended before the softirq is triggered. It makes sense for the "we're > in hardirq context", but it also does it for that (needs_exp && > !irqs_were_disabled()) case >=20 > So then it depends on the softirq checking it *again*. That all seems > a bit strange. >=20 > But it looks like rcu_core_si() does do that, and then falls back to >=20 > if (rcu_preempt_need_deferred_qs()) > set_need_resched_current(); >=20 > and it feels like maybe the rcu_read_unlock_special() should just have > done that itself instead of playing softirq games. >=20 > Maybe it's some off optimization, but going through the softirq code > doesn't _feel_ like it's the right thing to do. >=20 > I don't know this code well enough to judge. >=20 > Let's wait to see if Al's patch actually makes any difference. It > *shouldn't* make any difference, and maybe it doesn't. No need to > start chasing RCU bugs that may not even be there. >=20 > Jeff? >=20 > > @Jeff, just want to confirm, this UAF can be reproduced without > > PREEMPT_RT=3Dy, right? >=20 > I haven't seen Jeff's config, but some of the syzbot reports that > *may* be related have configs that look pretty normal and didn't have > PREEMPT_RT. >=20 >=20 I've not been able to reproduce the UAF at all. It was possible to reproduce the hang that we were seeing by injecting an mdelay() just after the cond_resched() in __dentry_kill(), but Al's other series seems to have fixed that. The UAF seems to be dependent on those hangs somehow, but we were never able to get one to fire from a reproducer attempt. They just occasionally pop up when these stalls occur. --=20 Jeff Layton