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 704A7395D8E for ; Thu, 9 Apr 2026 22:38:32 +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=1775774312; cv=none; b=ucE74Oke+3l+wfj3a6KeqMJNdy84mLrZWoTbO+4rNAsFY8fAou/9kDniDbiJr32wCbzCskRuwIH5RQO4walC/Vqx9E9eTWu0lS1chsyRhaxswOl0sI0WcVjOtopAw0dLVQHHluFD8fA2cZOcu8qLRLMYr6X3rbdRnMK4NWf3Ycw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775774312; c=relaxed/simple; bh=4xNYaMgsrGst+UGtpewW8fKJVzFO6/Su9kQMgjL8K/Y=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=AxPUNHvuKjIdOX2S6V4t8To+0Nlc5EWmFi/vz20xSMIDPJIexj5Sf5qx/1e1qEf++r0scx+h1GEY1USx9r9xsOUSnmD8Dmym/aOaJcQ6r4iuygCnyegCBnLZBulYKxQswYbiI2gmbZDNOM7+/NJOb8LyXAvdszzV3CdI30i7bRQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KTihKM4S; 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="KTihKM4S" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 47623C4CEF7; Thu, 9 Apr 2026 22:38:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775774312; bh=4xNYaMgsrGst+UGtpewW8fKJVzFO6/Su9kQMgjL8K/Y=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=KTihKM4SI4pPd6cJ04k2xXCauOzEXVHfI2aAK5sD2P//Pec6tDi2qQDbnQwxZbRPl F4NtkrlcPMXjOusqBPhRVUb1s1dvv3+nLSHZn7i2NJgS/GdqX/cTX6EKblqs4yTlca VNwR8wMUWVvWKnyOjmx8Q5pza9DYPzE+eV6EOrVNj8zwRhlcLoGXjYd0b8bA2gC7T+ vC52dH+9mdHPMixqqEfsDRJRZBeq70NDR+KQtVrOaJi8XpY5tGeiTp/TaKeHM7Xkgp /WZ5D9+SBgeJQoA2jOzM0hHi525oUqdkLxEeRVvRJJktS7AuSuSmthl4TOsyEBCnVV dy9xv7jX0Tq/g== Message-ID: Subject: Re: [RFC PATCH v3 0/4] getting rid of busy-wait in shrink_dcache_parent() From: Jeff Layton To: Al Viro Cc: Linus Torvalds , linux-fsdevel@vger.kernel.org, Christian Brauner , Jan Kara , Nikolay Borisov , Max Kellermann , Eric Sandeen , Paulo Alcantara Date: Thu, 09 Apr 2026 18:38:28 -0400 In-Reply-To: <20260409215733.GS3836593@ZenIV> 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> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw 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Evolution 3.58.3 (3.58.3-1.fc43) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-04-09 at 22:57 +0100, Al Viro wrote: > On Thu, Apr 09, 2026 at 04:10:41PM -0400, Jeff Layton wrote: >=20 > > head_A =E2=86=94 UAF =E2=86=94 mountinfo(1) =E2=86=94 swaps(/) =E2=86= =94 ... =E2=86=94 > > head_B(STACK) =E2=86=94 status(5962) =E2=86=94 exe(5962) =E2=86=94 ..= . =E2=86=94 > > stat(8808) =E2=86=94 ... =E2=86=94 mountinfo(1) =E2=86=94 UAF =E2=86= =94 head_A > > ``` >=20 I think the above is BS. Claude says: In the earlier analysis I walked the list both forward from=20 head_B (the stack entry) and backward from head_A That was misleading =E2=80=94 there's only one mountinfo node. The ring passes through it once.= The duplicate in my listing was an artifact of walking the ring from two entry points and not deduplicating. =20 > Wait a bloody minute; *two* UAF and two mountinfo there? Are the links > in that cyclic list consistent? ->next and ->prev, I mean. No: The list was NOT a proper ring: - FWD (head.next): LOOP at entry 1 =E2=80=94 immediately hits a self-loop= (the processed "status" dentry) - BWD (head.prev): LOOP at entry 55 =E2=80=94 walks 55 entries then hits = a cycle > If I understand the notation correctly... are there two different > dentries, both with "mountinfo" for name and PROC_I(_->d_inode)->pid > being PID 1? What are their ->d_parent pointing to? Are they hashed? >=20 > > ### d_walk Escaped Its Starting Dentry > > `d_walk` was called with `parent =3D /proc/4530/task/5964` (`data.start= ` > > confirmed in stack frame). It should only traverse descendants of 5964. > > But the dispose list contains entries from: > > - `/proc/4530/task/5962/*` (151 children =E2=80=94 sibling of 5964) > > - `/proc/4530/task/6830`, `/proc/4530/task/8808` =E2=80=94 other task e= ntries > > - `/proc/1/mountinfo`, `/proc/1/status`, `/proc/1/net` =E2=80=94 PID 1 = entries > > - `/proc/sys/vm/overcommit_memory`, `/proc/sys/fs/*` =E2=80=94 sysctl e= ntries > > - `/proc/pressure/{cpu,io,memory}` =E2=80=94 PSI entries > > - `/proc/swaps`, `/proc/cpuinfo`, `/proc/kcore` =E2=80=94 root proc ent= ries >=20 > Not at all obvious; there's a list from another thread mixed into that, > and we've no idea what had the root been for that one. For that matter, > I'd check ->d_flags on the entries, just to verify that those are > from shrink lists and not from LRU - the fact that these UAF still > have pointers to plausible dentries does not mean they are from the > same moment in time; if dentry in question had been on a different > list before getting freed... That's why the question about ->prev > and ->next consistncy. > > And seeing that procfs has zero callers of d_splice_alias() or d_move(), > I would expect ->d_parent on all dentries in there to be constant over > the entire lifetime; would be rather hard for d_walk() to escape in > such conditions... Definitely not legitimately. I suspect some other sort of mem corruption. /proc/1/mountinfo is on the list though: mountinfo dentries on dispose list: 1 =20 = =20 /proc/1/mountinfo (dentry 0xffff88823deef800) =20 d_parent: 0xffff891373c06a80 "1" -> "/" (root) =20 hashed: yes (d_hash.pprev=3D0xffff893d337264d0) =20 PROC_I->pid nr: 3217077 =20 d_inode: 0xffff88ea55a44508 =20 Claude's theory (maybe garbage): PID 1's mountinfo being on the list is actually a strong clue =E2=80=94 it'= s a dentry that would be on the global LRU (negative or zero-count after someone closed /proc/1/mountinfo), and drop_caches would pull it off the LRU via prune_dcache_sb. That's likely how it ended up on Thread B's dispose list. Here's a cursory comparitive analysis across the 3 vmcores I have so far: Comparative analysis across three vmcores = =20 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D = =20 = =20 Kernel 6.13.2-0_fbk5-hf2 6.13.2-0_fbk12-hf1 6.13.= 2-0_fbk5-hf2 =20 Date Mar 31 Apr 9 Apr= 2 =20 Process SREventBase61 tokio-runtime-w TTL= SFWorkerU151 =20 d_invalidate target /task/5964 /task/3555587 /ta= sk/1102 =20 had_submounts False False Fal= se =20 UAF slab kmalloc-96 kmalloc-64 kma= lloc-96 =20 UAF d_flags 0x0 0x81e73ce0 0x0= =20 UAF d_sb 0x0 0x0 0x0= =20 UAF d_lockref.count 0 -14080 -77= 1573072 =20 Consistent findings across all three cores: = =20 =20 - had_submounts is False in every case. detach_mounts() and = =20 namespace_unlock() are not involved in any of these crashes. = =20 - No mounts under /proc//... in any mount namespace (checked = =20 on the od0103 core across 49 namespaces, zero proc submounts). = =20 - All three are in the same call path: = =20 release_task -> proc_flush_pid -> d_invalidate -> = =20 shrink_dcache_parent -> shrink_dentry_list -> spin_lock (UAF) = =20 =20 - The UAF'd dentry is in a non-dentry slab in every case (kmalloc-64 = =20 or kmalloc-96), confirming the dentry was freed and its page reused = =20 by a different slab cache before shrink_dentry_list tried to lock it.= =20 = =20 - Different applications triggered the crash (Rust async runtime, = =20 C++ worker, Java-like worker) -- not application-specific.