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 BCE312EA749 for ; Thu, 9 Apr 2026 20:10:44 +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=1775765444; cv=none; b=c0ifqBVwYf2+X3MS+ogiBgcl1HJrqPF6Oaf+riT9PV7ntHOt3RJvtQejobFfaLbX+RnTAj+f8J9VPn84puSBNO6+2z01arg6B7cFFVoDX6JurmK/CB2dak1kl82PYR2w1eQDIwpm6wlrGwZKiZ52dgDVFCVEntqKTrLW/IL4O5w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775765444; c=relaxed/simple; bh=eg6USMUwKMGcmo4SxhxiHpEO7qtj99Hcbo8wLwy+XHU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ksHpYKKIgFC7K/zVDI0dUm9xppAqFAf3l/Qed8WQLxy4O7+I1ZqyJuG5bzH9H7oGesdzQwA5N1o3PeRufFGVZ/jCSlijBst5PGll8i7KrQSHYFsJnrR9um0rU26AUVJwid/Bv6LH9XVy+iNcm218roKzwd9tOAKWg3ydvLLmxUk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RyU25tcH; 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="RyU25tcH" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7C851C4CEF7; Thu, 9 Apr 2026 20:10:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775765444; bh=eg6USMUwKMGcmo4SxhxiHpEO7qtj99Hcbo8wLwy+XHU=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=RyU25tcHAPnyzMiCrBTAEuI1LjM2HxwYmlrPVVL2oSqOxgpFWVhpvuXS5Co4zBijg yVl6JTN4AqcptXvtmPywPKcpbb3X29AKb7eSj1YnAUHC5N2n8ipXFPpF6C6p1CZZJd VNIQ2UFBuvgFKMDMLOnA84JQpJh97wzAZT2zTSvf7LbVn1sKGaKb36kTPGTVpj6Il6 /5YCBUojguBR1ABeX1bLE25RGUjdF+N7qw+apILmvBCIwS0YyrIGofVs0UZ7TSi3Yg mQW5Pf0C0uuGkNOZlMJ6ZLw1SjDigwee2CKjfBmiD0lS+SMWVtJSGtolTnpWzdHR4m CkHPdL/pn1i0A== Message-ID: <41cfd0f95b7fde411c0d59463dce979be89cb8ef.camel@kernel.org> 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 16:10:41 -0400 In-Reply-To: <20260409190213.GQ3836593@ZenIV> References: <20260122202025.GG3183987@ZenIV> <20260404080751.2526990-1-viro@zeniv.linux.org.uk> <53b4795292d1df2fe1569fc724325ab52fcab322.camel@kernel.org> <20260409190213.GQ3836593@ZenIV> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-04-09 at 20:02 +0100, Al Viro wrote: > On Thu, Apr 09, 2026 at 12:51:38PM -0400, Jeff Layton wrote: >=20 > > I can also confirm that this fixes the livelock we were seeing as a > > precursor to this UAF [1]. I'm still not 100% sure how that livelock > > leads to a UAF. Claude thinks is has to do with the livelock making the > > loop outlive an RCU grace period. >=20 > Claude is full of substance likely to cause CoC complaints when mentioned= , > film at 11. I agree with the "livelock opens up some window" part, > what with the stack traces there, but I very much doubt that mechanism > has anything to do with the stuff on a shrink list staying around from > one iteration to another. The thing is, *nothing* is held between the > iterations of loop in there; data.dispose is empty at the end of the loop > body in all cases and reference in data.victim is not retained either > across the iterations either. We do have a weird RCU use pattern there, > all right (unbalanced acquire in select_collect2() paired with drop in > the caller of caller of select_collect2()), but the scope is neither > long nor extended by the livelock - in that case the function between > shrink_dentry_tree() and select_collect2() (d_walk()) is told to drop > the locks and return to its caller immediately. >=20 > What I suspect might be getting opened up is the possibility > of d_invalidate() (e.g. from pathname resolution trying > to walk into /proc//...) hitting in the middle of > proc_invalidate_siblings_dcache(). >=20 > Incidentally, one thing to check would be whether anything on affected > boxen had things mounted under /proc//... - that would give > some idea whether detach_mounts() (and namespace_unlock() with its > handling of the list of mountpoints) might be needed in the mix. >=20 =E2=97=8F had_submounts =3D false. The d_invalidate code never found submou= nts, so detach_mounts() was never called for this dentry. The find_submount / namespace_unlock path was not involved in this crash. Summary for Al: On this crashed machine, there were no mounts under /proc//... in any of the 49 mount namespaces checked (from surviving tasks). The d_invalidate stack frame shows had_submounts =3D false, confirming that detach_mounts() and namespace_unlock() were not in the mix for this crash. The code path was purely shrink_dcache_parent =E2=86=92 shrink_dentry_list =E2=86=92 lock_for_kill= , without any mount detachment involved. > > I did eventually find a vmcore and was able to confirm some things: > >=20 > > - in the freed/reallocated dentry, most of the fields are clobbered, > > but the d_lru list pointers seemed to be intact. That tells me that the > > sucker was added to the shrink list after being freed and reallocated. > > That means that the WARN_ON_ONCE() wouldn't have caught this anyway. > >=20 > > - there was some list corruption too: it looked like there were two > > different (on-stack) list heads in the list. There were also some > > dentries in there that were outside the=20 ...directories being shrunk from processes exiting. Here's what it found when I told it to validate the integrity of the d_lru that the realloc'ed entry was on. More claude slop from the earlier vmcore analysis: ------------------8<------------------ ### Dispose List: Two Merged Lists The dispose list contains **55 unique entries** and **two stack-based list heads** forming a single ring: 1. **head_A** at `0xffffc900c0e8fcc0` =E2=80=94 crashed thread's `data.disp= ose` 2. **head_B** at `0xffffc900c0e7fcc0` =E2=80=94 another thread's `data.disp= ose` (exited, stack freed, exactly 0x10000 bytes apart) ``` 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 h= ead_A ``` ### 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 entri= es - `/proc/1/mountinfo`, `/proc/1/status`, `/proc/1/net` =E2=80=94 PID 1 entr= ies - `/proc/sys/vm/overcommit_memory`, `/proc/sys/fs/*` =E2=80=94 sysctl entri= es - `/proc/pressure/{cpu,io,memory}` =E2=80=94 PSI entries - `/proc/swaps`, `/proc/cpuinfo`, `/proc/kcore` =E2=80=94 root proc entries ------------------8<------------------ >=20 > =20 > > Note that this was an older (v6.13-based) kernel though, and it's > > possible there are other unfixed bugs in here. > >=20 > > I'm currently trying to reproduce the UAF using that mdelay() based > > fault injection patch on something closer to mainline. ...and so far it's not trivially reproducible. I can reproduce the stalls on a recent kernel without your patches, but whatever causes this UAF is not easily triggerable by just stalling things out. I think I'm going to have to hunt for more vmcores... --=20 Jeff Layton