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 F34BC339B3D; Wed, 8 Apr 2026 21:05:44 +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=1775682345; cv=none; b=cyHCyNWN/ROSPPjvE8F0aC2+BDVoJTr4Fz8QTcLGVDWuc+3rF7y34xxeW6Ebjr9sCU2ksclVey9Hm4rnZnN0yhd4xPsFgNTuJEvQj1lizpCa+uNS4F0fXBk5XvO/EsOizoe8HUF5oih2yD5UCPIlVHw4u2bqCvchpDcm2UpvXAo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775682345; c=relaxed/simple; bh=5pF7YtRERMCa2hZ6asD7YIvR0VEUx5fH/DbdVR1U3sM=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=OA4C7n/FWhaWzzi6jptBHQHpb0WQbFQsZhcVRl2b88Pl0mQpe8gwNR3FoMgPV8W7NUvKVAWppX7ZBWpft2vBCwrYrPIApjbkTiYVTDTbYlVSrSWSExOOFOb1RGfA4O5Kg17A+b98UpxDh5DvAj+K0YWT/xRnwph89C33TggMKyY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pgbVqIt8; 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="pgbVqIt8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E0E94C19421; Wed, 8 Apr 2026 21:05:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775682344; bh=5pF7YtRERMCa2hZ6asD7YIvR0VEUx5fH/DbdVR1U3sM=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=pgbVqIt8PsgqIb3tULWeSN7IEY8hsVgaRKoEj1MPGy/LRfj3P4rsAmtHW0dlLcfVO /T17xYwcQArALHinozD1q1g+sgs06uZQKs1I/JIQrlUzJXu1QkUPB0c28HtpGJxNv3 W4Z2WL6ek785Y+RALuAwIx4JaUgAHU15EwVjl7+hFRHII6NFLlqSPysYEFqfZqu78R vEMy7X7Y+8U7K/rB4Ht4yxtGD9LNLA2d6pQqA2bWv8arjeuGxEvATcfGkDPe3mywNz wcSSv4zMuITqCBNf8Fgi5w7onhNhXgE/w12Gs8uwpuf6zNZwIB7oFB3xjbDtUXiEuV lSj0HN/jRgEsg== Message-ID: <10d8499d60824f319e32f52fd6a196b3645f6652.camel@kernel.org> Subject: Re: [PATCH] dcache: warn when a dentry is freed with a non-empty ->d_lru From: Jeff Layton To: Al Viro Cc: Christian Brauner , Jan Kara , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, clm@meta.com, gustavold@meta.com Date: Wed, 08 Apr 2026 17:05:41 -0400 In-Reply-To: <20260408192620.GI3836593@ZenIV> References: <20260406-dcache-warn-v1-1-c665efbc005f@kernel.org> <20260408064251.GE3836593@ZenIV> <20260408192620.GI3836593@ZenIV> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Wed, 2026-04-08 at 20:26 +0100, Al Viro wrote: > On Wed, Apr 08, 2026 at 02:28:20PM -0400, Jeff Layton wrote: >=20 > > ...it turns out that Gustavo had been chasing this independently to me, > > and had Claude do a bit more analysis. I included it below, but here's > > a link that may be more readable. Any thoughts? >=20 > Other than rather uncharitable ones about the usefulness of the Turing Te= st, > you mean? >=20 > > **In production** (narrow race window): The livelock occasionally > > resolves through specific timing that allows a parent dentry to be > > freed and its slab page reused. >=20 > Livelock is real and known, all right, but do explain what does "resolves > through specific timing that allows a parent dentry to be freed" mean. > Especially since the reference to parent is *not* dropped until after > having child detached from the tree and DCACHE_DENTRY_KILLED on it, > with ->d_lock on child held over that. So select_collect2() seeing > the victim still locked and attached to the tree has to happen before > the grace period for parent has a chance to begin. And rcu_read_lock() > grabbed there prevents that grace period from completing until we > do the matching rcu_read_unlock() in shrink_dcache_parent(). >=20 > > In `select_collect()` (the `d_walk` callback used by > > `shrink_dcache_parent`), two types of dentries are incorrectly counted > really? > > as "found": >=20 > > 1. **Dead dentries** (`d_lockref.count < 0`): Another CPU called > > `lockref_mark_dead()` in `__dentry_kill()` but hasn't yet called > > `dentry_unlist()` to remove the dentry from the parent's children list. > > With the debug delay, the dentry stays dead-but-visible for 5ms. >=20 > Yes. And? That's the livelock, all right, and it needs fixing, > but how does busy-wait here lead to UAF on anything? >=20 > > 2. **`DCACHE_SHRINK_LIST` dentries**: Already isolated by another > > shrinker path (e.g., the global LRU shrinker from `drop_caches`) to its > > own dispose list. These are being processed by that other path but > > slowly (5ms per proc dentry with the debug delay). > >=20 > > When `select_collect` counts these as `found++`, > > `shrink_dcache_parent()` sees `data.found > 0` and loops again. But > > these dentries can never be collected onto `data.dispose` (dead ones > > have count < 0, shrink-list ones already have `DCACHE_SHRINK_LIST` > > set), so the loop never makes progress =E2=86=92 **infinite loop**. >=20 > They have no business going into data.dispose; for fuck sake, dentries > on somebody else's shrink list are explicitly fed to shrink_kill(). >=20 > > **Why this is correct:** >=20 > It is not. >=20 > > - **Dead dentries (`count < 0`)**: These are being killed by another > > CPU's `__dentry_kill()`. That CPU will call `dentry_unlist()` to remove > > them from the parent's children list. `shrink_dcache_parent()` doesn't > > need to wait for them =E2=80=94 they'll disappear from the tree on thei= r own. >=20 > ... and since they keep their parents busy, we should not leave until > they are gone. For real fun, consider calls from shrink_dcache_for_umoun= t() - > and yes, it *is* possible for another thread's shrink list to contain > dentries from filesystem being shut down. Legitimately so. >=20 > > - **`DCACHE_SHRINK_LIST` dentries**: These are already on another > > shrinker's dispose list and will be processed by that path. Counting > > them as "found" forces `shrink_dcache_parent()` to wait for the other > > shrinker to finish, which can take arbitrarily long (especially with > > filesystem callbacks or the debug delay). >=20 > Ditto. >=20 > > - **The `select_collect2` path** (used when `data.found > 0` but > > `data.dispose` is empty) handles `DCACHE_SHRINK_LIST` dentries > > separately by setting `data->victim` and processing them directly. With > > this fix, `select_collect2` is only reached when there are genuinely > > unprocessable dentries (count > 0, not dead, not on shrink list), not > > when there are merely in-flight kills or concurrent shrinkers. >=20 > Bollocks, due to above. >=20 > > ### Relationship to the production UAF crash > >=20 > > The livelock is the **precursor** to the use-after-free crash seen in > > production (P2260313060): >=20 > ... and it still offers zero explanation of the path from livelock to > UAF. It may or may not be real, but there's nothing in all that verbiage > even suggesting what it might be. And proposed analysis is flat-out > wrong. >=20 > As for the livelock, see viro/vfs.git #work.dcache-busy-wait (in -next > as of today). Thanks for taking a look, Al. We'll keep looking at this thing and see if we can collect more info, and come up with a better theory of the crash. I'll dig deeper into the vmcore tomorrow. FWIW, I've been running a bpftrace script across a swath of machines that emulates the WARN_ON_ONCE() in the original patch I proposed. We've had a few crashes while it's running and haven't yet collected any stack traces. The script only runs about 80% of the time though, so it's possible we're just getting unlucky, but I sort of doubt it. It seems more likely that these dentries are racing onto the list after we've passed the check. --=20 Jeff Layton