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 DC34B3537EF; Wed, 8 Apr 2026 18:28:23 +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=1775672903; cv=none; b=fYNLhHeofhdBBfFKbyC5G9zkgpXoI3f5rXOeXWr9fGk0+3nNHblHVnACjdzQQGrNLocTVkOpwpTVSesO4O7v506VF4Nm6su7mUqmj6ft4Jubzq+3i3FxBFDkNiQkoyOC1JLwwPf0Y/NZSccjOBhpFogFzFX05FPrK/OF9cNl9oI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775672903; c=relaxed/simple; bh=CL0FXxVPzsCOjfk/Q8/ufWpILq4QDS39p3t7y0XPEBU=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=a7dnnZ/WkgtTpf8PgUtdkcsVN32TInriRJuDymsUBy8xiDkRYnZQmgeC+we8i9S7Rn5JOIFyIczkkY7p7D+HpFW069x0/ZiDOxx+Q615tE27wg3+5N0nql94CpbXEufNgquM1pV7OeRhpaBsbZerq/HT9J7WY6ZQYQxPI67gfkk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Y/ozZPU5; 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="Y/ozZPU5" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BE596C19425; Wed, 8 Apr 2026 18:28:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775672903; bh=CL0FXxVPzsCOjfk/Q8/ufWpILq4QDS39p3t7y0XPEBU=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Y/ozZPU5Hcpqslp1lyzE8AWvm534QM39SJuMnHK6MyCvZKzA0pfB+7kFn5V+qeoxZ OG3FEN76Ir1XTtW+Yh2/8zv4YzkqV7C/Sz8/iGQa7TpRiGlxV55A/T3S+0hc8Ee6PT qV12F4y6rGQPd92mIJNZDuub0i0ShZo2MLsR8g2FUCaujfPyxG5r9983R0nSz/w8oT vBy6IOkobtXB4uxu/ah3H27S1dlhRPbNj3J77/8l3WL/8oIdrK7OAoM6iWhylmb5wP 5rqwasCXCk2ShHQzgag25JUadbwKZAEUIonEmE71+BEVodXoZMVg7NA60DYWbLQ3T3 a5SdTUYY/duRg== Message-ID: 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 14:28:20 -0400 In-Reply-To: <20260408064251.GE3836593@ZenIV> References: <20260406-dcache-warn-v1-1-c665efbc005f@kernel.org> <20260408064251.GE3836593@ZenIV> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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12:44:13PM -0400, Jeff Layton wrote: > > We've had a number of panics that seem to occur on hosts with heavy > > process churn. The symptoms are a panic when invalidating /proc entries > > as a task is exiting: > >=20 > > queued_spin_lock_slowpath+0x153/0x270 > > shrink_dentry_list+0x11d/0x220 > > shrink_dcache_parent+0x68/0x110 > > d_invalidate+0x90/0x170 > > proc_invalidate_siblings_dcache+0xc8/0x140 > > release_task+0x41b/0x510 > > do_exit+0x3d8/0x9d0 > > do_group_exit+0x7d/0xa0 > > get_signal+0x2a9/0x6a0 > > arch_do_signal_or_restart+0x1a/0x1c0 > > syscall_exit_to_user_mode+0xe6/0x1c0 > > do_syscall_64+0x74/0x130 > > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > >=20 > > The problem appears to be a UAF. It's freeing a shrink list of > > dentries, but one of the dentries on it has already been freed. >=20 > That, or dentry pointer passed to shrink_dcache_parent() is a > complete garbage - e.g. due to struct pid having already been > freed. Might make sense to try and get a crash dump and poke > around... >=20 Chris was able to track me down a vmcore. No, it actually does seem to be what we thought originally. The parent is fine, but one of the dentries under it has been freed and reallocated: >>> stack #0 queued_spin_lock_slowpath (kernel/locking/qspinlock.c:471:3) #1 spin_lock (./include/linux/spinlock.h:351:2) #2 lock_for_kill (fs/dcache.c:675:3) #3 shrink_dentry_list (fs/dcache.c:1086:8) #4 shrink_dcache_parent (fs/dcache.c:0) #5 d_invalidate (fs/dcache.c:1614:2) #6 proc_invalidate_siblings_dcache (fs/proc/inode.c:142:5) #7 proc_flush_pid (fs/proc/base.c:3478:2) #8 release_task (kernel/exit.c:279:2) #9 exit_notify (kernel/exit.c:775:3) #10 do_exit (kernel/exit.c:958:2) #11 do_group_exit (kernel/exit.c:1087:2) #12 get_signal (kernel/signal.c:3036:3) #13 arch_do_signal_or_restart (arch/x86/kernel/signal.c:337:6) #14 exit_to_user_mode_loop (kernel/entry/common.c:111:4) #15 exit_to_user_mode_prepare (./include/linux/entry-common.h:329:13) #16 __syscall_exit_to_user_mode_work (kernel/entry/common.c:207:2) #17 syscall_exit_to_user_mode (kernel/entry/common.c:218:2) #18 do_syscall_64 (arch/x86/entry/common.c:89:2) #19 entry_SYSCALL_64+0x6c/0xaa (arch/x86/entry/entry_64.S:121) #20 0x7f49ead2c482 >>> identify_address(stack[3]["dentry"]) 'slab object: kmalloc-96+0x48' >>> identify_address(stack[4]["parent"]) 'slab object: dentry+0x0' ...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? https://markdownpastebin.com/?id=3D7c258413493b4144ab27d5cdcb8ae5b4 -------------8<---------------------- ## dcache: `shrink_dcache_parent()` livelock leading to use-after-free ### Summary A race between concurrent proc dentry invalidation (`proc_flush_pid` =E2=86= =92 `d_invalidate` =E2=86=92 `shrink_dcache_parent`) and the global dentry shri= nker (`drop_caches` / memory pressure =E2=86=92 `prune_dcache_sb`) causes `shrink_dcache_parent()` to loop indefinitely. This livelock is the root cause of the use-after-free crash observed in production (see P2260313060 for the original crash analysis). ### How the bug manifests **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. When a sibling's `__dentry_kill` then tries `spin_lock(&parent->d_lock)` on the reused memory =E2=86=92 page faul= t in `queued_spin_lock_slowpath` (Oops). **With `CONFIG_DCACHE_SHRINK_RACE_DEBUG`** (5ms delay in `__dentry_kill`): The race is deterministic. `shrink_dcache_parent()` livelocks on the first iteration and never completes. ### Root cause In `select_collect()` (the `d_walk` callback used by `shrink_dcache_parent`), two types of dentries are incorrectly counted as "found": 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. 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). 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**. ``` shrink_dcache_parent() loop: for (;;) { d_walk(parent, &data, select_collect); if (!list_empty(&data.dispose)) { shrink_dentry_list(&data.dispose); // never reached continue; } if (!data.found) break; // never reached because found > 0 // ... loops forever } ``` ### Reproducer **Requirements:** - `CONFIG_DCACHE_SHRINK_RACE_DEBUG=3Dy` (injects 5ms `mdelay()` in `__dentry_kill` for proc dentries) - `CONFIG_KASAN=3Dy` (optional, for UAF detection) - `CONFIG_DEBUG_KERNEL=3Dy` **Debug patch** (apply to `fs/Kconfig` and `fs/dcache.c`): ```diff --- a/fs/Kconfig +++ b/fs/Kconfig @@ -9,6 +9,15 @@ menu "File systems" config DCACHE_WORD_ACCESS bool =20 +config DCACHE_SHRINK_RACE_DEBUG + bool "Debug: inject delay in __dentry_kill to widen race window" + depends on DEBUG_KERNEL + default n + help + Inject a delay in __dentry_kill() between releasing d_lock and + re-acquiring it, to make the shrink_dentry_list race reproducible + in test environments. Only enable for testing. + config VALIDATE_FS_PARSER --- a/fs/dcache.c +++ b/fs/dcache.c @@ -32,6 +32,7 @@ #include +#include #include "internal.h" =20 @@ -630,6 +631,16 @@ static struct dentry *__dentry_kill(...) cond_resched(); +#ifdef CONFIG_DCACHE_SHRINK_RACE_DEBUG + /* + * Delay proc dentry kills to keep dead dentries in the tree + * longer. With the bug (count < 0 counted as "found" in + * select_collect), d_walk keeps re-finding dead dentries and + * shrink_dcache_parent() loops forever. + */ + if (dentry->d_sb->s_magic =3D=3D 0x9fa0 /* PROC_SUPER_MAGIC */) + mdelay(5); +#endif /* now that it's negative, ->d_parent is stable */ ``` **Test program** (`test_dcache_race.sh`): The reproducer creates multi-threaded processes, populates their `/proc//task//...` dcache entries, then SIGKILLs them while simultaneously running `drop_caches` in tight loops. This creates concurrent `proc_flush_pid` (from dying threads) and `prune_dcache_sb` (from `drop_caches`) paths competing on the same proc dentries. ```c /* Key structure: * - Fork child with N threads (creates /proc//task//... entries) * - Parent reads all /proc entries to populate dcache * - Background threads continuously do: echo 2 > /proc/sys/vm/drop_caches * - SIGKILL child -> all threads exit -> concurrent proc_flush_pid * - drop_caches shrinker races with proc_flush_pid on same dentries */ ``` Parameters used: 50 threads/process, 200 iterations, 4 shrinker threads, 4 reader threads. **vmtest.toml:** ```toml [[target]] name =3D "dcache-shrink-race" kernel =3D "arch/x86/boot/bzImage" kernel_args =3D "hung_task_panic=3D0 softlockup_panic=3D0 rcupdate.rcu_cpu_stall_suppress=3D1" command =3D "/mnt/vmtest/test_dcache_race.sh" [target.vm] memory =3D "16G" # KASAN needs extra memory num_cpus =3D 8 timeout =3D 1200 ``` ### Reproduction results | Kernel | Result | |---|---| | Unfixed + debug delay + KASAN | **FAIL**: livelock on iteration 1, test timed out at 750s | | Fixed + debug delay + KASAN | **PASS**: all 200 iterations completed, no KASAN/warnings | ### Fix The fix is in `select_collect()` =E2=80=94 stop counting dead dentries and `DCACHE_SHRINK_LIST` dentries as "found": ```diff --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1448,13 +1459,27 @@ static enum d_walk_ret select_collect(void *_data, struct dentry *dentry) if (data->start =3D=3D dentry) goto out; =20 - if (dentry->d_flags & DCACHE_SHRINK_LIST) { - data->found++; + if (dentry->d_lockref.count < 0) { + /* + * Dead dentry (lockref_mark_dead sets count negative). + * Another CPU is in the middle of __dentry_kill() and + * will shortly unlink it from the tree. Do not count + * it as "found" --- that causes shrink_dcache_parent() + * to loop indefinitely. + */ + } else if (dentry->d_flags & DCACHE_SHRINK_LIST) { + /* + * Already on a shrink list, being processed by another + * path (e.g., the global LRU shrinker). Do not count + * it as "found" --- if the other path is slow (e.g., + * contention on d_lock or filesystem callbacks), + * shrink_dcache_parent() would spin forever waiting for + * them to finish. The other shrinker will handle these + * dentries. + */ } else if (!dentry->d_lockref.count) { to_shrink_list(dentry, &data->dispose); data->found++; - } else if (dentry->d_lockref.count < 0) { - data->found++; } ``` **Why this is correct:** - **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 their ow= n. - **`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). - **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. ### Relationship to the production UAF crash The livelock is the **precursor** to the use-after-free crash seen in production (P2260313060): 1. Without the debug delay, the `__dentry_kill` race window is nanoseconds (just `cond_resched()`). 2. Most of the time, the dead dentry is unlinked before `select_collect` finds it =E2=86=92 no issue. 3. Occasionally, `select_collect` finds dead dentries and spins briefly. During this spinning, the specific timing allows a parent dentry to be fully freed (via `dentry_free` =E2=86=92 `call_rcu` =E2=86=92 = slab reclaim) and its slab page reused for `kmalloc-96`. 4. When the spinning `shrink_dcache_parent` or a concurrent `__dentry_kill` then accesses the freed parent =E2=86=92 UAF crash. The fix prevents the spinning entirely, eliminating both the livelock and the UAF.