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 C05B11FBEBC; Tue, 21 Apr 2026 01:23:51 +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=1776734631; cv=none; b=tLbwx3vtOBQVMm29w4lI5i3VoOQ0Bj4G5H33GT18G8k9FbrtEnqx3AJGHlOoMtXc2adViOEonePXb7z2le4EWU1G5Ls3v90Ja8W2DLZeCzxzrHpLlHuNHc0YTlBTD7sC3O+6pQOZGCjRSB4iu95VsCQIButGaB/15r+a+mWs3XU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776734631; c=relaxed/simple; bh=XkZKTo3VZh3AYgCplCQOoxro1b6UUyXleYklHwpf71g=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=pSKakr7rXcjwtgC6gzN3is3e+iE9+YRaCt79370bSPABXEkxt0WbDoGDbyRi7pOM3mw9ofRUaz2KxiL2iG9ES16XNBl1FGODe4wunJ391Y58CGAH/OUsDuGagXu/E/NLrUUI9SxQwfejLISygPyZrZTn26cnqCZV8CNDWPwWOhk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=t4QKkzyS; 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="t4QKkzyS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2796CC19425; Tue, 21 Apr 2026 01:23:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776734631; bh=XkZKTo3VZh3AYgCplCQOoxro1b6UUyXleYklHwpf71g=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=t4QKkzySJTVFWSG6qXBEtMZA1cpdyGlOQbuhbGRDIL24YZvl9tChkDN0Ocz0zP97n P8NCc+OM3xopArrqeOrHdAnRV8mju919uSsNztfhtEuSqZtVC6BfAyElhgRdQBnxTf dl6gh4rxFfZaH+cQjlR4nd6ZOqzl5WxoXh864DmlDgD1UjYe8uvWflK4+YZFMZB6LS SWi3zaoJycfpTUD5ISRvY9sEYuesfo9fRjT5YMhqETwG0ytt8ZURC9+gcTfoteYAMz hjMY8HjQXsmUQTqjAD2MxIWYFQZxSiYJEOnXh3K6+84vhPGbdhDkfeCVcgsaeBkgHw 0tgVka4Nxej3Q== From: SeongJae Park To: SeongJae Park Cc: Junxi Qian , linux-mm@kvack.org, akpm@linux-foundation.org, damon@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/damon/sysfs-schemes: fix use-after-free on memcg_path and goal path Date: Mon, 20 Apr 2026 18:23:46 -0700 Message-ID: <20260421012346.67274-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260421012028.67143-1-sj@kernel.org> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Mon, 20 Apr 2026 18:20:27 -0700 SeongJae Park wrote: > Hello Junxi, > > > When you send a new version of a patch, please send it as a new mail with > changes log [1], rather than as a reply to the old version. Also, please share your revision plan first, and wait for possible others' comments for about a day before posting a new version. Thanks, SJ > > On Mon, 20 Apr 2026 20:54:05 +0800 Junxi Qian wrote: > > > memcg_path_store() and path_store() free and replace their respective > > string pointers without holding damon_sysfs_lock. This creates a race > > with the kdamond thread, which reads these pointers in > > damon_sysfs_add_scheme_filters() and damos_sysfs_add_quota_score() > > via damon_call() during a "commit" operation. > > > > The race window is as follows: > > Thread A (commit): holds damon_sysfs_lock, triggers damon_call() > > kdamond thread: reads sysfs_filter->memcg_path in > > damon_sysfs_memcg_path_to_id() > > Thread B (sysfs): calls memcg_path_store() WITHOUT lock, > > kfree(filter->memcg_path), replaces pointer > > > > Since Thread B does not hold damon_sysfs_lock, the kdamond thread can > > observe a freed pointer, resulting in a use-after-free when > > damon_sysfs_memcg_path_eq() dereferences it for string comparison. > > Thank you for finding and sharing this bug! > > > > Similarly, memcg_path_show() and path_show() read the string pointers > > without holding the lock, so a concurrent store can free the string > > just before sysfs_emit() dereferences it, causing a use-after-free. > > Is this correct? Isn't sysfs prohibiting such concurrent read/write using > kernfs_open_file->mutex? Were you able to trigger this? > > > > > KASAN reports the following on v7.0.0-rc5 with CONFIG_KASAN=y: > > > > ================================================================== > > BUG: KASAN: double-free in memcg_path_store+0x6e/0xc0 > > > > Free of addr ffff888100a086c0 by task exp/149 > > > > CPU: 1 UID: 0 PID: 149 Comm: exp Not tainted 7.0.0-rc5-gd38efd7c139a #18 PREEMPT(lazy) > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 > > Call Trace: > > > > dump_stack_lvl+0x55/0x70 > > print_report+0xcb/0x5d0 > > kasan_report_invalid_free+0x94/0xc0 > > check_slab_allocation+0xde/0x110 > > kfree+0x114/0x3b0 > > memcg_path_store+0x6e/0xc0 > > kernfs_fop_write_iter+0x2fc/0x490 > > vfs_write+0x8e1/0xcc0 > > ksys_write+0xee/0x1c0 > > do_syscall_64+0xfc/0x580 > > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > > > > > Allocated by task 147 on cpu 0 at 12.364295s: > > kasan_save_stack+0x24/0x50 > > kasan_save_track+0x17/0x60 > > __kasan_kmalloc+0x7f/0x90 > > __kmalloc_noprof+0x191/0x490 > > memcg_path_store+0x32/0xc0 > > kernfs_fop_write_iter+0x2fc/0x490 > > vfs_write+0x8e1/0xcc0 > > ksys_write+0xee/0x1c0 > > > > Freed by task 150 on cpu 0 at 13.373810s: > > kasan_save_stack+0x24/0x50 > > kasan_save_track+0x17/0x60 > > kasan_save_free_info+0x3b/0x60 > > __kasan_slab_free+0x43/0x70 > > kfree+0x137/0x3b0 > > memcg_path_store+0x6e/0xc0 > > kernfs_fop_write_iter+0x2fc/0x490 > > vfs_write+0x8e1/0xcc0 > > ksys_write+0xee/0x1c0 > > > > The buggy address belongs to the object at ffff888100a086c0 > > which belongs to the cache kmalloc-8 of size 8 > > ================================================================== > > > > Fix this by holding damon_sysfs_lock while swapping the path pointer > > in both memcg_path_store() and path_store(), > > I'd like to avoid use of the lock in long term. But this sounds good and > simple for hotfixes. > > > and while reading the > > path pointer in memcg_path_show() and path_show(). > > As I commented above, I'm not sure if this is really needed. > > > The actual kfree() > > is moved outside the lock since the old pointer is no longer reachable > > once replaced. > > > > Fixes: 490a43d07f16 ("mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write") > > I think this deserves Cc-ing stable@. > > Also, this patch is fixing two bugs. For making backport of the fixes easy, > could we split this patch into two patches, one for memcg_path_{show,store}(), > and the other one for path_{show,store}()? > > > Signed-off-by: Junxi Qian > > > > --- > > Changes in v2: > > - Also protect memcg_path_show() and path_show() with damon_sysfs_lock, > > since sysfs show and store callbacks can run concurrently and a > > lockless read in show can race with the kfree in store. (Sashiko AI) > > As I abovely commented, I'm not sure that Sashiko comment is correct. > > > > > mm/damon/sysfs-schemes.c | 36 ++++++++++++++++++++++++++++-------- > > 1 file changed, 28 insertions(+), 8 deletions(-) > > > > diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c > > index 5186966da..1a890e2a4 100644 > > --- a/mm/damon/sysfs-schemes.c > > +++ b/mm/damon/sysfs-schemes.c > > @@ -533,9 +533,14 @@ static ssize_t memcg_path_show(struct kobject *kobj, > > { > > struct damon_sysfs_scheme_filter *filter = container_of(kobj, > > struct damon_sysfs_scheme_filter, kobj); > > + ssize_t len; > > > > - return sysfs_emit(buf, "%s\n", > > + mutex_lock(&damon_sysfs_lock); > > I read Sashiko is commenting [2] about possible ABBA deadlock, and suggesting > mutex_trylock(). I think that makes sense. damon_sysfs_lock is always only > mutex_trylock()-ed for the reason. > > > + len = sysfs_emit(buf, "%s\n", > > filter->memcg_path ? filter->memcg_path : ""); > > + mutex_unlock(&damon_sysfs_lock); > > + > > + return len; > > } > > And I'm unsure if this change for memcg_path_show() is really needed. > > > > > static ssize_t memcg_path_store(struct kobject *kobj, > > @@ -543,15 +548,20 @@ static ssize_t memcg_path_store(struct kobject *kobj, > > { > > struct damon_sysfs_scheme_filter *filter = container_of(kobj, > > struct damon_sysfs_scheme_filter, kobj); > > - char *path = kmalloc_array(size_add(count, 1), sizeof(*path), > > - GFP_KERNEL); > > + char *path, *old; > > > > + path = kmalloc_array(size_add(count, 1), sizeof(*path), GFP_KERNEL); > > if (!path) > > return -ENOMEM; > > > > strscpy(path, buf, count + 1); > > - kfree(filter->memcg_path); > > + > > + mutex_lock(&damon_sysfs_lock); > > Let's use mutex_trylock(). > > > + old = filter->memcg_path; > > filter->memcg_path = path; > > + mutex_unlock(&damon_sysfs_lock); > > + > > + kfree(old); > > return count; > > } > > Same comments for path_show() and path_store() changes. > > [1] https://docs.kernel.org/process/submitting-patches.html#commentary > [2] https://lore.kernel.org/20260420183146.0DA7AC2BCB0@smtp.kernel.org > > > Thanks, > SJ > > [...] > Sent using hkml (https://github.com/sjp38/hackermail)