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b=r0NYO6QgOQnz7w0hGW6fe+snBccFuqDWJoNXEh5XR3kO9qkpDi240W2SQeqIiWg7n VxSvuCD3UHDRTxvB9HlEISY1HtCjI8EKNHdWnN4nhBGWax3+i7Oh2fpFLAoFxL1I/F WFzcldkI++wq3fxJp0UXiujq1YwUhbt7YTAa7bpKr6JUfNncHcwKv/R/iHsroFU0Rp on1bUR8C9SU6AfcPxOOCt5eTaQT1zaioluBPSDg+iWQVAahgq+Vnw+TeJISnj0zmAp L1mE3xskQMApGmmc/1iFqbGOh5s3YqHnPBw5LJzmeTvutQtvch5ASKwEp3N3dFq2KS dG9qLfd0NQeXA== From: SeongJae Park To: Junxi Qian Cc: SeongJae Park , 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 18:20:27 -0700 Message-ID: <20260421012028.67143-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260420125405.362137-1-qjx1298677004@gmail.com> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: xjq88cu4d8xohe9wa38jee75kewmsxis X-Rspam-User: X-Rspamd-Queue-Id: D5E394000A X-Rspamd-Server: rspam05 X-HE-Tag: 1776734438-886710 X-HE-Meta: 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 uK9lYlIq 9V90qgREcgpKJnpCjgOU0d0XrN28otMibGY/iEyAS2vitVVwYx0MAOcP0J8UZ3WweJrBelbqVjFyKKr4nXnZYm5BbRuelDz21FH7hkd336NcIvYj3wkgmiFIE0hK7BkwZ3bfVyekoKUwQrdmZiVLsvK6Hf4cSdCsRiUFzGggVeGrOTlNbsguEXf9z9D42dcH5Q/crBHPgJSNMMCXW2VqsHKJtMv26ouA0u+xU7WGxiBogwilArbUEZZatZhpPOQzjMpyS3LpzHRqWKaxadvOSQhKF8XlcsEjm8L8ujNB4p0fcLw5VxW+FdudLRm3NdbopwXDq Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. 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 [...]