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Mon, 20 Apr 2026 01:53:35 -0700 (PDT) Received: from localhost.localdomain ([14.218.34.206]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-36141990bb6sm10104820a91.17.2026.04.20.01.53.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2026 01:53:34 -0700 (PDT) From: Junxi Qian To: linux-mm@kvack.org Cc: sj@kernel.org, akpm@linux-foundation.org, damon@lists.linux.dev, linux-kernel@vger.kernel.org Subject: [PATCH] mm/damon/sysfs-schemes: fix use-after-free on memcg_path and goal path Date: Mon, 20 Apr 2026 16:53:32 +0800 Message-Id: <20260420085332.178473-1-qjx1298677004@gmail.com> X-Mailer: git-send-email 2.34.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: A605418000D X-Rspamd-Server: rspam07 X-Stat-Signature: hqgr8751z89x5tzuhkk8q51eiwnzxxxf X-Rspam-User: X-HE-Tag: 1776675216-519992 X-HE-Meta: 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 YM0XZdj0 Ihl+Ubu15CTKOBE7umz+IGnW3CsxiKwvomC5hgEoQx+58DBXRHyn9Q9ECv+dcQqq4yZmY3tr963Zdp51QuegI+4MIyVRoitE57uqM+0WSGhlG9hbIFR6dnZUh1SDdarAheaOSOY811DCkOLFrVYs+c5ujf6UOnZE7S8TJ/hIjySR+RfhKGHXA16ENwbK+RRd3Pv+ij4VVBPHbDQSDvjn48qWOpHjrH2XqGmIQvr4rbzK5NPCBUMEYsO45/qch6WJLOERhwaenZTZ74P7wc7ZQKjRPKE898uKpKTpbV0a8+2sfi5ZzBRofk+qAeQ+n9L0HwbOct5xLJNQEvhC3kxMLgRR+XvDcqwa+ULOfYMXsSrWVJfKtFTteN1Ld3WJhxwHVVjc95nSSZYGKCpLkX+z3uHxDFpUtMYXx/8Bz1L5nlkyZWnYhWdRyOz1kJDZjEKSpseONWwee1SJiUxg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. 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(). 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") Signed-off-by: Junxi Qian --- #define _GNU_SOURCE #include #include #include #include #include #include #include #include #define DAMON_BASE "/sys/kernel/mm/damon/admin" static volatile int stop; static char memcg_path_file[1024], state_file[1024]; static char real_memcg_path[512] = "/"; static int num_extra_cgroups = 200; static char cgroup_base[256]; static int write_str(const char *path, const char *val) { int fd = open(path, O_WRONLY); if (fd < 0) return -errno; ssize_t r = write(fd, val, strlen(val)); int e = errno; close(fd); return r > 0 ? 0 : -e; } static int create_extra_cgroups(void) { struct stat st; char path[512]; int created = 0; if (stat("/sys/fs/cgroup/cgroup.controllers", &st) == 0) strncpy(cgroup_base, "/sys/fs/cgroup", sizeof(cgroup_base)); else if (stat("/sys/fs/cgroup/memory", &st) == 0) strncpy(cgroup_base, "/sys/fs/cgroup/memory", sizeof(cgroup_base)); else return 0; for (int i = 0; i < num_extra_cgroups; i++) { snprintf(path, sizeof(path), "%s/damon_poc_%d", cgroup_base, i); if (mkdir(path, 0755) == 0) created++; } if (created > 0) snprintf(real_memcg_path, sizeof(real_memcg_path), "%s/damon_poc_0", cgroup_base); return created; } static void cleanup_extra_cgroups(void) { char path[512]; if (!cgroup_base[0]) return; for (int i = 0; i < num_extra_cgroups; i++) { snprintf(path, sizeof(path), "%s/damon_poc_%d", cgroup_base, i); rmdir(path); } } static void *path_writer(void *arg) { long tid = (long)arg; int i = 0; char buf[512]; while (!stop) { switch (i % 4) { case 0: snprintf(buf,sizeof(buf),"%s",real_memcg_path); break; case 1: snprintf(buf,sizeof(buf), "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX_%d_%ld",i,tid); break; case 2: snprintf(buf,sizeof(buf),"YYYYYYYYYYYYYY_%d",i); break; case 3: snprintf(buf,sizeof(buf),"Z_%d",i); break; } write_str(memcg_path_file, buf); i++; } return NULL; } static void *committer(void *arg) { while (!stop) { if (write_str(state_file, "commit") == 0) usleep(10); } return NULL; } static int setup_damon(void) { char path[1024]; int err; snprintf(path,sizeof(path),"%s/kdamonds/nr_kdamonds",DAMON_BASE); write_str(path,"1"); snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/nr_contexts",DAMON_BASE); write_str(path,"1"); snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/operations",DAMON_BASE); if (write_str(path,"paddr")) { write_str(path,"vaddr"); snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/targets/nr_targets",DAMON_BASE); write_str(path,"1"); char pid[32]; snprintf(pid,sizeof(pid),"%d",getpid()); snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/targets/0/pid_target",DAMON_BASE); write_str(path,pid); } snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/schemes/nr_schemes",DAMON_BASE); write_str(path,"1"); snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/schemes/0/action",DAMON_BASE); write_str(path,"stat"); snprintf(state_file,sizeof(state_file), "%s/kdamonds/0/state",DAMON_BASE); if ((err = write_str(state_file,"on"))) return err; snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/schemes/0/filters/nr_filters",DAMON_BASE); write_str(path,"1"); snprintf(path,sizeof(path), "%s/kdamonds/0/contexts/0/schemes/0/filters/0/type",DAMON_BASE); write_str(path,"memcg"); snprintf(memcg_path_file,sizeof(memcg_path_file), "%s/kdamonds/0/contexts/0/schemes/0/filters/0/memcg_path", DAMON_BASE); write_str(memcg_path_file, real_memcg_path); return 0; } int main(int argc, char *argv[]) { int duration = 60, n_writers = 4; if (argc > 1) duration = atoi(argv[1]); if (argc > 2) num_extra_cgroups = atoi(argv[2]); if (getuid()) { fprintf(stderr,"Need root\n"); return 1; } create_extra_cgroups(); if (setup_damon()) { cleanup_extra_cgroups(); return 1; } sleep(1); pthread_t w[8], c; for (int i = 0; i < n_writers; i++) pthread_create(&w[i],NULL,path_writer,(void*)(long)i); pthread_create(&c,NULL,committer,NULL); sleep(duration); stop = 1; for (int i = 0; i < n_writers; i++) pthread_join(w[i],NULL); pthread_join(c,NULL); write_str(state_file,"off"); cleanup_extra_cgroups(); printf("Check: dmesg | grep -A20 'BUG: KASAN'\n"); } --- mm/damon/sysfs-schemes.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c index 5186966da..005cfecb1 100644 --- a/mm/damon/sysfs-schemes.c +++ b/mm/damon/sysfs-schemes.c @@ -543,15 +543,20 @@ 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); + old = filter->memcg_path; filter->memcg_path = path; + mutex_unlock(&damon_sysfs_lock); + + kfree(old); return count; } @@ -1196,15 +1201,20 @@ static ssize_t path_store(struct kobject *kobj, { struct damos_sysfs_quota_goal *goal = container_of(kobj, struct damos_sysfs_quota_goal, 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(goal->path); + + mutex_lock(&damon_sysfs_lock); + old = goal->path; goal->path = path; + mutex_unlock(&damon_sysfs_lock); + + kfree(old); return count; } -- 2.34.1