CVE-2025-39881 — linux / linux_kernel
Use After FreeIn the Linux kernel, the following vulnerability has been resolved:
kernfs: Fix UAF in polling when open file is released
A use-after-free (UAF) vulnerability was identified in the PSI (Pressure Stall Information) monitoring mechanism:
BUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140 Read of size 8 at addr ffff3de3d50bd308 by task systemd/1
psi_trigger_poll+0x3c/0x140 cgroup_pressure_poll+0x70/0xa0 cgroup_file_poll+0x8c/0x100 kernfs_fop_poll+0x11c/0x1c0 ep_item_poll.isra.0+0x188/0x2c0
Allocated by task 1: cgroup_file_open+0x88/0x388 kernfs_fop_open+0x73c/0xaf0 do_dentry_open+0x5fc/0x1200 vfs_open+0xa0/0x3f0 do_open+0x7e8/0xd08 path_openat+0x2fc/0x6b0 do_filp_open+0x174/0x368
Freed by task 8462: cgroup_file_release+0x130/0x1f8 kernfs_drain_open_files+0x17c/0x440 kernfs_drain+0x2dc/0x360 kernfs_show+0x1b8/0x288 cgroup_file_show+0x150/0x268 cgroup_pressure_write+0x1dc/0x340 cgroup_file_write+0x274/0x548
Reproduction Steps:
- Open test/cpu.pressure and establish epoll monitoring
- Disable monitoring: echo 0 > test/cgroup.pressure
- Re-enable monitoring: echo 1 > test/cgroup.pressure
The race condition occurs because:
- When cgroup.pressure is disabled (echo 0 > cgroup.pressure), it:
- Releases PSI triggers via cgroup_file_release()
- Frees of->priv through kernfs_drain_open_files()
- While epoll still holds reference to the file and continues polling
- Re-enabling (echo 1 > cgroup.pressure) accesses freed of->priv
epolling disable/enable cgroup.pressure fd=open(cpu.pressure) while(1) ... epoll_wait kernfs_fop_poll kernfs_get_active = true echo 0 > cgroup.pressure ... cgroup_file_show kernfs_show // inactive kn kernfs_drain_open_files cft->release(of); kfree(ctx); ... kernfs_get_active = false echo 1 > cgroup.pressure kernfs_show kernfs_activate_one(kn); kernfs_fop_poll kernfs_get_active = true cgroup_file_poll psi_trigger_poll // UAF ... end: close(fd)
To address this issue, introduce kernfs_get_active_of() for kernfs open files to obtain active references. This function will fail if the open file has been released. Replace kernfs_get_active() with kernfs_get_active_of() to prevent further operations on released file descriptors.
| Software | From | Fixed in |
|---|---|---|
| linux / linux_kernel | 6.1 | 6.1.153 |
| linux / linux_kernel | 6.2 | 6.6.107 |
| linux / linux_kernel | 6.7 | 6.12.48 |
| linux / linux_kernel | 6.13 | 6.16.8 |
| linux / linux_kernel | 6.17-rc1 | 6.17-rc1.x |
| linux / linux_kernel | 6.17-rc2 | 6.17-rc2.x |
| linux / linux_kernel | 6.17-rc3 | 6.17-rc3.x |
| linux / linux_kernel | 6.17-rc4 | 6.17-rc4.x |
| linux / linux_kernel | 6.17-rc5 | 6.17-rc5.x |
| debian / debian_linux | 11.0 | 11.0.x |
- https://git.kernel.org/stable/c/34d9cafd469c69ad85e6a36b4303c78382cf5c79
- https://git.kernel.org/stable/c/3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f
- https://git.kernel.org/stable/c/7e64474aba78d240f7804f48f2d454dcca78b15f
- https://git.kernel.org/stable/c/854baafc00c433cccbe0ab4231b77aeb9b637b77
- https://git.kernel.org/stable/c/ac5cda4fae8818cf1963317bb699f7f2f85b60af
- https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html
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