CVE-2025-21796: nfsd: clear acl_access/acl_default after releasing them
First published: Thu Feb 27 2025(Updated: )
In the Linux kernel, the following vulnerability has been resolved: nfsd: clear acl_access/acl_default after releasing them If getting acl_default fails, acl_access and acl_default will be released simultaneously. However, acl_access will still retain a pointer pointing to the released posix_acl, which will trigger a WARNING in nfs3svc_release_getacl like this: ------------[ cut here ]------------ refcount_t: underflow; use-after-free. WARNING: CPU: 26 PID: 3199 at lib/refcount.c:28 refcount_warn_saturate+0xb5/0x170 Modules linked in: CPU: 26 UID: 0 PID: 3199 Comm: nfsd Not tainted 6.12.0-rc6-00079-g04ae226af01f-dirty #8 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.1-2.fc37 04/01/2014 RIP: 0010:refcount_warn_saturate+0xb5/0x170 Code: cc cc 0f b6 1d b3 20 a5 03 80 fb 01 0f 87 65 48 d8 00 83 e3 01 75 e4 48 c7 c7 c0 3b 9b 85 c6 05 97 20 a5 03 01 e8 fb 3e 30 ff <0f> 0b eb cd 0f b6 1d 8a3 RSP: 0018:ffffc90008637cd8 EFLAGS: 00010282 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff83904fde RDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88871ed36380 RBP: ffff888158beeb40 R08: 0000000000000001 R09: fffff520010c6f56 R10: ffffc90008637ab7 R11: 0000000000000001 R12: 0000000000000001 R13: ffff888140e77400 R14: ffff888140e77408 R15: ffffffff858b42c0 FS: 0000000000000000(0000) GS:ffff88871ed00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000562384d32158 CR3: 000000055cc6a000 CR4: 00000000000006f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: <TASK> ? refcount_warn_saturate+0xb5/0x170 ? __warn+0xa5/0x140 ? refcount_warn_saturate+0xb5/0x170 ? report_bug+0x1b1/0x1e0 ? handle_bug+0x53/0xa0 ? exc_invalid_op+0x17/0x40 ? asm_exc_invalid_op+0x1a/0x20 ? tick_nohz_tick_stopped+0x1e/0x40 ? refcount_warn_saturate+0xb5/0x170 ? refcount_warn_saturate+0xb5/0x170 nfs3svc_release_getacl+0xc9/0xe0 svc_process_common+0x5db/0xb60 ? __pfx_svc_process_common+0x10/0x10 ? __rcu_read_unlock+0x69/0xa0 ? __pfx_nfsd_dispatch+0x10/0x10 ? svc_xprt_received+0xa1/0x120 ? xdr_init_decode+0x11d/0x190 svc_process+0x2a7/0x330 svc_handle_xprt+0x69d/0x940 svc_recv+0x180/0x2d0 nfsd+0x168/0x200 ? __pfx_nfsd+0x10/0x10 kthread+0x1a2/0x1e0 ? kthread+0xf4/0x1e0 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x34/0x60 ? __pfx_kthread+0x10/0x10 ret_from_fork_asm+0x1a/0x30 </TASK> Kernel panic - not syncing: kernel: panic_on_warn set ... Clear acl_access/acl_default after posix_acl_release is called to prevent UAF from being triggered.
Credit: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Linux Kernel | ||
| Linux Kernel | >=2.6.13<5.10.235 | |
| Linux Kernel | >=5.11<5.15.179 | |
| Linux Kernel | >=5.16<6.1.129 | |
| Linux Kernel | >=6.2<6.6.79 | |
| Linux Kernel | >=6.7<6.12.16 | |
| Linux Kernel | >=6.13<6.13.4 | |
| Linux Kernel | =6.14-rc1 | |
| Linux Kernel | =6.14-rc2 |
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Reference Links
- https://git.kernel.org/stable/c/1fd94884174bd20beb1773990fd3b1aa877688d9
- https://git.kernel.org/stable/c/2e59b2b68782519560b3d6a41dd66a3d01a01cd3
- https://git.kernel.org/stable/c/55d947315fb5f67a35e4e1d3e01bb886b9c6decf
- https://git.kernel.org/stable/c/7faf14a7b0366f153284db0ad3347c457ea70136
- https://git.kernel.org/stable/c/f8d871523142f7895f250a856f8c4a4181614510
- https://git.kernel.org/stable/c/8a1737ae42c928384ab6447f6ee1a882510e85fa
- https://git.kernel.org/stable/c/6f7cfee1a316891890c505563aa54f3476db52fd
Frequently Asked Questions
What is the severity of CVE-2025-21796?
CVE-2025-21796 has a moderate severity level due to its impact on access control list (ACL) handling in the Linux kernel.
How do I fix CVE-2025-21796?
To fix CVE-2025-21796, update to the latest version of the Linux kernel that includes the patch for this vulnerability.
What systems are affected by CVE-2025-21796?
CVE-2025-21796 affects various distributions of the Linux kernel that utilize the nfsd component.
What type of vulnerability is CVE-2025-21796?
CVE-2025-21796 is classified as an access control vulnerability related to improper handling of ACLs.
Is CVE-2025-21796 exploitable remotely?
CVE-2025-21796 could potentially be exploited remotely if the affected system's NFS service is exposed to untrusted networks.
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