CVE-2025-21796 in Linux
Summary
by MITRE • 02/27/2025
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:
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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 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: ? 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 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.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability described in CVE-2025-21796 resides within the Linux kernel's Network File System version 3 server implementation, specifically in how it handles access control lists. This flaw manifests as a use-after-free condition that can lead to system instability and potential privilege escalation. The issue occurs during the processing of NFSv3 GETACL operations, where the kernel attempts to manage POSIX access control lists for file permissions. When the system fails to retrieve the default ACL for a file, it releases both the access ACL and default ACL simultaneously, but fails to properly nullify the pointers to these released structures.
The technical root cause involves improper memory management in the nfs3svc_release_getacl function. When posix_acl_release is invoked to free memory associated with ACL structures, the code does not clear the references to these now-invalid memory locations. This creates a scenario where acl_access continues to point to memory that has already been freed, leading to a reference count underflow warning. The kernel's reference counting mechanism detects this invalid state and triggers a warning, which can escalate to a kernel panic when the panic_on_warn configuration is enabled. This behavior aligns with CWE-416, which addresses use-after-free vulnerabilities in software systems.
The operational impact of this vulnerability extends beyond simple system instability, as it represents a potential security risk for NFS server implementations. An attacker could potentially exploit this condition to cause denial of service or, in more sophisticated scenarios, manipulate the kernel's memory management to execute arbitrary code. The vulnerability is particularly concerning in environments where NFS services are heavily utilized, as it could be triggered through legitimate NFS client operations. The specific trigger involves NFSv3 GETACL requests where default ACL retrieval fails, making this issue relevant to systems running the affected Linux kernel versions.
Mitigation strategies for this vulnerability should focus on applying the kernel patch that ensures proper clearing of acl_access and acl_default pointers after posix_acl_release is called. System administrators should prioritize updating to kernel versions that contain this fix, particularly in production environments where NFS services are critical. Additionally, monitoring for kernel warnings related to reference count underflows should be implemented as part of security operations procedures. The fix directly addresses the underlying memory management error by ensuring that pointers are nullified immediately after the associated memory structures are released, preventing the scenario where freed memory is accessed through stale references. This approach aligns with ATT&CK technique T1068, which involves exploiting weaknesses in the kernel's memory management to gain unauthorized access or escalate privileges. Organizations should also consider implementing network segmentation and access controls to limit exposure to NFS services that might be vulnerable to such exploits.