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CVE-2025-21796

Description

In the Linux kernel, the following vulnerability has been resolved:nfsd: clear acl_access/acl_default after releasing themIf getting acl_default fails, acl_access and acl_default will be releasedsimultaneously. However, acl_access will still retain a pointer pointingto the released posix_acl, which will trigger a WARNING innfs3svc_release_getacl like this:------------[ cut here ]------------refcount_t: underflow; use-after-free.WARNING: CPU: 26 PID: 3199 at lib/refcount.c:28refcount_warn_saturate+0xb5/0x170Modules linked in:CPU: 26 UID: 0 PID: 3199 Comm: nfsd Not tainted6.12.0-rc6-00079-g04ae226af01f-dirty #8Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS1.16.1-2.fc37 04/01/2014RIP: 0010:refcount_warn_saturate+0xb5/0x170Code: cc cc 0f b6 1d b3 20 a5 03 80 fb 01 0f 87 65 48 d8 00 83 e3 01 75e4 48 c7 c7 c0 3b 9b 85 c6 05 97 20 a5 03 01 e8 fb 3e 30 ff <0f> 0b ebcd 0f b6 1d 8a3RSP: 0018:ffffc90008637cd8 EFLAGS: 00010282RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff83904fdeRDX: dffffc0000000000 RSI: 0000000000000008 RDI: ffff88871ed36380RBP: ffff888158beeb40 R08: 0000000000000001 R09: fffff520010c6f56R10: ffffc90008637ab7 R11: 0000000000000001 R12: 0000000000000001R13: ffff888140e77400 R14: ffff888140e77408 R15: ffffffff858b42c0FS: 0000000000000000(0000) GS:ffff88871ed00000(0000)knlGS:0000000000000000CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033CR2: 0000562384d32158 CR3: 000000055cc6a000 CR4: 00000000000006f0DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400Call 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 preventUAF from being triggered.

POC

Reference

No PoCs from references.

Github

- https://github.com/w4zu/Debian_security