Amazon Linux AMI: CVE-2025-21796: Security patch for kernel (ALAS-2025-1973)

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 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.