CVE-2025-40052 smb: client: fix crypto buffers in non-linear memory

Not scored Published 28 Oct 2025 Updated 11 May 2026

A bug in the Linux kernel SMB client causes a kernel crash when handling encrypted mounts with high parallel I/O, leading to denial of service.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2025-40052?

CVE-2025-40052 is a denial of service vulnerability in Linux, published on 28 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix crypto buffers in non-linear memory

The crypto API, through the scatterlist API, expects input buffers to be in linear memory. We handle this with the cifs_sg_set_buf() helper that converts vmalloc'd memory to their corresponding pages.

However, when we allocate our aead_request buffer (@creq in smb2ops.c::crypt_message()), we do so with kvzalloc(), which possibly puts aead_request->__ctx in vmalloc area.

AEAD algorithm then uses ->__ctx for its private/internal data and operations, and uses sg_set_buf() for such data on a few places.

This works fine as long as @creq falls into kmalloc zone (small requests) or vmalloc'd memory is still within linear range.

Tasks' stacks are vmalloc'd by default (CONFIG_VMAP_STACK=y), so too many tasks will increment the base stacks' addresses to a point where virt_addr_valid(buf) will fail (BUG() in sg_set_buf()) when that happens.

In practice: too many parallel reads and writes on an encrypted mount will trigger this bug.

To fix this, always alloc @creq with kmalloc() instead. Also drop the @sensitive_size variable/arguments since kfree_sensitive() doesn't need it.

Backtrace:

[ 945.272081] ------------[ cut here ]------------ [ 945.272774] kernel BUG at include/linux/scatterlist.h:209! [ 945.273520] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC NOPTI [ 945.274412] CPU: 7 UID: 0 PID: 56 Comm: kworker/u33:0 Kdump: loaded Not tainted 6.15.0-lku-11779-g8e9d6efccdd7-dirty #1 PREEMPT(voluntary) [ 945.275736] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-2-gc13ff2cd-prebuilt.qemu.org 04/01/2014 [ 945.276877] Workqueue: writeback wb_workfn (flush-cifs-2) [ 945.277457] RIP: 0010:crypto_gcm_init_common+0x1f9/0x220 [ 945.278018] Code: b0 00 00 00 48 83 c4 08 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc 48 c7 c0 00 00 00 80 48 2b 05 5c 58 e5 00 e9 58 ff ff ff <0f> 0b 0f 0b 0f 0b 0f 0b 0f 0b 0f 0b 48 c7 04 24 01 00 00 00 48 8b [ 945.279992] RSP: 0018:ffffc90000a27360 EFLAGS: 00010246 [ 945.280578] RAX: 0000000000000000 RBX: ffffc90001d85060 RCX: 0000000000000030 [ 945.281376] RDX: 0000000000080000 RSI: 0000000000000000 RDI: ffffc90081d85070 [ 945.282145] RBP: ffffc90001d85010 R08: ffffc90001d85000 R09: 0000000000000000 [ 945.282898] R10: ffffc90001d85090 R11: 0000000000001000 R12: ffffc90001d85070 [ 945.283656] R13: ffff888113522948 R14: ffffc90001d85060 R15: ffffc90001d85010 [ 945.284407] FS: 0000000000000000(0000) GS:ffff8882e66cf000(0000) knlGS:0000000000000000 [ 945.285262] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 945.285884] CR2: 00007fa7ffdd31f4 CR3: 000000010540d000 CR4: 0000000000350ef0 [ 945.286683] Call Trace: [ 945.286952] <TASK> [ 945.287184] ? crypt_message+0x33f/0xad0 [cifs] [ 945.287719] crypto_gcm_encrypt+0x36/0xe0 [ 945.288152] crypt_message+0x54a/0xad0 [cifs] [ 945.288724] smb3_init_transform_rq+0x277/0x300 [cifs] [ 945.289300] smb_send_rqst+0xa3/0x160 [cifs] [ 945.289944] cifs_call_async+0x178/0x340 [cifs] [ 945.290514] ? __pfx_smb2_writev_callback+0x10/0x10 [cifs] [ 945.291177] smb2_async_writev+0x3e3/0x670 [cifs] [ 945.291759] ? find_held_lock+0x32/0x90 [ 945.292212] ? netfs_advance_write+0xf2/0x310 [ 945.292723] netfs_advance_write+0xf2/0x310 [ 945.293210] netfs_write_folio+0x346/0xcc0 [ 945.293689] ? __pfx__raw_spin_unlock_irq+0x10/0x10 [ 945.294250] netfs_writepages+0x117/0x460 [ 945.294724] do_writepages+0xbe/0x170 [ 945.295152] ? find_held_lock+0x32/0x90 [ 945.295600] ? kvm_sched_clock_read+0x11/0x20 [ 945.296103] __writeback_single_inode+0x56/0x4b0 [ 945.296643] writeback_sb_inodes+0x229/0x550 [ 945.297140] __writeback_inodes_wb+0x4c/0xe0 [ 945.297642] wb_writeback+0x2f1/0x3f0 [ 945.298069] wb_workfn+0x300/0x490 [ 945.298472] process_one_work+0x1fe/0x590 [ 945.298949] worker_thread+0x1ce/0x3c0 [ 945.299397] ? __pfx_worker_thread+0x10/0x10 [ 945.299900] kthr ---truncated---

Which versions are affected by CVE-2025-40052?

How do I fix CVE-2025-40052?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2025-40052 being exploited?

There is no known exploitation of CVE-2025-40052 in the wild and no public exploit code.

Sources

Also known as AZL-68846, BELL-CVE-2025-40052, DEBIAN-CVE-2025-40052, GHSA-76hq-frpj-75r7, ROOT-OS-DEBIAN-13-CVE-2025-40052, ROOT-OS-UBUNTU-2404-CVE-2025-40052 and 1 more.