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