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net/smc: fix warning in smc_rx_splice() when calling get_page()

smc_lo_register_dmb() allocates DMB buffers with kzalloc(), which are
later passed to get_page() in smc_rx_splice(). Since kmalloc memory is
not page-backed, this triggers WARN_ON_ONCE() in get_page() and prevents
holding a refcount on the buffer. This can lead to use-after-free if
the memory is released before splice_to_pipe() completes.

Use folio_alloc() instead, ensuring DMBs are page-backed and safe for
get_page().

WARNING: CPU: 18 PID: 12152 at ./include/linux/mm.h:1330 smc_rx_splice+0xaf8/0xe20 [smc]
CPU: 18 UID: 0 PID: 12152 Comm: smcapp Kdump: loaded Not tainted 6.17.0-rc3-11705-g9cf4672ecfee #10 NONE
Hardware name: IBM 3931 A01 704 (z/VM 7.4.0)
Krnl PSW : 0704e00180000000 000793161032696c (smc_rx_splice+0xafc/0xe20 [smc])
R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
Krnl GPRS: 0000000000000000 001cee80007d3001 00077400000000f8 0000000000000005
0000000000000001 001cee80007d3006 0007740000001000 001c000000000000
000000009b0c99e0 0000000000001000 001c0000000000f8 001c000000000000
000003ffcc6f7c88 0007740003e98000 0007931600000005 000792969b2ff7b8
Krnl Code: 0007931610326960: af000000 mc 0,0
0007931610326964: a7f4ff43 brc 15,00079316103267ea
#0007931610326968: af000000 mc 0,0
>000793161032696c: a7f4ff3f brc 15,00079316103267ea
0007931610326970: e320f1000004 lg %r2,256(%r15)
0007931610326976: c0e53fd1b5f5 brasl %r14,000793168fd5d560
000793161032697c: a7f4fbb5 brc 15,00079316103260e6
0007931610326980: b904002b lgr %r2,%r11
Call Trace:
smc_rx_splice+0xafc/0xe20 [smc]
smc_rx_splice+0x756/0xe20 [smc])
smc_rx_recvmsg+0xa74/0xe00 [smc]
smc_splice_read+0x1ce/0x3b0 [smc]
sock_splice_read+0xa2/0xf0
do_splice_read+0x198/0x240
splice_file_to_pipe+0x7e/0x110
do_splice+0x59e/0xde0
__do_splice+0x11a/0x2d0
__s390x_sys_splice+0x140/0x1f0
__do_syscall+0x122/0x280
system_call+0x6e/0x90
Last Breaking-Event-Address:
smc_rx_splice+0x960/0xe20 [smc]
---[ end trace 0000000000000000 ]---

Fixes: f7a22071dbf3 ("net/smc: implement DMB-related operations of loopback-ism")
Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
Signed-off-by: Sidraya Jayagond <sidraya@linux.ibm.com>
Link: https://patch.msgid.link/20250917184220.801066-1-sidraya@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

authored by

Sidraya Jayagond and committed by
Jakub Kicinski a35c04de 1091860a

+9 -5
+9 -5
net/smc/smc_loopback.c
··· 56 56 { 57 57 struct smc_lo_dmb_node *dmb_node, *tmp_node; 58 58 struct smc_lo_dev *ldev = smcd->priv; 59 + struct folio *folio; 59 60 int sba_idx, rc; 60 61 61 62 /* check space for new dmb */ ··· 75 74 76 75 dmb_node->sba_idx = sba_idx; 77 76 dmb_node->len = dmb->dmb_len; 78 - dmb_node->cpu_addr = kzalloc(dmb_node->len, GFP_KERNEL | 79 - __GFP_NOWARN | __GFP_NORETRY | 80 - __GFP_NOMEMALLOC); 81 - if (!dmb_node->cpu_addr) { 77 + 78 + /* not critical; fail under memory pressure and fallback to TCP */ 79 + folio = folio_alloc(GFP_KERNEL | __GFP_NOWARN | __GFP_NOMEMALLOC | 80 + __GFP_NORETRY | __GFP_ZERO, 81 + get_order(dmb_node->len)); 82 + if (!folio) { 82 83 rc = -ENOMEM; 83 84 goto err_node; 84 85 } 86 + dmb_node->cpu_addr = folio_address(folio); 85 87 dmb_node->dma_addr = SMC_DMA_ADDR_INVALID; 86 88 refcount_set(&dmb_node->refcnt, 1); 87 89 ··· 126 122 write_unlock_bh(&ldev->dmb_ht_lock); 127 123 128 124 clear_bit(dmb_node->sba_idx, ldev->sba_idx_mask); 129 - kvfree(dmb_node->cpu_addr); 125 + folio_put(virt_to_folio(dmb_node->cpu_addr)); 130 126 kfree(dmb_node); 131 127 132 128 if (atomic_dec_and_test(&ldev->dmb_cnt))