net/smc: fix use-after-free of the LLC qentry in smc_llc_srv_add_link()

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

net/smc: fix use-after-free of the LLC qentry in smc_llc_srv_add_link()

smc_llc_srv_add_link() keeps add_llc pointing into the queue entry:

add_llc = &qentry->msg.add_link; smc_llc.c:1482
...
smc_llc_save_add_link_info(link_new, add_llc); smc_llc.c:1494
smc_llc_flow_qentry_del(&lgr->llc_flow_lcl); smc_llc.c:1495
...
u8 *llc_msg = smc_link_shared_v2_rxbuf(link) ?
(u8 *)lgr->wr_rx_buf_v2 : (u8 *)add_llc; smc_llc.c:1504
smc_llc_save_add_link_rkeys(link, link_new, llc_msg); smc_llc.c:1506

smc_llc_flow_qentry_del() kfree()s the entry, so on a link without a shared
v2 receive buffer the pointer handed to smc_llc_save_add_link_rkeys() is
already freed. Before the Fixes: commit that branch always used
lgr->wr_rx_buf_v2 and add_llc was not used after the free.

Reproduced on an unpatched tree over rxe, with KASAN, kasan_multi_shot
and a link forced to max_recv_sge == 1: the entry is freed and read by
the same call, and the freeing frame is smc_llc_srv_add_link() itself.

[ 2.523161] BUG: KASAN: slab-use-after-free in smc_llc_save_add_link_rkeys+0x333/0x350
[ 2.523499] Read of size 2 at addr ffff8880052194de by task kworker/0:1/11
[ 2.523789]
[ 2.523862] CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Not tainted 7.2.0-rc5-p0-g2c9dd296545d #35 PREEMPT(lazy)
[ 2.523865] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 2.523866] Workqueue: smc_hs_wq smc_listen_work
[ 2.523869] Call Trace:
[ 2.523870] <TASK>
[ 2.523871] dump_stack_lvl+0x53/0x70
[ 2.523872] print_report+0xd0/0x630
[ 2.523874] ? __pfx__raw_spin_lock_irqsave+0x10/0x10
[ 2.523876] ? smc_llc_save_add_link_rkeys+0x333/0x350
[ 2.523878] kasan_report+0xce/0x100
[ 2.523879] ? smc_llc_save_add_link_rkeys+0x333/0x350
[ 2.523881] smc_llc_save_add_link_rkeys+0x333/0x350
[ 2.523883] ? smcr_buf_reg_lgr+0x2a4/0x660
[ 2.523885] smc_llc_srv_add_link+0xaa2/0x1e50
[ 2.523888] ? _printk+0xba/0xf0
[ 2.523897] ? __pfx_smc_llc_srv_add_link+0x10/0x10
[ 2.523899] ? down_write+0xb0/0x130
[ 2.523903] ? __pfx_down_write+0x10/0x10
[ 2.523905] smc_listen_work+0x489e/0x4d00
[ 2.523907] ? kmem_cache_free+0x1c6/0x3a0
[ 2.523911] ? __pfx_smc_listen_work+0x10/0x10
[ 2.523913] ? release_sock+0x148/0x1d0
[ 2.523915] ? smc_tcp_listen_work+0xb4f/0xfc0
[ 2.523917] ? _raw_spin_lock_irq+0x80/0xe0
[ 2.523918] ? __pfx__raw_spin_lock_irq+0x10/0x10
[ 2.523920] process_one_work+0x633/0x1030
[ 2.523922] ? assign_work+0x11d/0x370
[ 2.523924] worker_thread+0x45b/0xd10
[ 2.523926] ? __pfx_worker_thread+0x10/0x10
[ 2.523928] ? __pfx_worker_thread+0x10/0x10
[ 2.523929] kthread+0x2c6/0x3b0
[ 2.523931] ? recalc_sigpending+0x15c/0x1e0
[ 2.523934] ? __pfx_kthread+0x10/0x10
[ 2.523935] ret_from_fork+0x36e/0x5a0
[ 2.523937] ? __pfx_ret_from_fork+0x10/0x10
[ 2.523938] ? __switch_to+0x572/0xdd0
[ 2.523943] ? __pfx_kthread+0x10/0x10
[ 2.523944] ret_from_fork_asm+0x1a/0x30
[ 2.523947] </TASK>
[ 2.523948]
[ 2.531253] Allocated by task 48:
[ 2.531399] kasan_save_stack+0x33/0x60
[ 2.531570] kasan_save_track+0x14/0x30
[ 2.531737] __kasan_kmalloc+0x8f/0xa0
[ 2.531905] __kmalloc_cache_noprof+0x158/0x370
[ 2.532100] smc_llc_enqueue+0x72/0x560
[ 2.532268] smc_wr_rx_tasklet_fn+0x474/0xa80
[ 2.532491] tasklet_action_common+0x20f/0x8a0
[ 2.532714] handle_softirqs+0x18e/0x590
[ 2.532886] do_softirq+0x3b/0x60
[ 2.533036] __local_bh_enable_ip+0x61/0x70
[ 2.533221] __alloc_skb+0x732/0x890
[ 2.533384] rxe_init_packet+0x16b/0x4f0
[ 2.533567] prepare_ack_packet+0xb8/0x830
[ 2.533760] rxe_receiver+0x495/0x96e0
[ 2.533933] do_work+0x144/0x470
[ 2
---truncated---