wifi: mt7601u: fix an integer underflow

Fix an integer underflow that leads to a null pointer dereference in
'mt7601u_rx_skb_from_seg()'. The variable 'dma_len' in the URB packet
could be manipulated, which could trigger an integer underflow of
'seg_len' in 'mt7601u_rx_process_seg()'. This underflow subsequently
causes the 'bad_frame' checks in 'mt7601u_rx_skb_from_seg()' to be
bypassed, eventually leading to a dereference of the pointer 'p', which
is a null pointer.

Ensure that 'dma_len' is greater than 'min_seg_len'.

Found by a modified version of syzkaller.

KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 PID: 12 Comm: ksoftirqd/0 Tainted: G        W  O      5.14.0+
#139
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
RIP: 0010:skb_add_rx_frag+0x143/0x370
Code: e2 07 83 c2 03 38 ca 7c 08 84 c9 0f 85 86 01 00 00 4c 8d 7d 08 44
89 68 08 48 b8 00 00 00 00 00 fc ff df 4c 89 fa 48 c1 ea 03 <80> 3c 02
00 0f 85 cd 01 00 00 48 8b 45 08 a8 01 0f 85 3d 01 00 00
RSP: 0018:ffffc900000cfc90 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffff888115520dc0 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffff8881118430c0 RDI: ffff8881118430f8
RBP: 0000000000000000 R08: 0000000000000e09 R09: 0000000000000010
R10: ffff888111843017 R11: ffffed1022308602 R12: 0000000000000000
R13: 0000000000000e09 R14: 0000000000000010 R15: 0000000000000008
FS:  0000000000000000(0000) GS:ffff88811a800000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000004035af40 CR3: 00000001157f2000 CR4: 0000000000750ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 mt7601u_rx_tasklet+0xc73/0x1270
 ? mt7601u_submit_rx_buf.isra.0+0x510/0x510
 ? tasklet_action_common.isra.0+0x79/0x2f0
 tasklet_action_common.isra.0+0x206/0x2f0
 __do_softirq+0x1b5/0x880
 ? tasklet_unlock+0x30/0x30
 run_ksoftirqd+0x26/0x50
 smpboot_thread_fn+0x34f/0x7d0
 ? smpboot_register_percpu_thread+0x370/0x370
 kthread+0x3a1/0x480
 ? set_kthread_struct+0x120/0x120
 ret_from_fork+0x1f/0x30
Modules linked in: 88XXau(O) 88x2bu(O)
---[ end trace 57f34f93b4da0f9b ]---
RIP: 0010:skb_add_rx_frag+0x143/0x370
Code: e2 07 83 c2 03 38 ca 7c 08 84 c9 0f 85 86 01 00 00 4c 8d 7d 08 44
89 68 08 48 b8 00 00 00 00 00 fc ff df 4c 89 fa 48 c1 ea 03 <80> 3c 02
00 0f 85 cd 01 00 00 48 8b 45 08 a8 01 0f 85 3d 01 00 00
RSP: 0018:ffffc900000cfc90 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffff888115520dc0 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffff8881118430c0 RDI: ffff8881118430f8
RBP: 0000000000000000 R08: 0000000000000e09 R09: 0000000000000010
R10: ffff888111843017 R11: ffffed1022308602 R12: 0000000000000000
R13: 0000000000000e09 R14: 0000000000000010 R15: 0000000000000008
FS:  0000000000000000(0000) GS:ffff88811a800000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000004035af40 CR3: 00000001157f2000 CR4: 0000000000750ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Signed-off-by: Jisoo Jang <jisoo.jang@yonsei.ac.kr>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221229092906.2328282-1-jisoo.jang@yonsei.ac.kr

drivers/net/wireless/mediatek/mt7601u/dma.c

@@ -123,7 +123,8 @@ static u16 mt7601u_rx_next_seg_len(u8 *data, u32 data_len)
 	if (data_len < min_seg_len ||
 	    WARN_ON_ONCE(!dma_len) ||
 	    WARN_ON_ONCE(dma_len + MT_DMA_HDRS > data_len) ||
-	    WARN_ON_ONCE(dma_len & 0x3))
+	    WARN_ON_ONCE(dma_len & 0x3) ||
+	    WARN_ON_ONCE(dma_len < min_seg_len))
 		return 0;
 
 	return MT_DMA_HDRS + dma_len;