PCI: Fix the int overflow in proc_bus_pci_write()

Following testcase can trigger a softlockup BUG.
syscall(__NR_pwritev, /*fd=*/..., /*vec=*/..., /*vlen=*/...,
        /*pos_l=*/0x80010000, /*pos_h=*/0x100);

watchdog: BUG: soft lockup - CPU#11 stuck for 22s! [test:537]
Modules linked in:
CPU: 11 PID: 537 Comm: test Not tainted 5.10.0+ torvalds#67
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
RIP: 0010:pci_user_write_config_dword+0x67/0xc0
Code: 00 00 44 89 e2 48 8b 87 e0 00 00 00 48 8b 40 20 e8 9e 54 7e 00 48 c7 c7 20 48 a2 83 41 89 c0 c6 07 00 0f 1f 40 00 fb 45 85 c0 <7e> 12 41 8d 80 7f ff ff ff 41 b8 de ff ff ff 83 f8 08 76 0c 5b 44
RSP: 0018:ffffc900016c3d30 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff888042058000 RCX: 0000000000000005
RDX: ffff888004058a00 RSI: 0000000000000046 RDI: ffffffff83a24820
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
R10: ffff888005c25900 R11: 0000000000000000 R12: 0000000080c48680
R13: 0000000020c38684 R14: 0000000080010000 R15: ffff888004702408
FS:  000000003ae91880(0000) GS:ffff88801f380000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020c00000 CR3: 0000000006f2c000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 proc_bus_pci_write+0x22c/0x260
 proc_reg_write+0x40/0x90
 do_loop_readv_writev.part.0+0x97/0xc0
 do_iter_write+0xf6/0x150
 vfs_writev+0x97/0x130
 ? files_cgroup_alloc_fd+0x5c/0x70
 ? do_sys_openat2+0x1c9/0x320
 __x64_sys_pwritev+0xb1/0x100
 do_syscall_64+0x2b/0x40
 entry_SYSCALL_64_after_hwframe+0x6c/0xd6

The pos_l parameter for pwritev syscall may be an integer negative value,
which will make the variable pos in proc_bus_pci_write() negative and
variable cnt a very large number.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>

Author: Wang ShaoBo

drivers/pci/proc.c

@@ -121,7 +121,7 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 	if (ret)
 		return ret;
 
-	if (pos >= size)
+	if (pos < 0 || pos >= size)
 		return 0;
 	if (nbytes >= size)
 		nbytes = size;