Re: [PATCH] driver: usb: nullify dangling pointer in cdc_ncm_free

[Johan Hovold]

On Sat, Apr 09, 2022 at 08:09:00PM +0800, Dongliang Mu wrote:
> From: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> 
> cdc_ncm_bind calls cdc_ncm_bind_common and sets dev->data[0]
> with ctx. However, in the unbind function - cdc_ncm_unbind,
> it calls cdc_ncm_free and frees ctx, leaving dev->data[0] as
> a dangling pointer. The following ioctl operation will trigger
> the UAF in the function cdc_ncm_set_dgram_size.
> 
> Fix this by setting dev->data[0] as zero.

This sounds like a poor band-aid. Please explain how this prevent the
ioctl() from racing with unbind(). 

Johan

> ==================================================================
> BUG: KASAN: use-after-free in cdc_ncm_set_dgram_size+0xc91/0xde0
> Read of size 8 at addr ffff8880755210b0 by task dhcpcd/3174
> 
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>  print_address_description.constprop.0.cold+0xeb/0x495 mm/kasan/report.c:313
>  print_report mm/kasan/report.c:429 [inline]
>  kasan_report.cold+0xf4/0x1c6 mm/kasan/report.c:491
>  cdc_ncm_set_dgram_size+0xc91/0xde0 drivers/net/usb/cdc_ncm.c:608
>  cdc_ncm_change_mtu+0x10c/0x140 drivers/net/usb/cdc_ncm.c:798
>  __dev_set_mtu net/core/dev.c:8519 [inline]
>  dev_set_mtu_ext+0x352/0x5b0 net/core/dev.c:8572
>  dev_set_mtu+0x8e/0x120 net/core/dev.c:8596
>  dev_ifsioc+0xb87/0x1090 net/core/dev_ioctl.c:332
>  dev_ioctl+0x1b9/0xe30 net/core/dev_ioctl.c:586
>  sock_do_ioctl+0x15a/0x230 net/socket.c:1136
>  sock_ioctl+0x2f1/0x640 net/socket.c:1239
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:870 [inline]
>  __se_sys_ioctl fs/ioctl.c:856 [inline]
>  __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0x80 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7f00859e70e7
> RSP: 002b:00007ffedd503dd8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00007f00858f96c8 RCX: 00007f00859e70e7
> RDX: 00007ffedd513fc8 RSI: 0000000000008922 RDI: 0000000000000018
> RBP: 00007ffedd524178 R08: 00007ffedd513f88 R09: 00007ffedd513f38
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007ffedd513fc8 R14: 0000000000000028 R15: 0000000000008922
>  </TASK>

> Reported-by: syzbot+eabbf2aaa999cc507108@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> ---
>  drivers/net/usb/cdc_ncm.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
> index 15f91d691bba..9fc2df9f0b63 100644
> --- a/drivers/net/usb/cdc_ncm.c
> +++ b/drivers/net/usb/cdc_ncm.c
> @@ -1019,6 +1019,7 @@ void cdc_ncm_unbind(struct usbnet *dev, struct usb_interface *intf)
>  
>  	usb_set_intfdata(intf, NULL);
>  	cdc_ncm_free(ctx);
> +	dev->data[0] = 0;
>  }
>  EXPORT_SYMBOL_GPL(cdc_ncm_unbind);