Re: net/gadget: slab-out-of-bounds write in dev_config

[Andrey Konovalov]

On Tue, Dec 6, 2016 at 4:30 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 6 Dec 2016, Andrey Konovalov wrote:
>
>> Hi!
>>
>> I've got the following error report while running the syzkaller fuzzer.
>>
>> ep0_write() doesn't check the length, so a user can cause an
>> out-of-bounds with both size and data controlled.
>> There's a comment which says "IN DATA+STATUS caller makes len <=
>> wLength". While I'm not exactly sure what that means, the length seems
>> to be passed unmodified directly from dev_config().
>
> You're right about the comment being misleading.  It looks like
> somebody forgot to actually do the check.
>
>> This doesn't seem to be a critical security issue since gadgetfs can't
>> be mounted from a user namespace.
>>
>> On commit 3c49de52d5647cda8b42c4255cf8a29d1e22eff5 (Dec 2).
>>
>> ==================================================================
>> BUG: KASAN: slab-out-of-bounds in dev_config+0x86f/0x1190 at addr
>> ffff88003c47e160
>> Write of size 65537 by task syz-executor0/6356
>> CPU: 3 PID: 6356 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #19
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>  ffff88003c107ad8 ffffffff81f96aba ffffffff3dc11ef0 1ffff10007820eee
>>  ffffed0007820ee6 ffff88003dc11f00 0000000041b58ab3 ffffffff8598b4c8
>>  ffffffff81f96828 ffffffff813fb4a0 ffff88003b6eadc0 ffff88003c107738
>> Call Trace:
>>  [<     inline     >] __dump_stack lib/dump_stack.c:15
>>  [<ffffffff81f96aba>] dump_stack+0x292/0x398 lib/dump_stack.c:51
>>  [<ffffffff817e4dec>] kasan_object_err+0x1c/0x70 mm/kasan/report.c:159
>>  [<     inline     >] print_address_description mm/kasan/report.c:197
>>  [<ffffffff817e5080>] kasan_report_error+0x1f0/0x4e0 mm/kasan/report.c:286
>>  [<ffffffff817e5705>] kasan_report+0x35/0x40 mm/kasan/report.c:306
>>  [<     inline     >] check_memory_region_inline mm/kasan/kasan.c:308
>>  [<ffffffff817e3fb9>] check_memory_region+0x139/0x190 mm/kasan/kasan.c:315
>>  [<ffffffff817e4044>] kasan_check_write+0x14/0x20 mm/kasan/kasan.c:326
>>  [<     inline     >] copy_from_user arch/x86/include/asm/uaccess.h:689
>>  [<     inline     >] ep0_write drivers/usb/gadget/legacy/inode.c:1135
>>  [<ffffffff83228caf>] dev_config+0x86f/0x1190
>> drivers/usb/gadget/legacy/inode.c:1759
>>  [<ffffffff817fdd55>] __vfs_write+0x5d5/0x760 fs/read_write.c:510
>>  [<ffffffff817ff650>] vfs_write+0x170/0x4e0 fs/read_write.c:560
>>  [<     inline     >] SYSC_write fs/read_write.c:607
>>  [<ffffffff81803a5b>] SyS_write+0xfb/0x230 fs/read_write.c:599
>>  [<ffffffff84f47ec1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
>
> How does this patch work out?

I believe it fixes the issue.

Thanks!

>
> Alan Stern
>
>
>
> Index: usb-4.x/drivers/usb/gadget/legacy/inode.c
> ===================================================================
> --- usb-4.x.orig/drivers/usb/gadget/legacy/inode.c
> +++ usb-4.x/drivers/usb/gadget/legacy/inode.c
> @@ -1126,7 +1126,7 @@ ep0_write (struct file *fd, const char _
>         /* data and/or status stage for control request */
>         } else if (dev->state == STATE_DEV_SETUP) {
>
> -               /* IN DATA+STATUS caller makes len <= wLength */
> +               len = min(len, (size_t) dev->setup_wLength);
>                 if (dev->setup_in) {
>                         retval = setup_req (dev->gadget->ep0, dev->req, len);
>                         if (retval == 0) {
>