Re: [PATCH] wext: use flex array destination for memcpy()

[Kees Cook <keescook@chromium.org>]

On Mon, Sep 26, 2022 at 11:09:06PM +0800, Hawkins Jiawei wrote:
> Syzkaller reports refcount bug as follows:

"buffer overflow false positive" instead of "refcount bug"

> ------------[ cut here ]------------
> memcpy: detected field-spanning write (size 8) of single field
> 	"&compat_event->pointer" at net/wireless/wext-core.c:623 (size 4)
> WARNING: CPU: 0 PID: 3607 at net/wireless/wext-core.c:623
> 	wireless_send_event+0xab5/0xca0 net/wireless/wext-core.c:623
> Modules linked in:
> CPU: 1 PID: 3607 Comm: syz-executor659 Not tainted
> 	6.0.0-rc6-next-20220921-syzkaller #0
> [...]
> Call Trace:
>  <TASK>
>  ioctl_standard_call+0x155/0x1f0 net/wireless/wext-core.c:1022
>  wireless_process_ioctl+0xc8/0x4c0 net/wireless/wext-core.c:955
>  wext_ioctl_dispatch net/wireless/wext-core.c:988 [inline]
>  wext_ioctl_dispatch net/wireless/wext-core.c:976 [inline]
>  wext_handle_ioctl+0x26b/0x280 net/wireless/wext-core.c:1049
>  sock_ioctl+0x285/0x640 net/socket.c:1220
>  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+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>  [...]
>  </TASK>
> 
> Wireless events will be sent on the appropriate channels in
> wireless_send_event(). Different wireless events may have different
> payload structure and size, so kernel uses **len** and **cmd** field
> in struct __compat_iw_event as wireless event common LCP part, uses
> **pointer** as a label to mark the position of remaining different part.
> 
> Yet the problem is that, **pointer** is a compat_caddr_t type, which may
> be smaller than the relative structure at the same position. So during
> wireless_send_event() tries to parse the wireless events payload, it may
> trigger the memcpy() run-time destination buffer bounds checking when the
> relative structure's data is copied to the position marked by **pointer**.
> 
> This patch solves it by introducing flexible-array field **ptr_bytes**,
> to mark the position of the wireless events remaining part next to
> LCP part. What's more, this patch also adds **ptr_len** variable in
> wireless_send_event() to improve its maintainability.
> 
> Reported-and-tested-by: syzbot+473754e5af963cf014cf@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/all/00000000000070db2005e95a5984@google.com/
> Suggested-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>

Thanks for spinning this and getting it tested!

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook