Re: [PATCH] usb: xhci: fix missing null termination after copy_from_user()

[David Laight <david.laight.linux@gmail.com>]

On Sat, 17 Jan 2026 10:58:41 +0100
Greg KH <gregkh@linuxfoundation.org> wrote:

> On Sat, Jan 17, 2026 at 09:46:31AM +0000, Weigang He wrote:
> > The buffer 'buf' is filled by copy_from_user() but is not properly
> > null-terminated before being used with strncmp(). If userspace provides
> > fewer than 10 bytes, strncmp() may read beyond the copied data into
> > uninitialized stack memory.  
> 
> But that's fine, it will not match the check, and so it will stop when
> told, so no overflow happens anywhere.

That's not entirely true.
If the user passes "complianc" (without a '\0') and the on-stack buf[9]
happens to be 'e' then the test will succeed rather than fail.

But the only thing that will get upset is KASAN.

More 'interestingly':
- why is it min_t() not min(), everything is size_t.
- why sizeof(buf) - 1, reading into the last byte won't matter.
- why buf[32] not buf[10], even [16] would be plenty for 'future expansion'.

	David