linux-kernel - Re: [PATCH] usb: ch9: Replace bmSublinkSpeedAttr 1-element array with flexible array

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Message-ID: <202307051408.E16A95E@keescook>
Date:   Wed, 5 Jul 2023 14:11:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Jó Ágila Bitsch <jgilab@...il.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] usb: ch9: Replace bmSublinkSpeedAttr 1-element array
 with flexible array

On Thu, Jun 29, 2023 at 09:17:23PM +0200, Greg Kroah-Hartman wrote:
> On Thu, Jun 29, 2023 at 12:09:00PM -0700, Kees Cook wrote:
> > Since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"),
> > UBSAN_BOUNDS no longer pretends 1-element arrays are unbounded. Walking
> > bmSublinkSpeedAttr will trigger a warning, so make it a proper flexible
> > array. Add a union to keep the struct size identical for userspace in
> > case anything was depending on the old size.
> > 
> > False positive warning was:
> > 
> > UBSAN: array-index-out-of-bounds in drivers/usb/host/xhci-hub.c:231:31 index 1 is out of range for type '__le32 [1]'
> > 
> > for this line of code:
> > 
> > 	ssp_cap->bmSublinkSpeedAttr[offset++] = cpu_to_le32(attr);
> > 
> > Reported-by: Borislav Petkov <bp@...en8.de>
> > Closes: https://lore.kernel.org/lkml/2023062945-fencing-pebble-0411@gregkh/
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  include/uapi/linux/usb/ch9.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> Thanks for the quick response, I'll queue it up after 6.5-rc1 is out.

I'm going to send this before -rc1, since we've had another report[1] that
was fixed by this. Given the verification there and Boris's testing, I
think this is good to land. I'll toss it in -next now and send it to
Linus on Friday after making sure there are no more surprises.

-Kees

[1] https://lore.kernel.org/lkml/DA3FEB08-DF39-406B-89CC-9076CFCF597A@kernel.org/

-- 
Kees Cook