linux-kernel - Re: [PATCH 5.14 018/334] nbd: add the check to prevent overflow in __nbd

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Date:   Mon, 13 Sep 2021 11:39:22 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Naresh Kamboju <naresh.kamboju@...aro.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Baokun Li <libaokun1@...wei.com>,
        open list <linux-kernel@...r.kernel.org>,
        linux-stable <stable@...r.kernel.org>,
        Hulk Robot <hulkci@...wei.com>,
        Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        lkft-triage@...ts.linaro.org, llvm@...ts.linux.dev,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 5.14 018/334] nbd: add the check to prevent overflow in __nbd_ioctl()

On Mon, Sep 13, 2021 at 10:58 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Mon, Sep 13, 2021 at 09:52:33PM +0530, Naresh Kamboju wrote:
> > [PATCH 00/10] raise minimum GCC version to 5.1
> > https://lore.kernel.org/lkml/20210910234047.1019925-1-ndesaulniers@google.com/
>
> Has anyone submitted a fix for this upstream yet?  I can't seem to find
> one :(

That lore link has a series to address this, though that's maybe
something we don't want to backport to stable.

I thought about this all weekend; I think I might be able to work
around the one concern I had with my other approach, using
__builtin_choose_expr().

There's an issue with my alternative approach
(https://gist.github.com/nickdesaulniers/2479818f4983bbf2d688cebbab435863)
with declaring the local variable z in div_64() since either operand
could be 64b, which result in an unwanted truncation if the dividend
is 32b (or less, and divisor is 64b). I think (what I realized this
weekend) is that we might be able to replace the `if` with
`__builtin_choose_expr`, then have that whole expression be the final
statement and thus the "return value" of the statement expression.

I need to play with that idea more; maybe that could be a more
manageable patch for stable.  But I also need to eat lunch, and I've
been in the Rust for Linux conference, and trying to organize 3 other
conferences for the next two weeks...
-- 
Thanks,
~Nick Desaulniers