linux-kernel - Re: [PATCH] kernel: sys.c: Avoid copying possible padding bytes in copy_to

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Date:   Sun, 27 Oct 2019 06:47:55 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Joe Perches <joe@...ches.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dan Carpenter <error27@...il.com>,
        Julia Lawall <julia.lawall@...6.fr>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] kernel: sys.c: Avoid copying possible padding bytes in
 copy_to_user

On Sat, 26 Oct 2019, Joe Perches wrote:

> Initialization is not guaranteed to zero padding bytes so
> use an explicit memset instead to avoid leaking any kernel
> content in any possible padding bytes.

Here is an extract of an email that I sent to Kees at one point that left
me unsure about what should be done about these situations:

>From Kees:

    The only way to correctly handle this is:

    memset(&instance, 0, sizeof(instance));
    instance.one = 1;

>From me:

Actually, this document:

https://wiki.sei.cmu.edu/confluence/display/c/DCL39-C.+Avoid+information+leakage+when+passing+a+structure+across+a+trust+boundary

says that memset is a "noncompliant solution".  They suggest declaring the
structure as packed, as well as some other more unpleasant solutions.
Their point is that 1 will be sitting in a register, and the assignment at
least might copy the upper bytes of the register into the padding space.

-------------------------

Is the memset solution nevertheless what is always wanted in the kernel
when there is padding?

thanks,
julia