linux-kernel - Re: [PATCH] uaccess: Add missing __must

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Message-ID: <201909301611.1363980D7@keescook>
Date:   Mon, 30 Sep 2019 16:17:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] uaccess: Add missing __must_check attributes

On Mon, Sep 30, 2019 at 12:33:19PM +0200, Arnd Bergmann wrote:
> On Wed, Aug 28, 2019 at 7:38 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > The usercopy implementation comments describe that callers of the
> > copy_*_user() family of functions must always have their return values
> > checked. This can be enforced at compile time with __must_check, so add
> > it where needed.
> >
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> 
> I can't find any other reports, so I'd point out here that this found what
> looks like a bug in the x86 math-emu code:

Oh interesting!

> arch/x86/math-emu/reg_ld_str.c:88:2: error: ignoring return value of
> function declared with 'warn_unused_result' attribute
> [-Werror,-Wunused-result]
>         __copy_from_user(sti_ptr, s, 10);
>         ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~
> arch/x86/math-emu/reg_ld_str.c:1129:2: error: ignoring return value of
> function declared with 'warn_unused_result' attribute
> [-Werror,-Wunused-result]
>         __copy_from_user(register_base + offset, s, other);
>         ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/math-emu/reg_ld_str.c:1131:3: error: ignoring return value of
> function declared with 'warn_unused_result' attribute
> [-Werror,-Wunused-result]
>                 __copy_from_user(register_base, s + other, offset);
>                 ^~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

What was the CONFIG for this? I didn't hit these in my build tests.

> Moreover, the same code also ignores the return value from most
> get_user()/put_user()/FPU_get_user()/FPU_put_user() calls,
> which have no warn_unused_result annotation (they are macros,
> but I think something could be done if we want to have that
> annotation to catch some of the other such users).

It would certainly make sense to mark those as __must_check too... now
tracking this here for anyone that wants to take a stab at it:
https://github.com/KSPP/linux/issues/16

-- 
Kees Cook