linux-hardening - Re: [PATCH 1/5] string.h: add array-wrappers for (v)memdup

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Date:   Wed, 30 Aug 2023 16:19:32 +0200
From:   pstanner@...hat.com
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Shevchenko <andy@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Christian Brauner <brauner@...nel.org>,
        David Disseldorp <ddiss@...e.de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Siddh Raman Pant <code@...dh.me>,
        Nick Alcock <nick.alcock@...cle.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Zack Rusin <zackr@...are.com>,
        VMware Graphics Reviewers 
        <linux-graphics-maintainer@...are.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kexec@...ts.infradead.org, linux-hardening@...r.kernel.org,
        David Airlie <airlied@...hat.com>
Subject: Re: [PATCH 1/5] string.h: add array-wrappers for (v)memdup_user()

On Wed, 2023-08-30 at 17:11 +0300, Andy Shevchenko wrote:
> On Wed, Aug 30, 2023 at 4:46 PM Philipp Stanner <pstanner@...hat.com>
> wrote:
> > 
> > Currently, user array duplications are sometimes done without an
> > overflow check. Sometimes the checks are done manually; sometimes
> > the
> > array size is calculated with array_size() and sometimes by
> > calculating
> > n * size directly in code.
> > 
> > Introduce wrappers for arrays for memdup_user() and vmemdup_user()
> > to
> > provide a standardized and safe way for duplicating user arrays.
> > 
> > This is both for new code as well as replacing usage of
> > (v)memdup_user()
> > in existing code that uses, e.g., n * size to calculate array
> > sizes.
> 
> ...
> 
> > --- a/include/linux/string.h
> > +++ b/include/linux/string.h
> 
> I'm wondering if this has no side-effects as string.h/string.c IIRC
> is
> used also for early stages where some of the APIs are not available.
> 
> > @@ -6,6 +6,8 @@
> >  #include <linux/types.h>       /* for size_t */
> >  #include <linux/stddef.h>      /* for NULL */
> >  #include <linux/errno.h>       /* for E2BIG */
> > +#include <linux/overflow.h>    /* for check_mul_overflow() */
> > +#include <linux/err.h>         /* for ERR_PTR() */
> 
> Can we preserve order (to some extent)?

Sure. I just put it there so the comments build a congruent block.
Which order would you prefer?

> 
> >  #include <linux/stdarg.h>
> >  #include <uapi/linux/string.h>
> 
> ...
> 
> > +/**
> > + * memdup_array_user - duplicate array from user space
> 
> > + *
> 
> Do we need this blank line?

I more or less directly copied the docstring format from the original
functions (v)memdup_user() in mm/util.c
I guess this is common style?

> 
> > + * @src: source address in user space
> > + * @n: number of array members to copy
> > + * @size: size of one array member
> > + *
> > + * Return: an ERR_PTR() on failure.  Result is physically
> > + * contiguous, to be freed by kfree().
> > + */
> 
> ...
> 
> > +/**
> > + * vmemdup_array_user - duplicate array from user space
> 
> > + *
> 
> Redundant?

No, there are two functions:
 * memdup_array_user()
 * vmemdup_array_user()

On the deeper layers they utilize kmalloc() or kvmalloc(),
respectively.


Greetings,
P.

> 
> > + * @src: source address in user space
> > + * @n: number of array members to copy
> > + * @size: size of one array member
> > + *
> > + * Return: an ERR_PTR() on failure.  Result may be not
> > + * physically contiguous.  Use kvfree() to free.
> > + */
>