linux-kernel - Re: [PATCH RFC 5/5] kdbus: improve tests on incrementing quota

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Date:	Thu, 2 Jul 2015 10:50:47 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Sergei Zviagintsev <sergei@...v.net>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Daniel Mack <daniel@...que.org>,
	David Herrmann <dh.herrmann@...glemail.com>,
	Djalal Harouni <tixxdz@...ndz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 5/5] kdbus: improve tests on incrementing quota

Hi

On Sun, Jun 28, 2015 at 3:17 PM, Sergei Zviagintsev <sergei@...v.net> wrote:
>  1) Rewrite
>
>         quota->memory + memory > U32_MAX
>
>     as
>         U32_MAX - quota->memory < memory
>
>     and provide the comment on why we need that check.
>
>     We have no overflow issue in the original expression when size_t is
>     32-bit because the previous one (available - quota->memory < memory)
>     guarantees that quota->memory + memory doesn't exceed `available'
>     which is <= U32_MAX in that case.
>
>     But lets stay explicit rather than implicit, it would save us from
>     describing HOW the code works.
>
>  2) Add WARN_ON when quota->msgs > KDBUS_CONN_MAX_MSGS
>
>     This is somewhat inconsistent, so we need to properly report it.

I don't see the purpose of this WARN_ON(). Sure, ">" should never
happen, but that doesn't mean we have to add a WARN_ON. I'd just keep
the code as it is.

>  3) Replace
>
>         quota->fds + fds < quota->fds ||
>         quota->fds + fds > KDBUS_CONN_MAX_FDS_PER_USER
>
>     with
>
>         KDBUS_CONN_MAX_FDS_PER_USER - quota->fds < fds
>
>     and add explicit WARN_ON in the case
>     quota->fds > KDBUS_CONN_MAX_FDS_PER_USER.
>
>     Reading the code one can assume that the first expression is
>     there to ensure that we won't have an overflow in quota->fds after
>     quota->fds += fds, but what it really does is testing for size_t
>     overflow in `quota->fds + fds' to be safe in the second expression
>     (as fds is size_t, quota->fds is converted to bigger type).
>
>     Rewrite it in more obvious way. KDBUS_CONN_MAX_FDS_PER_USER is
>     checked at compile time to fill in quota->fds type (there is
>     BUILD_BUG_ON), so no further checks for quota->fds overflow are
>     needed.
>
> Signed-off-by: Sergei Zviagintsev <sergei@...v.net>
> ---
>  ipc/kdbus/connection.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/ipc/kdbus/connection.c b/ipc/kdbus/connection.c
> index 12e32de310f5..6556a0f9d44c 100644
> --- a/ipc/kdbus/connection.c
> +++ b/ipc/kdbus/connection.c
> @@ -701,13 +701,21 @@ int kdbus_conn_quota_inc(struct kdbus_conn *c, struct kdbus_user *u,
>         available = (available - accounted + quota->memory) / 3;
>
>         if (available < quota->memory ||
> -           available - quota->memory < memory ||
> -           quota->memory + memory > U32_MAX)
> +           available - quota->memory < memory)
>                 return -ENOBUFS;
> -       if (quota->msgs >= KDBUS_CONN_MAX_MSGS)
> +
> +       /*
> +        * available is size_t and thus it could be greater than U32_MAX.
> +        * Ensure that quota->memory won't overflow.
> +        */
> +       if (U32_MAX - quota->memory < memory)
> +               return -ENOBUFS;

Can you drop the comment and integrate it into the condition above? I
mean this whole section is about overflow checks, I don't see the
point of explaining one of them specially.

> +
> +       if (WARN_ON(quota->msgs > KDBUS_CONN_MAX_MSGS) ||
> +           quota->msgs == KDBUS_CONN_MAX_MSGS)
>                 return -ENOBUFS;

This one I'd keep as it was. I don't really see the point in adding a WARN_ON().

> -       if (quota->fds + fds < quota->fds ||
> -           quota->fds + fds > KDBUS_CONN_MAX_FDS_PER_USER)
> +       if (WARN_ON(quota->fds > KDBUS_CONN_MAX_FDS_PER_USER) ||
> +           KDBUS_CONN_MAX_FDS_PER_USER - quota->fds < fds)
>                 return -EMFILE;

Not sure the WARN_ON is needed, but this one looks fine to me.

Thanks
David

>         quota->memory += memory;
> --
> 1.8.3.1
>
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