linux-kernel - Re: [PATCH] block: sed-opal: reduce stack size of ioctl handler

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Message-ID: <20170208221221.GA10983@sbauer-Z170X-UD5>
Date:   Wed, 8 Feb 2017 15:12:21 -0700
From:   Scott Bauer <scott.bauer@...el.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Rafael Antognolli <rafael.antognolli@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        linuxppc-dev@...ts.ozlabs.org, hch@....de,
        Jonathan Derrick <jonathan.derrick@...el.com>
Subject: Re: [PATCH] block: sed-opal: reduce stack size of ioctl handler

On Wed, Feb 08, 2017 at 02:58:28PM -0700, Scott Bauer wrote:
> On Wed, Feb 08, 2017 at 10:15:28PM +0100, Arnd Bergmann wrote:
> > When CONFIG_KASAN is in use, the sed_ioctl function uses unusually large stack,
> > as each possible ioctl argument gets its own stack area plus redzone:
> > 
> > block/sed-opal.c: In function 'sed_ioctl':
> > block/sed-opal.c:2447:1: error: the frame size of 2256 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> > 
> > Moving the copy_from_user() calls into the individual functions has little
> > effect on readablility, but significantly reduces the stack size, with the
> > largest individual function (opal_enable_disable_shadow_mbr) now at
> > reasonable 456 bytes.
> > 
> > Fixes: 455a7b238cd6 ("block: Add Sed-opal library")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> 
> Hi Arnd,
> 
> Thank you for the report. We want to keep the function calls agnostic to userland.
> In the future we will have in-kernel callers and I don't want to have to do any
> get_fs(KERNEL_DS) wizardry.
> 
> Instead I think we can use a union to lessen the stack burden. I tested this patch just now
> with config_ksasan and was able to build.

Nack on this patch, it only really masks the issue. Keith pointed out we have a call chain
up to this ioctl then deeper down into nvme then the block layer. If we use 25% of the stack
just for this function it's still too dangerous and we'll run into corruption later on and not
remember this fix. I'll come up with another solution.