linux-kernel - Re: [PATCH v1 1/1] vsprintf: Fix potential unaligned access

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Message-ID: <Yd2RvIeP3LRc/+gl@smile.fi.intel.com>
Date:   Tue, 11 Jan 2022 16:18:36 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v1 1/1] vsprintf: Fix potential unaligned access

On Tue, Jan 11, 2022 at 04:13:37PM +0200, Sakari Ailus wrote:
> On Tue, Jan 11, 2022 at 01:22:29PM +0200, Andy Shevchenko wrote:
> > On Tue, Jan 11, 2022 at 12:54:41PM +0200, Sakari Ailus wrote:
> > > On Tue, Jan 11, 2022 at 12:10:28PM +0200, Andy Shevchenko wrote:
> > > > On Tue, Jan 11, 2022 at 12:12:46AM +0200, Sakari Ailus wrote:
> > > > > On Mon, Jan 10, 2022 at 10:50:49PM +0200, Andy Shevchenko wrote:
> > > > > > The %p4cc specifier in some cases might get an unaligned pointer.
> > > > > > Due to this we need to make copy to local variable once to avoid
> > > > > > potential crashes on some architectures due to improper access.
> > > > > 
> > > > > I guess this problem exists virtually everywhere where pointers are being
> > > > > handled: the pointer could be unaligned.
> > > > 
> > > > True. And my patch improves the situation.
> > > > See, for example, 0f70fe605fad ("hexdump: fix for non-aligned buffers").
> > > 
> > > This is different since there's no guarantee of a void pointer's alignment.
> > > 
> > > The pixelformat used for %p4cc is a pointer to u32.
> > 
> > Oh, look at the %p, compiler doesn't know about the %p extensions and caller
> > may supply whatever they want, i.e. %p may take any address that can be kept
> > in void *. Actual argument _is_ void *. What you put there as u32 is just
> > personal expectation, and not the reality.
> 
> If you assume this, you should add get_unaligned() calls in all places
> where you're casting a type to another with higher alignment requirements.

Those places so far were not subject of any reports, but ideally yes, you are
right (in a scope of %p extensions, otherwise it's silly).

I will leave this to the VSPRINTF / PRINTK maintainers to proceed. Personally
I found this patch useful since we don't know what other user will do unaligned
call to it.

-- 
With Best Regards,
Andy Shevchenko