linux-kernel - Re: [RFC] efivars write(2) races

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Message-ID: <1359119883.2496.156.camel@mfleming-mobl1.ger.corp.intel.com>
Date:	Fri, 25 Jan 2013 13:18:03 +0000
From:	Matt Fleming <matt.fleming@...el.com>
To:	Lingzhu Xiang <lxiang@...hat.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Kerr <jeremy.kerr@...onical.com>
Subject: Re: [RFC] efivars write(2) races

On Fri, 2013-01-25 at 11:50 +0800, Lingzhu Xiang wrote:
> On 01/25/2013 08:25 AM, Al Viro wrote:
> > 	1) process A does write() on efivars file, reaches ->get_variable(),
> > gets newdatasize set, drops efivars->lock and loses CPU before an attempt to
> > grab ->i_mutex.  process B comes and does the same thing, replacing the
> > variable contents.  Then it grabs ->i_mutex, updates size, drops ->i_mutex
> > and buggers off.  At which point A gets CPU back and proceeds to set size
> > to whatever would be valid for its write.  Only the value is bogus now...
> 
> There are a few other things that makes size bogus now.
> 
> 1. truncate() never touches nvram but pretends to be changing size.

Good catch. Looks like we need to provide an implementation for this.

> 2. Empty files come back with non-zero size after remount. They are imported
>    from sysfs when mounting.

Eeek. The return value of get_variable() isn't checked in
efivarfs_fill_super().

> 3. Arguably reading empty files could just return empty instead of returning
>    EIO/EFI_NOT_FOUND from firmware. 

Yeah, I think Jeremy has a patch to fix this.

> 4. EFI_VARIABLE_APPEND_WRITE with EFI_OUT_OF_RESOURCES truncates size but you
>    can still read its content.

I'm not sure what you mean by this. Could you please explain?

Thanks for the feedback.

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