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

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Date:	Fri, 25 Jan 2013 11:50:56 +0800
From:	Lingzhu Xiang <lxiang@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Matt Fleming <matt.fleming@...el.com>, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] efivars write(2) races

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.

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

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

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