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To:     Christoph Hellwig <hch@lst.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Arnd Bergmann <arnd@arndb.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <d5f0bbdc-8b31-fc4c-a5b9-b63cba4ebffe@c-s.fr>
Date:   Sun, 19 Apr 2020 11:46:55 +0200
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Le 19/04/2020 à 10:13, Christoph Hellwig a écrit :
> On Sat, Apr 18, 2020 at 06:55:56AM -0500, Eric W. Biederman wrote:
>>> Is that really an issue to use that set_fs() in the coredump code ?
>>
>> Using set_fs() is pretty bad and something that we would like to remove
>> from the kernel entirely.  The fewer instances of set_fs() we have the
>> better.
>>
>> I forget all of the details but set_fs() is both a type violation and an
>> attack point when people are attacking the kernel.  The existence of
>> set_fs() requires somethings that should be constants to be variables.
>> Something about that means that our current code is difficult to protect
>> from spectre style vulnerabilities.
> 
> Yes, set_fs requires variable based address checking in the uaccess
> routines for architectures with a shared address space, or even entirely
> different code for architectures with separate kernel and user address
> spaces.  My plan is to hopefully kill set_fs in its current form a few
> merge windows down the road.  We'll probably still need some form of
> it to e.g. mark a thread as kernel thread vs also being able to execute
> user code, but it will be much ore limited than before, called from very
> few places and actually be a no-op for many architectures.
> 

Oh nice. Some time ago I proposed a patch to change set_fs() to a 
flip/flop flag based logic, see 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/dd2876b808ea38eb7b7f760ecd6ce06096c61fb5.1580295551.git.christophe.leroy@c-s.fr/

But if we manage to get rid of it completely, that's even better.