linux-kernel - Re: [PATCH 1/2] signal: Factor copy_siginfo_to_external32 from copy_siginfo_to

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Date:   Sun, 19 Apr 2020 10:13:53 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Christoph Hellwig <hch@....de>, Arnd Bergmann <arnd@...db.de>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Jeremy Kerr <jk@...abs.org>
Subject: Re: [PATCH 1/2] signal: Factor copy_siginfo_to_external32 from
 copy_siginfo_to_user32

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.