linux-kernel - Re: [PATCH] fs: fix possible Spectre V1 indexing in __close

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Date:   Mon, 15 Oct 2018 15:37:08 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Greg Hackmann <ghackmann@...roid.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Omer Tripp <trippo@...gle.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Greg Hackmann <ghackmann@...gle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] fs: fix possible Spectre V1 indexing in __close_fd()

On Mon, Sep 24, 2018 at 08:39:11PM +0200, Greg KH wrote:
> On Mon, Sep 24, 2018 at 11:15:00AM -0700, Greg Hackmann wrote:
> > __close_fd() is reachable via the close() syscall with a
> > userspace-controlled fd.  Ensure that it can't be used to speculatively
> > access past the end of current->fdt.
> > 
> > Reported-by: Omer Tripp <trippo@...gle.com>
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Greg Hackmann <ghackmann@...gle.com>
> > ---
> >  fs/file.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index 7ffd6e9d103d..a80cf82be96b 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/bitops.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/rcupdate.h>
> > +#include <linux/nospec.h>
> >  
> >  unsigned int sysctl_nr_open __read_mostly = 1024*1024;
> >  unsigned int sysctl_nr_open_min = BITS_PER_LONG;
> > @@ -626,6 +627,7 @@ int __close_fd(struct files_struct *files, unsigned fd)
> >  	fdt = files_fdtable(files);
> >  	if (fd >= fdt->max_fds)
> >  		goto out_unlock;
> > +	fd = array_index_nospec(fd, fdt->max_fds);
> >  	file = fdt->fd[fd];
> 
> Don't you need 2 "halfs" of a gadget in order to make it work?  This is
> one half, where is the second half?
> 
> Or am I reading this code wrong here somehow?
> 
> We don't want to play whack-a-mole with only 1/2 spectre gadgets,
> otherwise the 700+ patches that Red Hat added to their kernel would have
> been merged already.
> 
> Which reminds me, did the Red Hat tooling catch this one as well?  If
> not, someone need to go fix it :)

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