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SubjectRe: [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@google.com>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Greg Hackmann <ghackmann@google.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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