Re: [PATCH] mm/mempolicy.c: Fix out of bounds write in mpol_parse_str()

[Dmitry Vyukov]

On Wed, Jan 15, 2020 at 1:54 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 1/15/20 6:54 AM, Dan Carpenter wrote:
> > What we are trying to do is change the '=' character to a NUL terminator
> > and then at the end of the function we restore it back to an '='.  The
> > problem is there are two error paths where we jump to the end of the
> > function before we have replaced the '=' with NUL.  We end up putting
> > the '=' in the wrong place (possibly one element before the start of
> > the buffer).
>
> Bleh.
>
> > Reported-by: syzbot+e64a13c5369a194d67df@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
> > Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> CC stable perhaps? Can this (tmpfs mount options parsing AFAICS?) become
> part of unprivileged operation in some scenarios?

Yes, tmpfs can be mounted by any user inside of a user namespace.
Also I suspect there are cases where an unprivileged attacker can
trick some utility to mount tmpfs on their behalf and provide their
own mount options.

> > ---
> >  mm/mempolicy.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 067cf7d3daf5..1340c5c496b5 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2817,6 +2817,9 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
> >       char *flags = strchr(str, '=');
> >       int err = 1, mode;
> >
> > +     if (flags)
> > +             *flags++ = '\0';        /* terminate mode string */
> > +
> >       if (nodelist) {
> >               /* NUL-terminate mode or flags string */
> >               *nodelist++ = '\0';
> > @@ -2827,9 +2830,6 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
> >       } else
> >               nodes_clear(nodes);
> >
> > -     if (flags)
> > -             *flags++ = '\0';        /* terminate mode string */
> > -
> >       mode = match_string(policy_modes, MPOL_MAX, str);
> >       if (mode < 0)
> >               goto out;
> >
>
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