From: Libor Pechacek <lpechacek@suse.cz>
To: Nathan Lynch <nathanl@linux.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Michal Suchanek <msuchanek@suse.cz>,
Thomas Gleixner <tglx@linutronix.de>,
Allison Randal <allison@lohutok.net>,
Leonardo Bras <leonardo@linux.ibm.com>,
David Hildenbrand <david@redhat.com>,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc: drmem: avoid NULL pointer dereference when drmem is unavailable
Date: Tue, 28 Jan 2020 11:19:45 +0100 [thread overview]
Message-ID: <20200128101945.GA20336@fm.suse.cz> (raw)
In-Reply-To: <87o8uudv51.fsf@linux.ibm.com>
Hello Nathan,
On Thu 23-01-20 09:56:10, Nathan Lynch wrote:
> Libor Pechacek <lpechacek@suse.cz> writes:
> > In KVM guests drmem structure is only zero initialized. Trying to
> > manipulate DLPAR parameters results in a crash in this environment.
>
> I think this statement needs qualification. Unless I'm mistaken, this
> happens only when you boot a guest without any hotpluggable memory
> configured, and then try to add or remove memory.
Thanks for the review. The introductory statement can indeed be clearer.
[...]
> > diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
> > index c126b94d1943..4ea6af002e27 100644
> > --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> > +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> > @@ -236,9 +236,9 @@ static int get_lmb_range(u32 drc_index, int n_lmbs,
> > if (!start)
> > return -EINVAL;
> >
> > - end = &start[n_lmbs - 1];
> > + end = &start[n_lmbs];
> >
> > - last_lmb = &drmem_info->lmbs[drmem_info->n_lmbs - 1];
> > + last_lmb = &drmem_info->lmbs[drmem_info->n_lmbs];
> > if (end > last_lmb)
> > return -EINVAL;
>
> Is this not undefined behavior? I'd rather do this in a way that does
> not involve forming out-of-bounds pointers.
Well, this is a tough question for the case when drmem_info->lmbs was not
allocated. Given that the array does not exist, what bounds are we talking
about?
My patch builds on the fact that NULL[0] is NULL and NULL < NULL is false.
Talking about a pointer to one past the last element of an non-existent array
is too much philosophy for me.
For the case when drmem_info->lmbs is allocated, last_lmb is a pointer to one
past the last element of the array as Michal mentioned.
> Even if it's safe, naming that pointer "last_lmb" now actively hinders
> understanding of the code; it should be named "limit" or something.
Good catch.
[...]
> 1 file changed, 36 insertions(+), 7 deletions(-)
> arch/powerpc/include/asm/drmem.h | 43 +++++++++++++++++++++++++++++++++-------
>
> modified arch/powerpc/include/asm/drmem.h
> @@ -20,19 +20,48 @@ struct drmem_lmb {
>
> struct drmem_lmb_info {
> struct drmem_lmb *lmbs;
> - int n_lmbs;
> + unsigned int n_lmbs;
> u32 lmb_size;
> };
>
> extern struct drmem_lmb_info *drmem_info;
>
> -#define for_each_drmem_lmb_in_range(lmb, start, end) \
> - for ((lmb) = (start); (lmb) <= (end); (lmb)++)
> +static inline bool drmem_present(void)
> +{
> + return drmem_info->lmbs != NULL;
> +}
Yes, use of this test was also my first idea about the fix.
> +static inline struct drmem_lmb *drmem_lmb_index(unsigned int index)
> +{
> + if (!drmem_present())
> + return NULL;
>
> -#define for_each_drmem_lmb(lmb) \
> - for_each_drmem_lmb_in_range((lmb), \
> - &drmem_info->lmbs[0], \
> - &drmem_info->lmbs[drmem_info->n_lmbs - 1])
> + if (WARN_ON(index >= drmem_info->n_lmbs))
> + return NULL;
Why is this WARN_ON needed?
> +
> + return &drmem_info->lmbs[index];
> +}
> +
> +static inline struct drmem_lmb *drmem_first_lmb(void)
> +{
> + return drmem_lmb_index(0);
> +}
> +
> +static inline struct drmem_lmb *drmem_last_lmb(void)
> +{
> + if (!drmem_present())
> + return NULL;
> +
> + return drmem_lmb_index(drmem_info->n_lmbs - 1);
Is the unsigned integer wraparound intended in drmem_info->n_lmbs == 0 case?
> +}
> +
> +#define for_each_drmem_lmb(lmb) \
> + for ((lmb) = drmem_first_lmb(); \
drmem_first_lmb() is essentially a call to drmem_info->lmbs(0). What
happens if drmem_info->n_lmbs is zero and drmem_info->lmbs is not NULL?
> + (lmb) != NULL && (lmb) <= drmem_last_lmb(); \
> + (lmb)++)
> +
> +#define for_each_drmem_lmb_in_range(lmb, start, end) \
> + for ((lmb) = (start); (lmb) <= (end); (lmb)++)
>
> /*
> * The of_drconf_cell_v1 struct defines the layout of the LMB data
>
After all, I don't mind how the bug will be fixed. As you can see, my
preference is towards simpler solutions.
In my opinion your solution special-cased drmem_info->lmbs == NULL and opened
the doorway to the combination of drmem_info->lmbs != NULL &&
!drmem_info->n_lmbs. Maybe the condition can never become true but the code
should IMHO be robust enough to handle it.
Thanks!
Libor
--
Libor Pechacek
SUSE Labs Remember to have fun...
next prev parent reply other threads:[~2020-01-28 10:19 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-16 10:27 [PATCH] powerpc: drmem: avoid NULL pointer dereference when drmem is unavailable Libor Pechacek
2020-01-22 15:21 ` Michal Suchánek
2020-01-23 15:56 ` Nathan Lynch
2020-01-23 16:10 ` Michal Suchánek
2020-01-28 10:19 ` Libor Pechacek [this message]
2020-01-31 13:28 ` [PATCH v2] " Michal Suchanek
2020-02-05 16:55 ` Nathan Lynch
2020-03-06 0:27 ` Michael Ellerman
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