[PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Julius Werner]

xhci_alloc_segments_for_ring() builds a list of xhci_segments and links
the tail to head at the end (forming a ring). When it bails out for OOM
reasons half-way through, it tries to destroy its half-built list with
xhci_free_segments_for_ring(), even though it is not a ring yet. This
causes a null-pointer dereference upon hitting the last element.

Furthermore, one of its callers (xhci_ring_alloc()) mistakenly believes
the output parameters to be valid upon this kind of OOM failure, and
calls xhci_ring_free() on them. Since the (incomplete) list/ring should
already be destroyed in that case, this would lead to a use after free.

This patch fixes those issues by having xhci_alloc_segments_for_ring()
destroy its half-built, non-circular list manually and destroying the
invalid struct xhci_ring in xhci_ring_alloc() with a plain kfree().

Signed-off-by: Julius Werner <jwerner@chromium.org>
---
 drivers/usb/host/xhci-mem.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 487bc08..420ba37 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -205,7 +205,11 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
 
 		next = xhci_segment_alloc(xhci, cycle_state, flags);
 		if (!next) {
-			xhci_free_segments_for_ring(xhci, *first);
+			prev = *first;
+			do {
+				next = prev->next;
+				xhci_segment_free(xhci, prev);
+			} while ((prev = next));
 			return -ENOMEM;
 		}
 		xhci_link_segments(xhci, prev, next, type);
@@ -258,7 +262,7 @@ static struct xhci_ring *xhci_ring_alloc(struct xhci_hcd *xhci,
 	return ring;
 
 fail:
-	xhci_ring_free(xhci, ring);
+	kfree(ring);
 	return NULL;
 }
 
-- 
1.7.8.6

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Sergei Shtylyov]

Hello.

On 10/29/2012 08:00 PM, Julius Werner wrote:

> xhci_alloc_segments_for_ring() builds a list of xhci_segments and links
> the tail to head at the end (forming a ring). When it bails out for OOM
> reasons half-way through, it tries to destroy its half-built list with
> xhci_free_segments_for_ring(), even though it is not a ring yet. This
> causes a null-pointer dereference upon hitting the last element.

> Furthermore, one of its callers (xhci_ring_alloc()) mistakenly believes
> the output parameters to be valid upon this kind of OOM failure, and
> calls xhci_ring_free() on them. Since the (incomplete) list/ring should
> already be destroyed in that case, this would lead to a use after free.

> This patch fixes those issues by having xhci_alloc_segments_for_ring()
> destroy its half-built, non-circular list manually and destroying the
> invalid struct xhci_ring in xhci_ring_alloc() with a plain kfree().

> Signed-off-by: Julius Werner <jwerner@chromium.org>
> ---
>  drivers/usb/host/xhci-mem.c |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)

> diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> index 487bc08..420ba37 100644
> --- a/drivers/usb/host/xhci-mem.c
> +++ b/drivers/usb/host/xhci-mem.c
> @@ -205,7 +205,11 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
>  
>  		next = xhci_segment_alloc(xhci, cycle_state, flags);
>  		if (!next) {
> -			xhci_free_segments_for_ring(xhci, *first);
> +			prev = *first;
> +			do {
> +				next = prev->next;
> +				xhci_segment_free(xhci, prev);
> +			} while ((prev = next));

   It's preferred that the assignments are done outside the *if* and *while*
statements. In fact, at least for the *if* statements scripts/checkpatch.pl
gives a warning (it was silent in this case).

>  			return -ENOMEM;
>  		}
>  		xhci_link_segments(xhci, prev, next, type);
> @@ -258,7 +262,7 @@ static struct xhci_ring *xhci_ring_alloc(struct xhci_hcd *xhci,
>  	return ring;
>  
>  fail:
> -	xhci_ring_free(xhci, ring);
> +	kfree(ring);
>  	return NULL;
>  }

[headless@wasted linux]$ scripts/checkpatch.pl
patches/xhci-fix-null-pointer-dereference-when-destroying-half-built-segment-rings.patch

ERROR: DOS line endings
#30: FILE: drivers/usb/host/xhci-mem.c:208:
+^I^I^Iprev = *first;^M$

ERROR: DOS line endings
#31: FILE: drivers/usb/host/xhci-mem.c:209:
+^I^I^Ido {^M$

ERROR: DOS line endings
#32: FILE: drivers/usb/host/xhci-mem.c:210:
+^I^I^I^Inext = prev->next;^M$

ERROR: DOS line endings
#33: FILE: drivers/usb/host/xhci-mem.c:211:
+^I^I^I^Ixhci_segment_free(xhci, prev);^M$

ERROR: DOS line endings
#34: FILE: drivers/usb/host/xhci-mem.c:212:
+^I^I^I} while ((prev = next));^M$

ERROR: DOS line endings
#43: FILE: drivers/usb/host/xhci-mem.c:265:
+^Ikfree(ring);^M$

total: 6 errors, 0 warnings, 20 lines checked

patches/xhci-fix-null-pointer-dereference-when-destroying-half-built-segment-rings.patch
has style problems, please review.

If any of these errors are false positives, please report
them to the maintainer, see CHECKPATCH in MAINTAINERS.

   I have noticed that the patch description has DOS line endings as well.

WBR, Sergei

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Julius Werner]

>    I have noticed that the patch description has DOS line endings as well.

Sorry about those, Gmail adds them automatically. According to RFC
2046 (section 4.1.1), text/plain content must use CRLFs to encode line
breaks, so I guess this is the right thing. Your MUA should be
responsible for converting them to native endings upon retrieval (and
if it doesn't, git-am will).

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Sarah Sharp]

On Mon, Oct 29, 2012 at 09:35:15PM +0300, Sergei Shtylyov wrote:
> Hello.
> 
> On 10/29/2012 08:00 PM, Julius Werner wrote:
> >  		next = xhci_segment_alloc(xhci, cycle_state, flags);
> >  		if (!next) {
> > -			xhci_free_segments_for_ring(xhci, *first);
> > +			prev = *first;
> > +			do {
> > +				next = prev->next;
> > +				xhci_segment_free(xhci, prev);
> > +			} while ((prev = next));
> 
>    It's preferred that the assignments are done outside the *if* and *while*
> statements. In fact, at least for the *if* statements scripts/checkpatch.pl
> gives a warning (it was silent in this case).

Hi Julius,

I agree with Sergei (for once).  The assignment in the while conditional
is confusing, and everyone reading the code will wonder if you meant
(prev == next).  Putting extra parenthesis around it to avoid static
type checker warnings is kind of missing the point.

Your patch does apply fine with git-am, so don't worry about the line
endings.  Can you please move the assignment into the loop and resubmit
this patch?

Sarah Sharp

[PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Julius Werner]

xhci_alloc_segments_for_ring() builds a list of xhci_segments and links
the tail to head at the end (forming a ring). When it bails out for OOM
reasons half-way through, it tries to destroy its half-built list with
xhci_free_segments_for_ring(), even though it is not a ring yet. This
causes a null-pointer dereference upon hitting the last element.

Furthermore, one of its callers (xhci_ring_alloc()) mistakenly believes
the output parameters to be valid upon this kind of OOM failure, and
calls xhci_ring_free() on them. Since the (incomplete) list/ring should
already be destroyed in that case, this would lead to a use after free.

This patch fixes those issues by having xhci_alloc_segments_for_ring()
destroy its half-built, non-circular list manually and destroying the
invalid struct xhci_ring in xhci_ring_alloc() with a plain kfree().

Signed-off-by: Julius Werner <jwerner@chromium.org>
---
 drivers/usb/host/xhci-mem.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 487bc08..fb51c70 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -205,7 +205,12 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
 
 		next = xhci_segment_alloc(xhci, cycle_state, flags);
 		if (!next) {
-			xhci_free_segments_for_ring(xhci, *first);
+			prev = *first;
+			while (prev) {
+				next = prev->next;
+				xhci_segment_free(xhci, prev);
+				prev = next;
+			}
 			return -ENOMEM;
 		}
 		xhci_link_segments(xhci, prev, next, type);
@@ -258,7 +263,7 @@ static struct xhci_ring *xhci_ring_alloc(struct xhci_hcd *xhci,
 	return ring;
 
 fail:
-	xhci_ring_free(xhci, ring);
+	kfree(ring);
 	return NULL;
 }
 
-- 
1.7.8.6

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Andy Shevchenko]

On Thu, Nov 1, 2012 at 9:47 PM, Julius Werner <jwerner@chromium.org> wrote:
> xhci_alloc_segments_for_ring() builds a list of xhci_segments and links
> the tail to head at the end (forming a ring). When it bails out for OOM
> reasons half-way through, it tries to destroy its half-built list with
> xhci_free_segments_for_ring(), even though it is not a ring yet. This
> causes a null-pointer dereference upon hitting the last element.
>
> Furthermore, one of its callers (xhci_ring_alloc()) mistakenly believes
> the output parameters to be valid upon this kind of OOM failure, and
> calls xhci_ring_free() on them. Since the (incomplete) list/ring should
> already be destroyed in that case, this would lead to a use after free.
>
> This patch fixes those issues by having xhci_alloc_segments_for_ring()
> destroy its half-built, non-circular list manually and destroying the
> invalid struct xhci_ring in xhci_ring_alloc() with a plain kfree().
>
> Signed-off-by: Julius Werner <jwerner@chromium.org>
> ---
>  drivers/usb/host/xhci-mem.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> index 487bc08..fb51c70 100644
> --- a/drivers/usb/host/xhci-mem.c
> +++ b/drivers/usb/host/xhci-mem.c
> @@ -205,7 +205,12 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
>
>                 next = xhci_segment_alloc(xhci, cycle_state, flags);
>                 if (!next) {
> -                       xhci_free_segments_for_ring(xhci, *first);
> +                       prev = *first;
> +                       while (prev) {
> +                               next = prev->next;
> +                               xhci_segment_free(xhci, prev);
> +                               prev = next;
> +                       }

Is it just
for (prev = *first; prev; prev = prev->next)
    xhci_segment_free(xhci, prev);

?

>                         return -ENOMEM;
>                 }
>                 xhci_link_segments(xhci, prev, next, type);
> @@ -258,7 +263,7 @@ static struct xhci_ring *xhci_ring_alloc(struct xhci_hcd *xhci,
>         return ring;
>
>  fail:
> -       xhci_ring_free(xhci, ring);
> +       kfree(ring);
>         return NULL;
>  }
>
> --
> 1.7.8.6
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-usb" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Sarah Sharp]

On Thu, Nov 01, 2012 at 10:13:00PM +0200, Andy Shevchenko wrote:
> On Thu, Nov 1, 2012 at 9:47 PM, Julius Werner <jwerner@chromium.org> wrote:
> > xhci_alloc_segments_for_ring() builds a list of xhci_segments and links
> > the tail to head at the end (forming a ring). When it bails out for OOM
> > reasons half-way through, it tries to destroy its half-built list with
> > xhci_free_segments_for_ring(), even though it is not a ring yet. This
> > causes a null-pointer dereference upon hitting the last element.
> >
> > Furthermore, one of its callers (xhci_ring_alloc()) mistakenly believes
> > the output parameters to be valid upon this kind of OOM failure, and
> > calls xhci_ring_free() on them. Since the (incomplete) list/ring should
> > already be destroyed in that case, this would lead to a use after free.
> >
> > This patch fixes those issues by having xhci_alloc_segments_for_ring()
> > destroy its half-built, non-circular list manually and destroying the
> > invalid struct xhci_ring in xhci_ring_alloc() with a plain kfree().
> >
> > Signed-off-by: Julius Werner <jwerner@chromium.org>
> > ---
> >  drivers/usb/host/xhci-mem.c |    9 +++++++--
> >  1 files changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> > index 487bc08..fb51c70 100644
> > --- a/drivers/usb/host/xhci-mem.c
> > +++ b/drivers/usb/host/xhci-mem.c
> > @@ -205,7 +205,12 @@ static int xhci_alloc_segments_for_ring(struct xhci_hcd *xhci,
> >
> >                 next = xhci_segment_alloc(xhci, cycle_state, flags);
> >                 if (!next) {
> > -                       xhci_free_segments_for_ring(xhci, *first);
> > +                       prev = *first;
> > +                       while (prev) {
> > +                               next = prev->next;
> > +                               xhci_segment_free(xhci, prev);
> > +                               prev = next;
> > +                       }
> 
> Is it just
> for (prev = *first; prev; prev = prev->next)
>     xhci_segment_free(xhci, prev);
> 
> ?

Yeah, that seems cleaner.

Sarah Sharp

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Julius Werner]

>> Is it just
>> for (prev = *first; prev; prev = prev->next)
>>    xhci_segment_free(xhci, prev);
>> 
>> ?
> 
> Yeah, that seems cleaner.
> 
> Sarah Sharp

I can submit it that way if you want, but I would advise against it. This way you access the prev pointer after it has been freed already… that's probably not a problem in the current kernel, but it depends on how the underlying memory is managed and is generally just not a good idea in my opinion.

Re: [PATCH] xhci: fix null-pointer dereference when destroying half-built segment rings

[Sarah Sharp]

On Thu, Nov 01, 2012 at 01:28:46PM -0700, Julius Werner wrote:
> >> Is it just
> >> for (prev = *first; prev; prev = prev->next)
> >>    xhci_segment_free(xhci, prev);
> >> 
> >> ?
> > 
> > Yeah, that seems cleaner.
> > 
> > Sarah Sharp
> 
> I can submit it that way if you want, but I would advise against it. This way you access the prev pointer after it has been freed already… that's probably not a problem in the current kernel, but it depends on how the underlying memory is managed and is generally just not a good idea in my opinion.

Yep, you're right.  I'll take the second patch you submitted, thanks.

Sarah Sharp