[PATCH 0/5] fs, xfs refcount conversions

[PATCH 0/5] fs, xfs refcount conversions

[Elena Reshetova]

v3:
 * fixed header file inclusion

Now when new refcount_t type and API are finally merged
(see include/linux/refcount.h), the following
patches convert various refcounters in the xfs susystem from atomic_t
to refcount_t. By doing this we prevent intentional or accidental
underflows or overflows that can led to use-after-free vulnerabilities.


Elena Reshetova (5):
  fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to
    refcount_t
  fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to
    refcount_t
  fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t
  fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to
    refcount_t
  fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to
    refcount_t

 fs/xfs/xfs_bmap_item.c     |  4 ++--
 fs/xfs/xfs_bmap_item.h     |  2 +-
 fs/xfs/xfs_extfree_item.c  |  4 ++--
 fs/xfs/xfs_extfree_item.h  |  2 +-
 fs/xfs/xfs_linux.h         |  1 +
 fs/xfs/xfs_log.c           | 10 +++++-----
 fs/xfs/xfs_log_priv.h      |  2 +-
 fs/xfs/xfs_refcount_item.c |  4 ++--
 fs/xfs/xfs_refcount_item.h |  2 +-
 fs/xfs/xfs_rmap_item.c     |  4 ++--
 fs/xfs/xfs_rmap_item.h     |  2 +-
 11 files changed, 19 insertions(+), 18 deletions(-)

-- 
2.7.4

[PATCH 1/5] fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 fs/xfs/xfs_bmap_item.c | 4 ++--
 fs/xfs/xfs_bmap_item.h | 2 +-
 fs/xfs/xfs_linux.h     | 1 +
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c
index 9bf57c7..33104ad 100644
--- a/fs/xfs/xfs_bmap_item.c
+++ b/fs/xfs/xfs_bmap_item.c
@@ -199,7 +199,7 @@ xfs_bui_init(
 	buip->bui_format.bui_nextents = XFS_BUI_MAX_FAST_EXTENTS;
 	buip->bui_format.bui_id = (uintptr_t)(void *)buip;
 	atomic_set(&buip->bui_next_extent, 0);
-	atomic_set(&buip->bui_refcount, 2);
+	refcount_set(&buip->bui_refcount, 2);
 
 	return buip;
 }
@@ -215,7 +215,7 @@ void
 xfs_bui_release(
 	struct xfs_bui_log_item	*buip)
 {
-	if (atomic_dec_and_test(&buip->bui_refcount)) {
+	if (refcount_dec_and_test(&buip->bui_refcount)) {
 		xfs_trans_ail_remove(&buip->bui_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_bui_item_free(buip);
 	}
diff --git a/fs/xfs/xfs_bmap_item.h b/fs/xfs/xfs_bmap_item.h
index c867daa..7048b14 100644
--- a/fs/xfs/xfs_bmap_item.h
+++ b/fs/xfs/xfs_bmap_item.h
@@ -61,7 +61,7 @@ struct kmem_zone;
  */
 struct xfs_bui_log_item {
 	struct xfs_log_item		bui_item;
-	atomic_t			bui_refcount;
+	refcount_t			bui_refcount;
 	atomic_t			bui_next_extent;
 	unsigned long			bui_flags;	/* misc flags */
 	struct xfs_bui_log_format	bui_format;
diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
index 7a989de..09b0396 100644
--- a/fs/xfs/xfs_linux.h
+++ b/fs/xfs/xfs_linux.h
@@ -19,6 +19,7 @@
 #define __XFS_LINUX__
 
 #include <linux/types.h>
+#include <linux/refcount.h>
 
 /*
  * Kernel specific type declarations for XFS
-- 
2.7.4

[PATCH 2/5] fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 fs/xfs/xfs_extfree_item.c | 4 ++--
 fs/xfs/xfs_extfree_item.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c
index d7bc149..4e0acf0 100644
--- a/fs/xfs/xfs_extfree_item.c
+++ b/fs/xfs/xfs_extfree_item.c
@@ -220,7 +220,7 @@ xfs_efi_init(
 	efip->efi_format.efi_nextents = nextents;
 	efip->efi_format.efi_id = (uintptr_t)(void *)efip;
 	atomic_set(&efip->efi_next_extent, 0);
-	atomic_set(&efip->efi_refcount, 2);
+	refcount_set(&efip->efi_refcount, 2);
 
 	return efip;
 }
@@ -290,7 +290,7 @@ void
 xfs_efi_release(
 	struct xfs_efi_log_item	*efip)
 {
-	if (atomic_dec_and_test(&efip->efi_refcount)) {
+	if (refcount_dec_and_test(&efip->efi_refcount)) {
 		xfs_trans_ail_remove(&efip->efi_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_efi_item_free(efip);
 	}
diff --git a/fs/xfs/xfs_extfree_item.h b/fs/xfs/xfs_extfree_item.h
index a32c794..fadf736 100644
--- a/fs/xfs/xfs_extfree_item.h
+++ b/fs/xfs/xfs_extfree_item.h
@@ -64,7 +64,7 @@ struct kmem_zone;
  */
 typedef struct xfs_efi_log_item {
 	xfs_log_item_t		efi_item;
-	atomic_t		efi_refcount;
+	refcount_t		efi_refcount;
 	atomic_t		efi_next_extent;
 	unsigned long		efi_flags;	/* misc flags */
 	xfs_efi_log_format_t	efi_format;
-- 
2.7.4

[PATCH 3/5] fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 fs/xfs/xfs_log.c      | 10 +++++-----
 fs/xfs/xfs_log_priv.h |  2 +-
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index b1469f0..c127fa0 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -3500,8 +3500,8 @@ void
 xfs_log_ticket_put(
 	xlog_ticket_t	*ticket)
 {
-	ASSERT(atomic_read(&ticket->t_ref) > 0);
-	if (atomic_dec_and_test(&ticket->t_ref))
+	ASSERT(refcount_read(&ticket->t_ref) > 0);
+	if (refcount_dec_and_test(&ticket->t_ref))
 		kmem_zone_free(xfs_log_ticket_zone, ticket);
 }
 
@@ -3509,8 +3509,8 @@ xlog_ticket_t *
 xfs_log_ticket_get(
 	xlog_ticket_t	*ticket)
 {
-	ASSERT(atomic_read(&ticket->t_ref) > 0);
-	atomic_inc(&ticket->t_ref);
+	ASSERT(refcount_read(&ticket->t_ref) > 0);
+	refcount_inc(&ticket->t_ref);
 	return ticket;
 }
 
@@ -3632,7 +3632,7 @@ xlog_ticket_alloc(
 
 	unit_res = xfs_log_calc_unit_res(log->l_mp, unit_bytes);
 
-	atomic_set(&tic->t_ref, 1);
+	refcount_set(&tic->t_ref, 1);
 	tic->t_task		= current;
 	INIT_LIST_HEAD(&tic->t_queue);
 	tic->t_unit_res		= unit_res;
diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index c2604a5..3fc4aba 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -168,7 +168,7 @@ typedef struct xlog_ticket {
 	struct list_head   t_queue;	 /* reserve/write queue */
 	struct task_struct *t_task;	 /* task that owns this ticket */
 	xlog_tid_t	   t_tid;	 /* transaction identifier	 : 4  */
-	atomic_t	   t_ref;	 /* ticket reference count       : 4  */
+	refcount_t	   t_ref;	 /* ticket reference count       : 4  */
 	int		   t_curr_res;	 /* current reservation in bytes : 4  */
 	int		   t_unit_res;	 /* unit reservation in bytes    : 4  */
 	char		   t_ocnt;	 /* original count		 : 1  */
-- 
2.7.4

[PATCH 4/5] fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 fs/xfs/xfs_refcount_item.c | 4 ++--
 fs/xfs/xfs_refcount_item.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_refcount_item.c b/fs/xfs/xfs_refcount_item.c
index 6e4c744..61bc570 100644
--- a/fs/xfs/xfs_refcount_item.c
+++ b/fs/xfs/xfs_refcount_item.c
@@ -205,7 +205,7 @@ xfs_cui_init(
 	cuip->cui_format.cui_nextents = nextents;
 	cuip->cui_format.cui_id = (uintptr_t)(void *)cuip;
 	atomic_set(&cuip->cui_next_extent, 0);
-	atomic_set(&cuip->cui_refcount, 2);
+	refcount_set(&cuip->cui_refcount, 2);
 
 	return cuip;
 }
@@ -221,7 +221,7 @@ void
 xfs_cui_release(
 	struct xfs_cui_log_item	*cuip)
 {
-	if (atomic_dec_and_test(&cuip->cui_refcount)) {
+	if (refcount_dec_and_test(&cuip->cui_refcount)) {
 		xfs_trans_ail_remove(&cuip->cui_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_cui_item_free(cuip);
 	}
diff --git a/fs/xfs/xfs_refcount_item.h b/fs/xfs/xfs_refcount_item.h
index 5b74ddd..abc0377 100644
--- a/fs/xfs/xfs_refcount_item.h
+++ b/fs/xfs/xfs_refcount_item.h
@@ -63,7 +63,7 @@ struct kmem_zone;
  */
 struct xfs_cui_log_item {
 	struct xfs_log_item		cui_item;
-	atomic_t			cui_refcount;
+	refcount_t			cui_refcount;
 	atomic_t			cui_next_extent;
 	unsigned long			cui_flags;	/* misc flags */
 	struct xfs_cui_log_format	cui_format;
-- 
2.7.4

[PATCH 5/5] fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 fs/xfs/xfs_rmap_item.c | 4 ++--
 fs/xfs/xfs_rmap_item.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c
index 73c8278..5e1664a 100644
--- a/fs/xfs/xfs_rmap_item.c
+++ b/fs/xfs/xfs_rmap_item.c
@@ -204,7 +204,7 @@ xfs_rui_init(
 	ruip->rui_format.rui_nextents = nextents;
 	ruip->rui_format.rui_id = (uintptr_t)(void *)ruip;
 	atomic_set(&ruip->rui_next_extent, 0);
-	atomic_set(&ruip->rui_refcount, 2);
+	refcount_set(&ruip->rui_refcount, 2);
 
 	return ruip;
 }
@@ -243,7 +243,7 @@ void
 xfs_rui_release(
 	struct xfs_rui_log_item	*ruip)
 {
-	if (atomic_dec_and_test(&ruip->rui_refcount)) {
+	if (refcount_dec_and_test(&ruip->rui_refcount)) {
 		xfs_trans_ail_remove(&ruip->rui_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_rui_item_free(ruip);
 	}
diff --git a/fs/xfs/xfs_rmap_item.h b/fs/xfs/xfs_rmap_item.h
index 340c968..1e425a9 100644
--- a/fs/xfs/xfs_rmap_item.h
+++ b/fs/xfs/xfs_rmap_item.h
@@ -64,7 +64,7 @@ struct kmem_zone;
  */
 struct xfs_rui_log_item {
 	struct xfs_log_item		rui_item;
-	atomic_t			rui_refcount;
+	refcount_t			rui_refcount;
 	atomic_t			rui_next_extent;
 	unsigned long			rui_flags;	/* misc flags */
 	struct xfs_rui_log_format	rui_format;
-- 
2.7.4

Re: [PATCH 3/5] fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t

[Christoph Hellwig]

> -	ASSERT(atomic_read(&ticket->t_ref) > 0);
> -	atomic_inc(&ticket->t_ref);
> +	ASSERT(refcount_read(&ticket->t_ref) > 0);
> +	refcount_inc(&ticket->t_ref);

With strict refcount semantics refcount_inc should check that
the count is larger than 0, otherwise we'd need to use 
recount_inc_not_zero or whatever you're going to call it.

Is that something the recount code does / could do?

Re: [PATCH 3/5] fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t

[Kees Cook]

On Wed, Mar 8, 2017 at 7:50 AM, Christoph Hellwig <hch@infradead.org> wrote:
>> -     ASSERT(atomic_read(&ticket->t_ref) > 0);
>> -     atomic_inc(&ticket->t_ref);
>> +     ASSERT(refcount_read(&ticket->t_ref) > 0);
>> +     refcount_inc(&ticket->t_ref);
>
> With strict refcount semantics refcount_inc should check that
> the count is larger than 0, otherwise we'd need to use
> recount_inc_not_zero or whatever you're going to call it.
>
> Is that something the recount code does / could do?

Yes, refcount_inc() will not increment from 0 and WARNs. It looks like
xfs's ASSERT is also a warn (though with XFS-specific formatting), so
perhaps the ASSERT could be dropped? IIUC, Elena's approach to these
changes was to be conservative about removing the existing checks.

-Kees

-- 
Kees Cook
Pixel Security

RE: [PATCH 3/5] fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t

[Reshetova, Elena]

> On Wed, Mar 8, 2017 at 7:50 AM, Christoph Hellwig <hch@infradead.org> wrote:
> >> -     ASSERT(atomic_read(&ticket->t_ref) > 0);
> >> -     atomic_inc(&ticket->t_ref);
> >> +     ASSERT(refcount_read(&ticket->t_ref) > 0);
> >> +     refcount_inc(&ticket->t_ref);
> >
> > With strict refcount semantics refcount_inc should check that
> > the count is larger than 0, otherwise we'd need to use
> > recount_inc_not_zero or whatever you're going to call it.
> >
> > Is that something the recount code does / could do?
> 
> Yes, refcount_inc() will not increment from 0 and WARNs. It looks like
> xfs's ASSERT is also a warn (though with XFS-specific formatting), so
> perhaps the ASSERT could be dropped? IIUC, Elena's approach to these
> changes was to be conservative about removing the existing checks.

I am removing the existing WARNs now where they are 100% not needed, but leaving the ones like this ASSERT, 
because I am thinking that you might have test cases that are dependent on these ASSERTs with specific formatting and I don't want to break them. 
If you don't need them, I can remove them also. 

Best Regards,
Elena

> 
> -Kees
> 
> --
> Kees Cook
> Pixel Security

RE: [PATCH 0/5] fs, xfs refcount conversions

[Reshetova, Elena]

 
> v3:
>  * fixed header file inclusion

I don't think I have heard anything back on this v3 patch set. 
Is there still smth here to fix or could you take the changes in?

Best Regards,
Elena.

> 
> Now when new refcount_t type and API are finally merged
> (see include/linux/refcount.h), the following
> patches convert various refcounters in the xfs susystem from atomic_t
> to refcount_t. By doing this we prevent intentional or accidental
> underflows or overflows that can led to use-after-free vulnerabilities.
> 
> 
> Elena Reshetova (5):
>   fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to
>     refcount_t
>   fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to
>     refcount_t
>   fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t
>   fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to
>     refcount_t
>   fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to
>     refcount_t
> 
>  fs/xfs/xfs_bmap_item.c     |  4 ++--
>  fs/xfs/xfs_bmap_item.h     |  2 +-
>  fs/xfs/xfs_extfree_item.c  |  4 ++--
>  fs/xfs/xfs_extfree_item.h  |  2 +-
>  fs/xfs/xfs_linux.h         |  1 +
>  fs/xfs/xfs_log.c           | 10 +++++-----
>  fs/xfs/xfs_log_priv.h      |  2 +-
>  fs/xfs/xfs_refcount_item.c |  4 ++--
>  fs/xfs/xfs_refcount_item.h |  2 +-
>  fs/xfs/xfs_rmap_item.c     |  4 ++--
>  fs/xfs/xfs_rmap_item.h     |  2 +-
>  11 files changed, 19 insertions(+), 18 deletions(-)
> 
> --
> 2.7.4

Re: [PATCH 0/5] fs, xfs refcount conversions

[Darrick J. Wong]

On Thu, Apr 20, 2017 at 08:11:41AM +0000, Reshetova, Elena wrote:
> 
>  
> > v3:
> >  * fixed header file inclusion
> 
> I don't think I have heard anything back on this v3 patch set. 
> Is there still smth here to fix or could you take the changes in?

Generally I think it looks ok; it's running through shakedown testing as
we speak.

--D

> 
> Best Regards,
> Elena.
> 
> > 
> > Now when new refcount_t type and API are finally merged
> > (see include/linux/refcount.h), the following
> > patches convert various refcounters in the xfs susystem from atomic_t
> > to refcount_t. By doing this we prevent intentional or accidental
> > underflows or overflows that can led to use-after-free vulnerabilities.
> > 
> > 
> > Elena Reshetova (5):
> >   fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to
> >     refcount_t
> >   fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to
> >     refcount_t
> >   fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t
> >   fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to
> >     refcount_t
> >   fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to
> >     refcount_t
> > 
> >  fs/xfs/xfs_bmap_item.c     |  4 ++--
> >  fs/xfs/xfs_bmap_item.h     |  2 +-
> >  fs/xfs/xfs_extfree_item.c  |  4 ++--
> >  fs/xfs/xfs_extfree_item.h  |  2 +-
> >  fs/xfs/xfs_linux.h         |  1 +
> >  fs/xfs/xfs_log.c           | 10 +++++-----
> >  fs/xfs/xfs_log_priv.h      |  2 +-
> >  fs/xfs/xfs_refcount_item.c |  4 ++--
> >  fs/xfs/xfs_refcount_item.h |  2 +-
> >  fs/xfs/xfs_rmap_item.c     |  4 ++--
> >  fs/xfs/xfs_rmap_item.h     |  2 +-
> >  11 files changed, 19 insertions(+), 18 deletions(-)
> > 
> > --
> > 2.7.4
> 
> --
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Re: [PATCH 0/5] fs, xfs refcount conversions

[Darrick J. Wong]

On Thu, Apr 20, 2017 at 10:24:04AM -0700, Darrick J. Wong wrote:
> On Thu, Apr 20, 2017 at 08:11:41AM +0000, Reshetova, Elena wrote:
> > 
> >  
> > > v3:
> > >  * fixed header file inclusion
> > 
> > I don't think I have heard anything back on this v3 patch set. 
> > Is there still smth here to fix or could you take the changes in?
> 
> Generally I think it looks ok; it's running through shakedown testing as
> we speak.

Well, it survives the shakedown testing all right, but I can't shake the
feeling that this is overkill.  The xfs_{ef,bu,cu,ru}i_log_item
structures should only ever be referenced by the log itself and the
log-update-done log item: the refcount should only ever go 2 -> 1 -> 0.
We set the refcount to 2 and never increment it.

Given that I've seen occasional refcounting problems with the log intent
items, I think it would suffice to ASSERT in xfs_*i_release that the
refcount isn't already zero.  Using ASSERT is particularly useful
because ASSERT compiles out in release builds.

As for the xlog_ticket, we increment its refcount as part of duplicating
a transaction as a part of committing one transaction (which decrements
the xlog_ticket's refcount) and reusing the log reservation to create a
new transaction.  In this case the refcounting already seems to have
sufficient non-zero checks, and since only one thread can hold a
transaction at any given time, I don't see how overflow protection helps
here.

In other words, I'm ok with better refcount checking but need convincing
that we need to do more than what a simple ASSERT would provide.

--D

> 
> --D
> 
> > 
> > Best Regards,
> > Elena.
> > 
> > > 
> > > Now when new refcount_t type and API are finally merged
> > > (see include/linux/refcount.h), the following
> > > patches convert various refcounters in the xfs susystem from atomic_t
> > > to refcount_t. By doing this we prevent intentional or accidental
> > > underflows or overflows that can led to use-after-free vulnerabilities.
> > > 
> > > 
> > > Elena Reshetova (5):
> > >   fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to
> > >     refcount_t
> > >   fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to
> > >     refcount_t
> > >   fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t
> > >   fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to
> > >     refcount_t
> > >   fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to
> > >     refcount_t
> > > 
> > >  fs/xfs/xfs_bmap_item.c     |  4 ++--
> > >  fs/xfs/xfs_bmap_item.h     |  2 +-
> > >  fs/xfs/xfs_extfree_item.c  |  4 ++--
> > >  fs/xfs/xfs_extfree_item.h  |  2 +-
> > >  fs/xfs/xfs_linux.h         |  1 +
> > >  fs/xfs/xfs_log.c           | 10 +++++-----
> > >  fs/xfs/xfs_log_priv.h      |  2 +-
> > >  fs/xfs/xfs_refcount_item.c |  4 ++--
> > >  fs/xfs/xfs_refcount_item.h |  2 +-
> > >  fs/xfs/xfs_rmap_item.c     |  4 ++--
> > >  fs/xfs/xfs_rmap_item.h     |  2 +-
> > >  11 files changed, 19 insertions(+), 18 deletions(-)
> > > 
> > > --
> > > 2.7.4
> > 
> > --
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> > the body of a message to majordomo@vger.kernel.org
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Re: [PATCH 0/5] fs, xfs refcount conversions

[Kees Cook]

On Thu, Apr 20, 2017 at 3:04 PM, Darrick J. Wong
<darrick.wong@oracle.com> wrote:
> On Thu, Apr 20, 2017 at 10:24:04AM -0700, Darrick J. Wong wrote:
>> On Thu, Apr 20, 2017 at 08:11:41AM +0000, Reshetova, Elena wrote:
>> >
>> >
>> > > v3:
>> > >  * fixed header file inclusion
>> >
>> > I don't think I have heard anything back on this v3 patch set.
>> > Is there still smth here to fix or could you take the changes in?
>>
>> Generally I think it looks ok; it's running through shakedown testing as
>> we speak.
>
> Well, it survives the shakedown testing all right, but I can't shake the
> feeling that this is overkill.  The xfs_{ef,bu,cu,ru}i_log_item
> structures should only ever be referenced by the log itself and the
> log-update-done log item: the refcount should only ever go 2 -> 1 -> 0.
> We set the refcount to 2 and never increment it.

I haven't studied this code, but I think it'd likely be worth keeping
it just to catch any future bug that might appear. If it doesn't
create a problem, we should use refcount_t for anything that is being
used as a reference counter.

> Given that I've seen occasional refcounting problems with the log intent
> items, I think it would suffice to ASSERT in xfs_*i_release that the
> refcount isn't already zero.  Using ASSERT is particularly useful
> because ASSERT compiles out in release builds.

We _absolutely_ don't want to compile out these checks: the point is
to catch flaws that are discovered and could be exploited in the wild
before updates could be delivered to a system (if they ever are at
all).

> As for the xlog_ticket, we increment its refcount as part of duplicating
> a transaction as a part of committing one transaction (which decrements
> the xlog_ticket's refcount) and reusing the log reservation to create a
> new transaction.  In this case the refcounting already seems to have
> sufficient non-zero checks, and since only one thread can hold a
> transaction at any given time, I don't see how overflow protection helps
> here.
>
> In other words, I'm ok with better refcount checking but need convincing
> that we need to do more than what a simple ASSERT would provide.

If it doesn't get in your way, and it really is reference counting
(which it sounds like it is), we should be using refcount_t. The kinds
of bugs we've seen over the last decade continually prove that we'll
see refcount issues where we least expect it, and we need to catch
them.

Thanks!

-Kees

-- 
Kees Cook
Pixel Security

[PATCH 5/5] fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to refcount_t

[Elena Reshetova]

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable xfs_rui_log_item.rui_refcount is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

**Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.
The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.
Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the xfs_rui_log_item.rui_refcount it might make a difference
in following places:
 - xfs_rui_release(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and control dependency on success
   vs. fully ordered atomic counterpart

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
---
 fs/xfs/xfs_rmap_item.c | 6 +++---
 fs/xfs/xfs_rmap_item.h | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c
index f3b139c..f914829f 100644
--- a/fs/xfs/xfs_rmap_item.c
+++ b/fs/xfs/xfs_rmap_item.c
@@ -204,7 +204,7 @@ xfs_rui_init(
 	ruip->rui_format.rui_nextents = nextents;
 	ruip->rui_format.rui_id = (uintptr_t)(void *)ruip;
 	atomic_set(&ruip->rui_next_extent, 0);
-	atomic_set(&ruip->rui_refcount, 2);
+	refcount_set(&ruip->rui_refcount, 2);
 
 	return ruip;
 }
@@ -243,8 +243,8 @@ void
 xfs_rui_release(
 	struct xfs_rui_log_item	*ruip)
 {
-	ASSERT(atomic_read(&ruip->rui_refcount) > 0);
-	if (atomic_dec_and_test(&ruip->rui_refcount)) {
+	ASSERT(refcount_read(&ruip->rui_refcount) > 0);
+	if (refcount_dec_and_test(&ruip->rui_refcount)) {
 		xfs_trans_ail_remove(&ruip->rui_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_rui_item_free(ruip);
 	}
diff --git a/fs/xfs/xfs_rmap_item.h b/fs/xfs/xfs_rmap_item.h
index 340c968..1e425a9 100644
--- a/fs/xfs/xfs_rmap_item.h
+++ b/fs/xfs/xfs_rmap_item.h
@@ -64,7 +64,7 @@ struct kmem_zone;
  */
 struct xfs_rui_log_item {
 	struct xfs_log_item		rui_item;
-	atomic_t			rui_refcount;
+	refcount_t			rui_refcount;
 	atomic_t			rui_next_extent;
 	unsigned long			rui_flags;	/* misc flags */
 	struct xfs_rui_log_format	rui_format;
-- 
2.7.4

[PATCH 5/5] fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to refcount_t

[Elena Reshetova]

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable xfs_rui_log_item.rui_refcount is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
---
 fs/xfs/xfs_rmap_item.c | 6 +++---
 fs/xfs/xfs_rmap_item.h | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c
index f3b139c..f914829f 100644
--- a/fs/xfs/xfs_rmap_item.c
+++ b/fs/xfs/xfs_rmap_item.c
@@ -204,7 +204,7 @@ xfs_rui_init(
 	ruip->rui_format.rui_nextents = nextents;
 	ruip->rui_format.rui_id = (uintptr_t)(void *)ruip;
 	atomic_set(&ruip->rui_next_extent, 0);
-	atomic_set(&ruip->rui_refcount, 2);
+	refcount_set(&ruip->rui_refcount, 2);
 
 	return ruip;
 }
@@ -243,8 +243,8 @@ void
 xfs_rui_release(
 	struct xfs_rui_log_item	*ruip)
 {
-	ASSERT(atomic_read(&ruip->rui_refcount) > 0);
-	if (atomic_dec_and_test(&ruip->rui_refcount)) {
+	ASSERT(refcount_read(&ruip->rui_refcount) > 0);
+	if (refcount_dec_and_test(&ruip->rui_refcount)) {
 		xfs_trans_ail_remove(&ruip->rui_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_rui_item_free(ruip);
 	}
diff --git a/fs/xfs/xfs_rmap_item.h b/fs/xfs/xfs_rmap_item.h
index 340c968..1e425a9 100644
--- a/fs/xfs/xfs_rmap_item.h
+++ b/fs/xfs/xfs_rmap_item.h
@@ -64,7 +64,7 @@ struct kmem_zone;
  */
 struct xfs_rui_log_item {
 	struct xfs_log_item		rui_item;
-	atomic_t			rui_refcount;
+	refcount_t			rui_refcount;
 	atomic_t			rui_next_extent;
 	unsigned long			rui_flags;	/* misc flags */
 	struct xfs_rui_log_format	rui_format;
-- 
2.7.4

[PATCH 5/5] fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 fs/xfs/xfs_rmap_item.c | 4 ++--
 fs/xfs/xfs_rmap_item.h | 4 +++-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c
index 73c8278..5e1664a 100644
--- a/fs/xfs/xfs_rmap_item.c
+++ b/fs/xfs/xfs_rmap_item.c
@@ -204,7 +204,7 @@ xfs_rui_init(
 	ruip->rui_format.rui_nextents = nextents;
 	ruip->rui_format.rui_id = (uintptr_t)(void *)ruip;
 	atomic_set(&ruip->rui_next_extent, 0);
-	atomic_set(&ruip->rui_refcount, 2);
+	refcount_set(&ruip->rui_refcount, 2);
 
 	return ruip;
 }
@@ -243,7 +243,7 @@ void
 xfs_rui_release(
 	struct xfs_rui_log_item	*ruip)
 {
-	if (atomic_dec_and_test(&ruip->rui_refcount)) {
+	if (refcount_dec_and_test(&ruip->rui_refcount)) {
 		xfs_trans_ail_remove(&ruip->rui_item, SHUTDOWN_LOG_IO_ERROR);
 		xfs_rui_item_free(ruip);
 	}
diff --git a/fs/xfs/xfs_rmap_item.h b/fs/xfs/xfs_rmap_item.h
index 340c968..2529a35 100644
--- a/fs/xfs/xfs_rmap_item.h
+++ b/fs/xfs/xfs_rmap_item.h
@@ -20,6 +20,8 @@
 #ifndef	__XFS_RMAP_ITEM_H__
 #define	__XFS_RMAP_ITEM_H__
 
+#include <linux/refcount.h>
+
 /*
  * There are (currently) three pairs of rmap btree redo item types: map, unmap,
  * and convert.  The common abbreviations for these are RUI (rmap update
@@ -64,7 +66,7 @@ struct kmem_zone;
  */
 struct xfs_rui_log_item {
 	struct xfs_log_item		rui_item;
-	atomic_t			rui_refcount;
+	refcount_t			rui_refcount;
 	atomic_t			rui_next_extent;
 	unsigned long			rui_flags;	/* misc flags */
 	struct xfs_rui_log_format	rui_format;
-- 
2.7.4