[PATCH] eventfs: Use children field for rcu head and add memory barriers

[PATCH] eventfs: Use children field for rcu head and add memory barriers

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

When an eventfs inode is freed, it sets ei->is_freed and then uses its
ei->list to add it to the srcu link list as the list field is a union with
the rcu list head. As the ei->list is used to iterate over an SRCU
protected list without taking the eventfs_mutex, there's nothing stopping
the iteration over that list to see the ei->rcu instead of the ei->list
and it will read a corrupt target.

To fix this, change the union of the rcu list head with the children list.
On freeing the eventfs inode, set the is_free and execute a smp_wmb()
before adding the eventfs inode to the SRCU list.

On iteration of the ei->children list, at the start, execute a smp_rmb()
and then read the is_freed of the ei to see if the children list is still
valid. If is_freed is set, then the ei_child read is not valid and the
loop should exit immediately.

Cc: stable@vger.kernel.org
Fixes: 704f960dbee2f ("eventfs: Read ei->entries before ei->children in eventfs_iterate()")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260806022719.375354-1-shuangpeng.kernel%40gmail.com
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
Changes since v2: https://patch.msgid.link/20260807204441.75a7db89@gandalf.local.home

- Fix comment about smp_wmb() being in put_ei() and not free_ei().

 fs/tracefs/event_inode.c | 24 ++++++++++++++++++++++++
 fs/tracefs/internal.h    |  4 ++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 39c7a34531e8..bc82d3284a2e 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -124,7 +124,17 @@ static inline void put_ei(struct eventfs_inode *ei)
 static inline void free_ei(struct eventfs_inode *ei)
 {
 	if (ei) {
+		/* The ei should have no children if it is being freed. */
+		WARN_ON_ONCE(!list_empty(&ei->children));
 		ei->is_freed = 1;
+		/*
+		 * The SRCU iteration has a smp_rmb() to make sure it
+		 * sees a child (that may have already been freed)
+		 * before it reads is_free. If is_free is set, it must
+		 * not use the child it acquired from ei->children, as
+		 * the list may be used for SRCU.
+		 */
+		smp_wmb();
 		put_ei(ei);
 	}
 }
@@ -627,6 +637,20 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 	list_for_each_entry_srcu(ei_child, &ei->children, list,
 				 srcu_read_lock_held(&eventfs_srcu)) {
 
+		/*
+		 * If the ei is being freed, then the ei->children may be
+		 * being used as the rcu list, which means the next element
+		 * may be garbage. The ei->is_free is set before switching
+		 * the ei->children over to ei->rcu. The read memory barrier
+		 * here makes sure the ei_child is read before is_free is
+		 * updated.
+		 *
+		 * Matches the smp_wmb() in free_ei()
+		 */
+		smp_rmb();
+		if (ei->is_freed)
+			return -EINVAL;
+
 		if (c > 0) {
 			c--;
 			continue;
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index a4a7f8431aff..c61481d04c8e 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -46,11 +46,11 @@ struct eventfs_attr {
  * @ino:	The saved inode number
  */
 struct eventfs_inode {
+	struct list_head	list;
 	union {
-		struct list_head	list;
+		struct list_head	children;
 		struct rcu_head		rcu;
 	};
-	struct list_head		children;
 	const struct eventfs_entry	*entries;
 	const char			*name;
 	struct eventfs_attr		*entry_attrs;
-- 
2.53.0

Re: [PATCH v3] eventfs: Use children field for rcu head and add memory barriers

[Steven Rostedt]

I forgot to make the subject say "v3"

-- Steve

[PATCH] eventfs: Use children field for rcu head and add memory barriers

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

When an eventfs inode is freed, it sets ei->is_freed and then uses its
ei->list to add it to the srcu link list as the list field is a union with
the rcu list head. As the ei->list is used to iterate over an SRCU
protected list without taking the eventfs_mutex, there's nothing stopping
the iteration over that list to see the ei->rcu instead of the ei->list
and it will read a corrupt target.

To fix this, change the union of the rcu list head with the children list.
On freeing the eventfs inode, set the is_free and execute a smp_wmb()
before adding the eventfs inode to the SRCU list.

On iteration of the ei->children list, at the start, execute a smp_rmb()
and then read the is_freed of the ei to see if the children list is still
valid. If is_freed is set, then the ei_child read is not valid and the
loop should exit immediately.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260806022719.375354-1-shuangpeng.kernel%40gmail.com
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 fs/tracefs/event_inode.c | 24 ++++++++++++++++++++++++
 fs/tracefs/internal.h    |  4 ++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 39c7a34531e8..677c39b0d62f 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -124,6 +124,16 @@ static inline void put_ei(struct eventfs_inode *ei)
 static inline void free_ei(struct eventfs_inode *ei)
 {
 	if (ei) {
+		WARN_ON_ONCE(!list_empty(&ei->children));
+		/*
+		 * The ei should have no children if it is being freed.
+		 * The SRCU iteration has a smp_rmb() to make sure it
+		 * sees a child (that may have already been freed)
+		 * before it reads is_free. If is_free is set, it must
+		 * not use the child it acquired from ei->children, as
+		 * the list may be used for SRCU.
+		 */
+		smp_wmb();
 		ei->is_freed = 1;
 		put_ei(ei);
 	}
@@ -627,6 +637,20 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
 	list_for_each_entry_srcu(ei_child, &ei->children, list,
 				 srcu_read_lock_held(&eventfs_srcu)) {
 
+		/*
+		 * If the ei is being freed, then the ei->children may be
+		 * being used as the rcu list, which means the next element
+		 * may be garbage. The ei->is_free is set before switching
+		 * the ei->children over to ei->rcu. The read memory barrier
+		 * here makes sure the ei_child is read before is_free is
+		 * updated.
+		 *
+		 * Matches the smp_wmb() in put_ei()
+		 */
+		smp_rmb();
+		if (ei->is_freed)
+			return -EINVAL;
+
 		if (c > 0) {
 			c--;
 			continue;
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index a4a7f8431aff..c61481d04c8e 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -46,11 +46,11 @@ struct eventfs_attr {
  * @ino:	The saved inode number
  */
 struct eventfs_inode {
+	struct list_head	list;
 	union {
-		struct list_head	list;
+		struct list_head	children;
 		struct rcu_head		rcu;
 	};
-	struct list_head		children;
 	const struct eventfs_entry	*entries;
 	const char			*name;
 	struct eventfs_attr		*entry_attrs;
-- 
2.53.0

Re: [PATCH] eventfs: Use children field for rcu head and add memory barriers

[Masami Hiramatsu]

On Fri, 7 Aug 2026 17:04:08 -0400
Steven Rostedt <steven@rostedt.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> When an eventfs inode is freed, it sets ei->is_freed and then uses its
> ei->list to add it to the srcu link list as the list field is a union with
> the rcu list head. As the ei->list is used to iterate over an SRCU
> protected list without taking the eventfs_mutex, there's nothing stopping
> the iteration over that list to see the ei->rcu instead of the ei->list
> and it will read a corrupt target.
> 
> To fix this, change the union of the rcu list head with the children list.
> On freeing the eventfs inode, set the is_free and execute a smp_wmb()
> before adding the eventfs inode to the SRCU list.
> 
> On iteration of the ei->children list, at the start, execute a smp_rmb()
> and then read the is_freed of the ei to see if the children list is still
> valid. If is_freed is set, then the ei_child read is not valid and the
> loop should exit immediately.
> 

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

The critical issues Sashiko mentioned will be fixed by Shuangpeng's patch[1].
[1] https://lore.kernel.org/all/20260806022719.375354-1-shuangpeng.kernel@gmail.com/

Thank you,

> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20260806022719.375354-1-shuangpeng.kernel%40gmail.com
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  fs/tracefs/event_inode.c | 24 ++++++++++++++++++++++++
>  fs/tracefs/internal.h    |  4 ++--
>  2 files changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
> index 39c7a34531e8..677c39b0d62f 100644
> --- a/fs/tracefs/event_inode.c
> +++ b/fs/tracefs/event_inode.c
> @@ -124,6 +124,16 @@ static inline void put_ei(struct eventfs_inode *ei)
>  static inline void free_ei(struct eventfs_inode *ei)
>  {
>  	if (ei) {
> +		WARN_ON_ONCE(!list_empty(&ei->children));
> +		/*
> +		 * The ei should have no children if it is being freed.
> +		 * The SRCU iteration has a smp_rmb() to make sure it
> +		 * sees a child (that may have already been freed)
> +		 * before it reads is_free. If is_free is set, it must
> +		 * not use the child it acquired from ei->children, as
> +		 * the list may be used for SRCU.
> +		 */
> +		smp_wmb();
>  		ei->is_freed = 1;
>  		put_ei(ei);
>  	}
> @@ -627,6 +637,20 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
>  	list_for_each_entry_srcu(ei_child, &ei->children, list,
>  				 srcu_read_lock_held(&eventfs_srcu)) {
>  
> +		/*
> +		 * If the ei is being freed, then the ei->children may be
> +		 * being used as the rcu list, which means the next element
> +		 * may be garbage. The ei->is_free is set before switching
> +		 * the ei->children over to ei->rcu. The read memory barrier
> +		 * here makes sure the ei_child is read before is_free is
> +		 * updated.
> +		 *
> +		 * Matches the smp_wmb() in put_ei()
> +		 */
> +		smp_rmb();
> +		if (ei->is_freed)
> +			return -EINVAL;
> +
>  		if (c > 0) {
>  			c--;
>  			continue;
> diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
> index a4a7f8431aff..c61481d04c8e 100644
> --- a/fs/tracefs/internal.h
> +++ b/fs/tracefs/internal.h
> @@ -46,11 +46,11 @@ struct eventfs_attr {
>   * @ino:	The saved inode number
>   */
>  struct eventfs_inode {
> +	struct list_head	list;
>  	union {
> -		struct list_head	list;
> +		struct list_head	children;
>  		struct rcu_head		rcu;
>  	};
> -	struct list_head		children;
>  	const struct eventfs_entry	*entries;
>  	const char			*name;
>  	struct eventfs_attr		*entry_attrs;
> -- 
> 2.53.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH] eventfs: Use children field for rcu head and add memory barriers

[Steven Rostedt]

On Sat, 8 Aug 2026 07:15:06 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

 
> Looks good to me.
> 
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> The critical issues Sashiko mentioned will be fixed by Shuangpeng's patch[1].

No ;-) The critical issues Sashiko mentioned was from a review of that
patch which stated:

  "This isn't a bug introduced by this patch, but does this still leave
   concurrent SRCU readers vulnerable to list pointer corruption?"

Thus Shuangpeng's patch doesn't address it.

I was in the process of applying Shuangpeng's patch and I look at the
Sashiko review and wrote this patch to fix that issue too. I plan on
applying both.


> [1] https://lore.kernel.org/all/20260806022719.375354-1-shuangpeng.kernel@gmail.com/
> 
> Thank you,
> 
> > Reported-by: Sashiko <sashiko-bot@kernel.org>
> > Closes: https://sashiko.dev/#/patchset/20260806022719.375354-1-shuangpeng.kernel%40gmail.com
> > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> > ---
> >  fs/tracefs/event_inode.c | 24 ++++++++++++++++++++++++
> >  fs/tracefs/internal.h    |  4 ++--
> >  2 files changed, 26 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
> > index 39c7a34531e8..677c39b0d62f 100644
> > --- a/fs/tracefs/event_inode.c
> > +++ b/fs/tracefs/event_inode.c
> > @@ -124,6 +124,16 @@ static inline void put_ei(struct eventfs_inode *ei)
> >  static inline void free_ei(struct eventfs_inode *ei)
> >  {
> >  	if (ei) {
> > +		WARN_ON_ONCE(!list_empty(&ei->children));
> > +		/*
> > +		 * The ei should have no children if it is being freed.
> > +		 * The SRCU iteration has a smp_rmb() to make sure it
> > +		 * sees a child (that may have already been freed)
> > +		 * before it reads is_free. If is_free is set, it must
> > +		 * not use the child it acquired from ei->children, as
> > +		 * the list may be used for SRCU.
> > +		 */
> > +		smp_wmb();
> >  		ei->is_freed = 1;

And Sashiko found a bug here (from reading the comments). The smp_wmb() is
supposed to be *after* the ei->is_free = 1 !!!

I'll send a v2.

-- Steve

> >  		put_ei(ei);
> >  	}