Re: [PATCH] zram: fix use-after-free in zram_writeback_endio

[Minchan Kim <minchan@kernel.org>]

On Thu, May 07, 2026 at 03:56:52PM -0700, Minchan Kim wrote:
> On Thu, May 07, 2026 at 06:40:37PM +0900, Sergey Senozhatsky wrote:
> > On (26/05/05 09:37), Minchan Kim wrote:
> > > > @@ -966,9 +966,8 @@ static void zram_writeback_endio(struct bio *bio)
> > > >
> > > >  	spin_lock_irqsave(&wb_ctl->done_lock, flags);
> > > >  	list_add(&req->entry, &wb_ctl->done_reqs);
> > > > -	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
> > > > -
> > > >  	wake_up(&wb_ctl->done_wait);
> > > > +	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
> > > >  }
> > > >
> > >
> > > I agree this will fix the issue, but using a lock to extend the lifetime of
> > > an object to avoid a UAF is not a good pattern. Object lifetime shared between
> > > process and interrupt contexts should be managed explicitly using refcount.
> > 
> > ->num_inflight is a ref-counter, basically.  The problem is that
> > completion is a two-step process, only one part of each is synchronized
> > with the writeback context.  I honestly don't want to have two ref-counts:
> > one for requests pending zram completion and one for active endio contexts.
> > Maybe we can repurpose num_inflight instead.
> 
> If it can make the code much clearer and simpler, I have no objection.
> 
> > 
> > > Furthermore, keeping wake_up() outside the critical section minimizes
> > > interrupt-disabled latency
> > 
> > So I considered that, but isn't endio already called from IRQ context?
> > Just asking.  We wakeup only one waiter (writeback task), so it's not
> > that bad CPU-cycles wise.  Do you think it's really a concern?
> 
> I don't think it will have any measurable impact; I was just pointing out
> a theoretical one.
> 
> > 
> > wake_up() under spin-lock solves the problem of a unsynchronized
> > two-stages endio process.
> > 
> > > and avoids nesting spinlocks (done_lock -> done_wait.lock), reducing
> > > the risk of future lockdep issues, just in case.
> > 
> > I considered lockdep as well but ruled it out as impossible scenario,
> > nesting here is strictly uni-directional, we never call into zram from
> > the scheduler.  Just saying.
> 
> Sure. I just prefer to avoid adding more lock dependencies without a strong
> justification, to prevent potential locking issues in the future.
> 
> > 
> > > It definitely will add more overhead for the submission/completion paths to deal
> > > with the refcount, but I think we should go that way at the cost of runtime.
> > 
> > Dunno, something like below maybe?
> > 
> > ---
> >  drivers/block/zram/zram_drv.c | 14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > index ce2e1c79fc75..27fe50d666d7 100644
> > --- a/drivers/block/zram/zram_drv.c
> > +++ b/drivers/block/zram/zram_drv.c
> > @@ -967,7 +967,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
> >  static void zram_writeback_endio(struct bio *bio)
> >  {
> >  	struct zram_wb_req *req = container_of(bio, struct zram_wb_req, bio);
> > -	struct zram_wb_ctl *wb_ctl = bio->bi_private;
> > +	struct zram_wb_ctl *wb_ctl = READ_ONCE(bio->bi_private);
> >  	unsigned long flags;
> >  
> >  	spin_lock_irqsave(&wb_ctl->done_lock, flags);
> > @@ -975,6 +975,7 @@ static void zram_writeback_endio(struct bio *bio)
> >  	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
> >  
> >  	wake_up(&wb_ctl->done_wait);
> > +	atomic_dec(&wb_ctl->num_inflight);
> >  }
> >  
> >  static void zram_submit_wb_request(struct zram *zram,
> > @@ -998,7 +999,7 @@ static int zram_complete_done_reqs(struct zram *zram,
> >  	unsigned long flags;
> >  	int ret = 0, err;
> >  
> > -	while (atomic_read(&wb_ctl->num_inflight) > 0) {
> > +	for (;;) {
> >  		spin_lock_irqsave(&wb_ctl->done_lock, flags);
> >  		req = list_first_entry_or_null(&wb_ctl->done_reqs,
> >  					       struct zram_wb_req, entry);
> > @@ -1006,7 +1007,6 @@ static int zram_complete_done_reqs(struct zram *zram,
> >  			list_del(&req->entry);
> >  		spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
> >  
> > -		/* ->num_inflight > 0 doesn't mean we have done requests */
> >  		if (!req)
> >  			break;
> >  
> > @@ -1014,7 +1014,6 @@ static int zram_complete_done_reqs(struct zram *zram,
> >  		if (err)
> >  			ret = err;
> >  
> > -		atomic_dec(&wb_ctl->num_inflight);
> >  		release_pp_slot(zram, req->pps);
> >  		req->pps = NULL;
> >  
> > @@ -1129,8 +1128,11 @@ static int zram_writeback_slots(struct zram *zram,
> >  	if (req)
> >  		release_wb_req(req);
> >  
> > -	while (atomic_read(&wb_ctl->num_inflight) > 0) {
> > -		wait_event(wb_ctl->done_wait, !list_empty(&wb_ctl->done_reqs));
> > +	while (atomic_read(&wb_ctl->num_inflight) ||
> > +	       !list_empty(&wb_ctl->done_reqs)) {
> > +		wait_event_timeout(wb_ctl->done_wait,
> > +				   !list_empty(&wb_ctl->done_reqs),
> > +				   HZ);
> >  		err = zram_complete_done_reqs(zram, wb_ctl);
> >  		if (err)
> >  			ret = err;
> 
> I understand why you used a timeout here, but I still don't think it's a good
> idea since the user could wait for up to a second unnecessarily during the
> race.
> 
> What I prefer is simple and explicit lifetime management for wb_ctl using
> refcount. It directly addresses the core issue (UAF of wb_ctl) in a standard,
> robust way without needing workarounds like timeouts. The runtime overhead
> of kref will be negligible.
> 

The other standard way to deal with lifetime is RCU.
How about this?

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index a324ede6206d..28ab4a24e77f 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -33,6 +33,7 @@
 #include <linux/cpuhotplug.h>
 #include <linux/part_stat.h>
 #include <linux/kernel_read_file.h>
+#include <linux/rcupdate.h>
 
 #include "zram_drv.h"
 
@@ -504,6 +505,7 @@ struct zram_wb_ctl {
 	wait_queue_head_t done_wait;
 	spinlock_t done_lock;
 	atomic_t num_inflight;
+	struct rcu_head rcu;
 };
 
 struct zram_wb_req {
@@ -829,14 +831,8 @@ static void release_wb_req(struct zram_wb_req *req)
 	kfree(req);
 }
 
 static void release_wb_ctl(struct zram_wb_ctl *wb_ctl)
 {
-	if (!wb_ctl)
-		return;
-
 	/* We should never have inflight requests at this point */
 	WARN_ON(atomic_read(&wb_ctl->num_inflight));
 	WARN_ON(!list_empty(&wb_ctl->done_reqs));
@@ -850,7 +849,7 @@ static void release_wb_ctl(struct zram_wb_ctl *wb_ctl)
 		release_wb_req(req);
 	}
 
-	kfree(wb_ctl);
+	kfree_rcu(wb_ctl, rcu);
 }
 
 static struct zram_wb_ctl *init_wb_ctl(struct zram *zram)
@@ -985,6 +997,7 @@ static void zram_writeback_endio(struct bio *bio)
 	struct zram_wb_ctl *wb_ctl = bio->bi_private;
 	unsigned long flags;
 
+	rcu_read_lock();
 	spin_lock_irqsave(&wb_ctl->done_lock, flags);
 	list_add(&req->entry, &wb_ctl->done_reqs);
 	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
@@ -991,5 +1004,6 @@ static void zram_writeback_endio(struct bio *bio)
 
 	wake_up(&wb_ctl->done_wait);
+	rcu_read_unlock();
 }
 
 static void zram_submit_wb_request(struct zram *zram,
@@ -1276,8 +1290,8 @@ static ssize_t writeback_store(struct device *dev,
 
 	wb_ctl = init_wb_ctl(zram);
 	if (!wb_ctl) {
-		ret = -ENOMEM;
-		goto out;
+		release_pp_ctl(zram, pp_ctl);
+		return -ENOMEM;
 	}
 
 	args = skip_spaces(buf);