Messages in this thread Patch in this message |  | | Date | Thu, 7 May 2026 15:56:52 -0700 | | From | Minchan Kim <> | | Subject | Re: [PATCH] zram: fix use-after-free in zram_writeback_endio |
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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.
Something like 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/kref.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 kref kref; }; struct zram_wb_req { @@ -829,11 +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) +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)); @@ -850,6 +849,18 @@ static void release_wb_ctl(struct zram_wb_ctl *wb_ctl) kfree(wb_ctl); } +static void release_wb_ctl_kref(struct kref *kref) +{ + struct zram_wb_ctl *wb_ctl = container_of(kref, struct zram_wb_ctl, kref); + + __release_wb_ctl(wb_ctl); +} + +static void release_wb_ctl(struct zram_wb_ctl *wb_ctl) +{ + kref_put(&wb_ctl->kref, release_wb_ctl_kref); +} + static struct zram_wb_ctl *init_wb_ctl(struct zram *zram) { struct zram_wb_ctl *wb_ctl; @@ -864,6 +875,7 @@ static struct zram_wb_ctl *init_wb_ctl(struct zram *zram) atomic_set(&wb_ctl->num_inflight, 0); init_waitqueue_head(&wb_ctl->done_wait); spin_lock_init(&wb_ctl->done_lock); + kref_init(&wb_ctl->kref); for (i = 0; i < zram->wb_batch_size; i++) { struct zram_wb_req *req; @@ -985,6 +997,7 @@ static void zram_writeback_endio(struct bio *bio) spin_unlock_irqrestore(&wb_ctl->done_lock, flags); wake_up(&wb_ctl->done_wait); + kref_put(&wb_ctl->kref, release_wb_ctl_kref); } static void zram_submit_wb_request(struct zram *zram, @@ -996,6 +1009,7 @@ static void zram_submit_wb_request(struct zram *zram, * so that we don't over-submit. */ zram_account_writeback_submit(zram); + kref_get(&wb_ctl->kref); atomic_inc(&wb_ctl->num_inflight); req->bio.bi_private = wb_ctl; submit_bio(&req->bio); @@ -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);
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