Messages in this thread Patch in this message |  | | Date | Thu, 7 May 2026 16:38:57 -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 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);
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