Re: scheduling while atomic in z3fold

[Vitaly Wool <vitaly.wool@konsulko.com>]

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On 2020-12-06 10:18, Mike Galbraith wrote:

> On Thu, 2020-12-03 at 14:39 +0100, Sebastian Andrzej Siewior wrote:
>> On 2020-12-03 09:18:21 [+0100], Mike Galbraith wrote:
>>> On Thu, 2020-12-03 at 03:16 +0100, Mike Galbraith wrote:
>>>> On Wed, 2020-12-02 at 23:08 +0100, Sebastian Andrzej Siewior wrote: 
>>>> Looks like... d8f117abb380 z3fold: fix use-after-free when freeing 
>>>> handles ...wasn't completely effective...
>>> The top two hunks seem to have rendered the thing RT tolerant.
>> Yes, it appears to. I have no idea if this is a proper fix or not. 
>> Without your write lock, after a few attempts, KASAN says: | BUG: 
>> KASAN: use-after-free in __pv_queued_spin_lock_slowpath+0x293/0x770 | 
>> Write of size 2 at addr ffff88800e0e10aa by task kworker/u16:3/237
> Things that make ya go hmmm... I started poking at it thinking ok, 
> given write_lock() fixes it, bad juju must be happening under another 
> read_lock(), so I poisoned the structure about to be freed by 
> __release_z3fold_page() under lock, and opened a delay window for bad 
> juju to materialize, but it didn't, so I just poisoned instead, and 
> sprinkled landmines all over the place. My landmines are not 
> triggering but the detonator is materializing inside the structure 
> containing the lock we explode on. Somebody blasts a recycled 
> z3fold_buddy_slots into ram we were working on?

Could you please try the following patch in your setup:

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 18feaa0bc537..efe9a012643d 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -544,12 +544,17 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
  			break;
  		}
  	}
-	if (!is_free)
+	if (!is_free) {
  		set_bit(HANDLES_ORPHANED, &zhdr->slots->pool);
-	read_unlock(&zhdr->slots->lock);
-
-	if (is_free)
+		read_unlock(&zhdr->slots->lock);
+	} else {
+		zhdr->slots->slot[0] =
+			zhdr->slots->slot[1] =
+			zhdr->slots->slot[2] =
+			zhdr->slots->slot[3] = 0xdeadbeef;
+		read_unlock(&zhdr->slots->lock);
  		kmem_cache_free(pool->c_handle, zhdr->slots);
+	}
  
  	if (locked)
  		z3fold_page_unlock(zhdr);


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