From: Domenico Cerasuolo <cerasuolodomenico@gmail.com>

Date: Wed, 3 May 2023 17:12:00 +0200

Subject: [PATCH] mm: fix zswap writeback race condition

References: bsc#1012628

Patch-mainline: 6.3.4

Git-commit: 04fc7816089c5a32c29a04ec94b998e219dfb946

commit 04fc7816089c5a32c29a04ec94b998e219dfb946 upstream.

The zswap writeback mechanism can cause a race condition resulting in

memory corruption, where a swapped out page gets swapped in with data that

was written to a different page.

The race unfolds like this:

1. a page with data A and swap offset X is stored in zswap

2. page A is removed off the LRU by zpool driver for writeback in

   zswap-shrink work, data for A is mapped by zpool driver

3. user space program faults and invalidates page entry A, offset X is

   considered free

4. kswapd stores page B at offset X in zswap (zswap could also be

   full, if so, page B would then be IOed to X, then skip step 5.)

5. entry A is replaced by B in tree->rbroot, this doesn't affect the

   local reference held by zswap-shrink work

6. zswap-shrink work writes back A at X, and frees zswap entry A

7. swapin of slot X brings A in memory instead of B

The fix:

Once the swap page cache has been allocated (case ZSWAP_SWAPCACHE_NEW),

zswap-shrink work just checks that the local zswap_entry reference is

still the same as the one in the tree.  If it's not the same it means that

it's either been invalidated or replaced, in both cases the writeback is

aborted because the local entry contains stale data.

Reproducer:

I originally found this by running `stress` overnight to validate my work

on the zswap writeback mechanism, it manifested after hours on my test

machine.  The key to make it happen is having zswap writebacks, so

whatever setup pumps /sys/kernel/debug/zswap/written_back_pages should do

the trick.

In order to reproduce this faster on a vm, I setup a system with ~100M of

available memory and a 500M swap file, then running `stress --vm 1

--vm-bytes 300000000 --vm-stride 4000` makes it happen in matter of tens

of minutes.  One can speed things up even more by swinging

/sys/module/zswap/parameters/max_pool_percent up and down between, say, 20

and 1; this makes it reproduce in tens of seconds.  It's crucial to set

`--vm-stride` to something other than 4096 otherwise `stress` won't

realize that memory has been corrupted because all pages would have the

same data.

Link: https://lkml.kernel.org/r/20230503151200.19707-1-cerasuolodomenico@gmail.com

Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Reviewed-by: Chris Li (Google) <chrisl@kernel.org>

Cc: Dan Streetman <ddstreet@ieee.org>

Cc: Johannes Weiner <hannes@cmpxchg.org>

Cc: Minchan Kim <minchan@kernel.org>

Cc: Nitin Gupta <ngupta@vflare.org>

Cc: Seth Jennings <sjenning@redhat.com>

Cc: Vitaly Wool <vitaly.wool@konsulko.com>

Cc: <stable@vger.kernel.org>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

---

 mm/zswap.c | 16 ++++++++++++++++

 1 file changed, 16 insertions(+)

diff --git a/mm/zswap.c b/mm/zswap.c

index f6c89049..5d5977c9 100644

--- a/mm/zswap.c

+++ b/mm/zswap.c

@@ -995,6 +995,22 @@ static int zswap_writeback_entry(struct zpool *pool, unsigned long handle)

 		goto fail;

 	case ZSWAP_SWAPCACHE_NEW: /* page is locked */

+		/*

+		 * Having a local reference to the zswap entry doesn't exclude

+		 * swapping from invalidating and recycling the swap slot. Once

+		 * the swapcache is secured against concurrent swapping to and

+		 * from the slot, recheck that the entry is still current before

+		 * writing.

+		 */

+		spin_lock(&tree->lock);

+		if (zswap_rb_search(&tree->rbroot, entry->offset) != entry) {

+			spin_unlock(&tree->lock);

+			delete_from_swap_cache(page_folio(page));

+			ret = -ENOMEM;

+			goto fail;

+		}

+		spin_unlock(&tree->lock);

+

 		/* decompress */

 		acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);

 		dlen = PAGE_SIZE;

-- 

2.35.3