[PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff()

[Ryan Roberts <ryan.roberts@arm.com>]

There was previously a theoretical window where swapoff() could run and
teardown a swap_info_struct while a call to free_swap_and_cache() was
running in another thread. This could cause, amongst other bad
possibilities, swap_page_trans_huge_swapped() (called by
free_swap_and_cache()) to access the freed memory for swap_map.

This is a theoretical problem and I haven't been able to provoke it from
a test case. But there has been agreement based on code review that this
is possible (see link below).

Fix it by using get_swap_device()/put_swap_device(), which will stall
swapoff(). There was an extra check in _swap_info_get() to confirm that
the swap entry was valid. This wasn't present in get_swap_device() so
I've added it. I couldn't find any existing get_swap_device() call sites
where this extra check would cause any false alarms.

Details of how to provoke one possible issue (thanks to David Hilenbrand
for deriving this):

--8<-----

__swap_entry_free() might be the last user and result in
"count == SWAP_HAS_CACHE".

swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.

So the question is: could someone reclaim the folio and turn
si->inuse_pages==0, before we completed swap_page_trans_huge_swapped().

Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are
still references by swap entries.

Process 1 still references subpage 0 via swap entry.
Process 2 still references subpage 1 via swap entry.

Process 1 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE
[then, preempted in the hypervisor etc.]

Process 2 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE

Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls
__try_to_reclaim_swap().

__try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->
put_swap_folio()->free_swap_slot()->swapcache_free_entries()->
swap_entry_free()->swap_range_free()->
...
WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);

What stops swapoff to succeed after process 2 reclaimed the swap cache
but before process1 finished its call to swap_page_trans_huge_swapped()?

--8<-----

Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch")
Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@redhat.com/
Cc: stable@vger.kernel.org
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

Applies on top of v6.8-rc6 and mm-unstable (b38c34939fe4).

Thanks,
Ryan

 mm/swapfile.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2b3a2d85e350..f580e6abc674 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1281,7 +1281,9 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
 	smp_rmb();
 	offset = swp_offset(entry);
 	if (offset >= si->max)
-		goto put_out;
+		goto bad_offset;
+	if (data_race(!si->swap_map[swp_offset(entry)]))
+		goto bad_free;

 	return si;
 bad_nofile:
@@ -1289,9 +1291,14 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
 out:
 	return NULL;
 put_out:
-	pr_err("%s: %s%08lx\n", __func__, Bad_offset, entry.val);
 	percpu_ref_put(&si->users);
 	return NULL;
+bad_offset:
+	pr_err("%s: %s%08lx\n", __func__, Bad_offset, entry.val);
+	goto put_out;
+bad_free:
+	pr_err("%s: %s%08lx\n", __func__, Unused_offset, entry.val);
+	goto put_out;
 }

 static unsigned char __swap_entry_free(struct swap_info_struct *p,
@@ -1609,13 +1616,14 @@ int free_swap_and_cache(swp_entry_t entry)
 	if (non_swap_entry(entry))
 		return 1;

-	p = _swap_info_get(entry);
+	p = get_swap_device(entry);
 	if (p) {
 		count = __swap_entry_free(p, entry);
 		if (count == SWAP_HAS_CACHE &&
 		    !swap_page_trans_huge_swapped(p, entry))
 			__try_to_reclaim_swap(p, swp_offset(entry),
 					      TTRS_UNMAPPED | TTRS_FULL);
+		put_swap_device(p);
 	}
 	return p != NULL;
 }
--
2.25.1