Patch "mm: swap: fix race between free_swap_and_cache() and swapoff()" has been added to the 6.8-stable tree

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This is a note to let you know that I've just added the patch titled

    mm: swap: fix race between free_swap_and_cache() and swapoff()

to the 6.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mm-swap-fix-race-between-free_swap_and_cache-and-swa.patch
and it can be found in the queue-6.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 72f0aee33da6f9326a79a2f5adf8327cc755dc9c
Author: Ryan Roberts <ryan.roberts@xxxxxxx>
Date:   Wed Mar 6 14:03:56 2024 +0000

    mm: swap: fix race between free_swap_and_cache() and swapoff()
    
    [ Upstream commit 82b1c07a0af603e3c47b906c8e991dc96f01688e ]
    
    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 not free.  This isn't present in get_swap_device()
    because it doesn't make sense in general due to the race between getting
    the reference and swapoff.  So I've added an equivalent check directly in
    free_swap_and_cache().
    
    Details of how to provoke one possible issue (thanks to David Hildenbrand
    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<-----
    
    Link: https://lkml.kernel.org/r/20240306140356.3974886-1-ryan.roberts@xxxxxxx
    Fixes: 7c00bafee87c ("mm/swap: free swap slots in batch")
    Closes: https://lore.kernel.org/linux-mm/65a66eb9-41f8-4790-8db2-0c70ea15979f@xxxxxxxxxx/
    Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
    Cc: David Hildenbrand <david@xxxxxxxxxx>
    Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
    Cc: <stable@xxxxxxxxxxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 746aa9da53025..6fe0cc25535f5 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1227,6 +1227,11 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
  * with get_swap_device() and put_swap_device(), unless the swap
  * functions call get/put_swap_device() by themselves.
  *
+ * Note that when only holding the PTL, swapoff might succeed immediately
+ * after freeing a swap entry. Therefore, immediately after
+ * __swap_entry_free(), the swap info might become stale and should not
+ * be touched without a prior get_swap_device().
+ *
  * Check whether swap entry is valid in the swap device.  If so,
  * return pointer to swap_info_struct, and keep the swap entry valid
  * via preventing the swap device from being swapoff, until
@@ -1604,13 +1609,19 @@ 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) {
+		if (WARN_ON(data_race(!p->swap_map[swp_offset(entry)]))) {
+			put_swap_device(p);
+			return 0;
+		}
+
 		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;
 }




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