nilfs2: do not force clear folio if buffer is referenced

Patch series "nilfs2: protect busy buffer heads from being force-cleared".

This series fixes the buffer head state inconsistency issues reported by
syzbot that occurs when the filesystem is corrupted and falls back to
read-only, and the associated buffer head use-after-free issue.

This patch (of 2):

Syzbot has reported that after nilfs2 detects filesystem corruption and
falls back to read-only, inconsistencies in the buffer state may occur.

One of the inconsistencies is that when nilfs2 calls mark_buffer_dirty()
to set a data or metadata buffer as dirty, but it detects that the buffer
is not in the uptodate state:

 WARNING: CPU: 0 PID: 6049 at fs/buffer.c:1177 mark_buffer_dirty+0x2e5/0x520
  fs/buffer.c:1177
 ...
 Call Trace:
  <TASK>
  nilfs_palloc_commit_alloc_entry+0x4b/0x160 fs/nilfs2/alloc.c:598
  nilfs_ifile_create_inode+0x1dd/0x3a0 fs/nilfs2/ifile.c:73
  nilfs_new_inode+0x254/0x830 fs/nilfs2/inode.c:344
  nilfs_mkdir+0x10d/0x340 fs/nilfs2/namei.c:218
  vfs_mkdir+0x2f9/0x4f0 fs/namei.c:4257
  do_mkdirat+0x264/0x3a0 fs/namei.c:4280
  __do_sys_mkdirat fs/namei.c:4295 [inline]
  __se_sys_mkdirat fs/namei.c:4293 [inline]
  __x64_sys_mkdirat+0x87/0xa0 fs/namei.c:4293
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
  entry_SYSCALL_64_after_hwframe+0x77/0x7f

The other is when nilfs_btree_propagate(), which propagates the dirty
state to the ancestor nodes of a b-tree that point to a dirty buffer,
detects that the origin buffer is not dirty, even though it should be:

 WARNING: CPU: 0 PID: 5245 at fs/nilfs2/btree.c:2089
  nilfs_btree_propagate+0xc79/0xdf0 fs/nilfs2/btree.c:2089
 ...
 Call Trace:
  <TASK>
  nilfs_bmap_propagate+0x75/0x120 fs/nilfs2/bmap.c:345
  nilfs_collect_file_data+0x4d/0xd0 fs/nilfs2/segment.c:587
  nilfs_segctor_apply_buffers+0x184/0x340 fs/nilfs2/segment.c:1006
  nilfs_segctor_scan_file+0x28c/0xa50 fs/nilfs2/segment.c:1045
  nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1216 [inline]
  nilfs_segctor_collect fs/nilfs2/segment.c:1540 [inline]
  nilfs_segctor_do_construct+0x1c28/0x6b90 fs/nilfs2/segment.c:2115
  nilfs_segctor_construct+0x181/0x6b0 fs/nilfs2/segment.c:2479
  nilfs_segctor_thread_construct fs/nilfs2/segment.c:2587 [inline]
  nilfs_segctor_thread+0x69e/0xe80 fs/nilfs2/segment.c:2701
  kthread+0x2f0/0x390 kernel/kthread.c:389
  ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
  </TASK>

Both of these issues are caused by the callbacks that handle the
page/folio write requests, forcibly clear various states, including the
working state of the buffers they hold, at unexpected times when they
detect read-only fallback.

Fix these issues by checking if the buffer is referenced before clearing
the page/folio state, and skipping the clear if it is.

Link: https://lkml.kernel.org/r/20250107200202.6432-1-konishi.ryusuke@gmail.com
Link: https://lkml.kernel.org/r/20250107200202.6432-2-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+b2b14916b77acf8626d7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b2b14916b77acf8626d7
Reported-by: syzbot+d98fd19acd08b36ff422@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=d98fd19acd08b36ff422
Fixes: 8c26c4e2694a ("nilfs2: fix issue with flush kernel thread after remount in RO mode because of driver's internal error or metadata corruption")
Tested-by: syzbot+b2b14916b77acf8626d7@syzkaller.appspotmail.com
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/nilfs2/page.c b/fs/nilfs2/page.c
index 9de2a494a069426a1a9bd8e38fbf7483483ce435..899686d2e5f71df06c2913dd91d7fe4a197394bc 100644 (file)
--- a/fs/nilfs2/page.c
+++ b/fs/nilfs2/page.c
@@ -392,6 +392,11 @@ void nilfs_clear_dirty_pages(struct address_space *mapping)
 /**
  * nilfs_clear_folio_dirty - discard dirty folio
  * @folio: dirty folio that will be discarded
+ *
+ * nilfs_clear_folio_dirty() clears working states including dirty state for
+ * the folio and its buffers.  If the folio has buffers, clear only if it is
+ * confirmed that none of the buffer heads are busy (none have valid
+ * references and none are locked).
  */
 void nilfs_clear_folio_dirty(struct folio *folio)
 {
@@ -399,10 +404,6 @@ void nilfs_clear_folio_dirty(struct folio *folio)
 
        BUG_ON(!folio_test_locked(folio));
 
-       folio_clear_uptodate(folio);
-       folio_clear_mappedtodisk(folio);
-       folio_clear_checked(folio);
-
        head = folio_buffers(folio);
        if (head) {
                const unsigned long clear_bits =
@@ -410,6 +411,25 @@ void nilfs_clear_folio_dirty(struct folio *folio)
                         BIT(BH_Async_Write) | BIT(BH_NILFS_Volatile) |
                         BIT(BH_NILFS_Checked) | BIT(BH_NILFS_Redirected) |
                         BIT(BH_Delay));
+               bool busy, invalidated = false;
+
+recheck_buffers:
+               busy = false;
+               bh = head;
+               do {
+                       if (atomic_read(&bh->b_count) | buffer_locked(bh)) {
+                               busy = true;
+                               break;
+                       }
+               } while (bh = bh->b_this_page, bh != head);
+
+               if (busy) {
+                       if (invalidated)
+                               return;
+                       invalidate_bh_lrus();
+                       invalidated = true;
+                       goto recheck_buffers;
+               }
 
                bh = head;
                do {
@@ -419,6 +439,9 @@ void nilfs_clear_folio_dirty(struct folio *folio)
                } while (bh = bh->b_this_page, bh != head);
        }
 
+       folio_clear_uptodate(folio);
+       folio_clear_mappedtodisk(folio);
+       folio_clear_checked(folio);
        __nilfs_clear_folio_dirty(folio);
 }