[PATCH v3] ocfs2: fix use-after-free in ocfs2_fault() when VM_FAULT_RETRY

[Tejas Bharambe]

filemap_fault() may drop the mmap_lock before returning VM_FAULT_RETRY,
as documented in mm/filemap.c:

  "If our return value has VM_FAULT_RETRY set, it's because the mmap_lock
  may be dropped before doing I/O or by lock_folio_maybe_drop_mmap()."

When this happens, a concurrent munmap() can call remove_vma() and free
the vm_area_struct via RCU. The saved 'vma' pointer in ocfs2_fault() then
becomes a dangling pointer, and the subsequent trace_ocfs2_fault() call
dereferences it -- a use-after-free.

Fix this by saving the inode reference before calling filemap_fault(),
and removing vma from the trace event. The inode remains valid across
the lock drop since the file is still open, so the trace can fire in
all cases without dereferencing the potentially freed vma.

Reported-by: syzbot+a49010a0e8fcdeea075f@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=a49010a0e8fcdeea075f
Suggested-by: Joseph Qi <joseph.qi@xxxxxxxxxxxxxxxxx>
Signed-off-by: Tejas Bharambe <tejas.bharambe@xxxxxxxxxxx>
---
 fs/ocfs2/mmap.c        |  6 +++---
 fs/ocfs2/ocfs2_trace.h | 10 ++++------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/fs/ocfs2/mmap.c b/fs/ocfs2/mmap.c
index 50e2faf64c..41c08c5a3d 100644
--- a/fs/ocfs2/mmap.c
+++ b/fs/ocfs2/mmap.c
@@ -30,7 +30,7 @@
 
 static vm_fault_t ocfs2_fault(struct vm_fault *vmf)
 {
-	struct vm_area_struct *vma = vmf->vma;
+	struct inode *inode = file_inode(vmf->vma->vm_file);
 	sigset_t oldset;
 	vm_fault_t ret;
 
@@ -38,8 +38,8 @@ static vm_fault_t ocfs2_fault(struct vm_fault *vmf)
 	ret = filemap_fault(vmf);
 	ocfs2_unblock_signals(&oldset);
 
-	trace_ocfs2_fault(OCFS2_I(vma->vm_file->f_mapping->host)->ip_blkno,
-			  vma, vmf->page, vmf->pgoff);
+	trace_ocfs2_fault(OCFS2_I(inode)->ip_blkno,
+			  vmf->page, vmf->pgoff);
 	return ret;
 }
 
diff --git a/fs/ocfs2/ocfs2_trace.h b/fs/ocfs2/ocfs2_trace.h
index 4b32fb5658..6c2c97a980 100644
--- a/fs/ocfs2/ocfs2_trace.h
+++ b/fs/ocfs2/ocfs2_trace.h
@@ -1246,22 +1246,20 @@ TRACE_EVENT(ocfs2_write_end_inline,
 
 TRACE_EVENT(ocfs2_fault,
 	TP_PROTO(unsigned long long ino,
-		 void *area, void *page, unsigned long pgoff),
-	TP_ARGS(ino, area, page, pgoff),
+		 void *page, unsigned long pgoff),
+	TP_ARGS(ino, page, pgoff),
 	TP_STRUCT__entry(
 		__field(unsigned long long, ino)
-		__field(void *, area)
 		__field(void *, page)
 		__field(unsigned long, pgoff)
 	),
 	TP_fast_assign(
 		__entry->ino = ino;
-		__entry->area = area;
 		__entry->page = page;
 		__entry->pgoff = pgoff;
 	),
-	TP_printk("%llu %p %p %lu",
-		  __entry->ino, __entry->area, __entry->page, __entry->pgoff)
+	TP_printk("%llu %p %lu",
+		  __entry->ino, __entry->page, __entry->pgoff)
 );
 
 /* End of trace events for fs/ocfs2/mmap.c. */
-- 
2.53.0