[PATCH v6] ocfs2: fix use-after-free in ocfs2_fault() when VM_FAULT_RETRY

[Tejas Bharambe <thbharam@gmail.com>]

From: Tejas Bharambe <tejas.bharambe@outlook.com>

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 ip_blkno as a plain integer before calling
filemap_fault(), and removing vma from the trace event. Since
ip_blkno is copied by value before the lock can be dropped, it
remains valid regardless of what happens to the vma or inode
afterward.

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

diff --git a/fs/ocfs2/mmap.c b/fs/ocfs2/mmap.c
index 50e2faf64c..6c570157ca 100644
--- a/fs/ocfs2/mmap.c
+++ b/fs/ocfs2/mmap.c
@@ -30,7 +30,8 @@
 
 static vm_fault_t ocfs2_fault(struct vm_fault *vmf)
 {
-	struct vm_area_struct *vma = vmf->vma;
+	unsigned long long ip_blkno =
+		OCFS2_I(file_inode(vmf->vma->vm_file))->ip_blkno;
 	sigset_t oldset;
 	vm_fault_t ret;
 
@@ -38,11 +39,9 @@ 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(ip_blkno, vmf->page, vmf->pgoff);
 	return ret;
 }
-
 static vm_fault_t __ocfs2_page_mkwrite(struct file *file,
 			struct buffer_head *di_bh, struct folio *folio)
 {
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