[PATCH] shmem: Pin the file in shmem_fault() if mmap_sem is dropped

[Kirill A. Shutemov]

syzbot found the following crash:

 BUG: KASAN: use-after-free in perf_trace_lock_acquire+0x401/0x530
 include/trace/events/lock.h:13
 Read of size 8 at addr ffff8880a5cf2c50 by task syz-executor.0/26173

 CPU: 0 PID: 26173 Comm: syz-executor.0 Not tainted 5.3.0-rc6 #146
 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
 Google 01/01/2011
 Call Trace:
   __dump_stack lib/dump_stack.c:77 [inline]
   dump_stack+0x172/0x1f0 lib/dump_stack.c:113
   print_address_description.cold+0xd4/0x306 mm/kasan/report.c:351
   __kasan_report.cold+0x1b/0x36 mm/kasan/report.c:482
   kasan_report+0x12/0x17 mm/kasan/common.c:618
   __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
   perf_trace_lock_acquire+0x401/0x530 include/trace/events/lock.h:13
   trace_lock_acquire include/trace/events/lock.h:13 [inline]
   lock_acquire+0x2de/0x410 kernel/locking/lockdep.c:4411
   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
   _raw_spin_lock+0x2f/0x40 kernel/locking/spinlock.c:151
   spin_lock include/linux/spinlock.h:338 [inline]
   shmem_fault+0x5ec/0x7b0 mm/shmem.c:2034
   __do_fault+0x111/0x540 mm/memory.c:3083
   do_shared_fault mm/memory.c:3535 [inline]
   do_fault mm/memory.c:3613 [inline]
   handle_pte_fault mm/memory.c:3840 [inline]
   __handle_mm_fault+0x2adf/0x3f20 mm/memory.c:3964
   handle_mm_fault+0x1b5/0x6b0 mm/memory.c:4001
   do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
   __do_page_fault+0x536/0xdd0 arch/x86/mm/fault.c:1506
   do_page_fault+0x38/0x590 arch/x86/mm/fault.c:1530
   page_fault+0x39/0x40 arch/x86/entry/entry_64.S:1202

It happens if the VMA got unmapped under us while we dropped mmap_sem
and inode got freed.

Pinning the file if we drop mmap_sem fixes the issue.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: syzbot+03ee87124ee05af991bd@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/shmem.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 30ce722c23fa..f672e4145cfd 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2022,16 +2022,14 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
 		    shmem_falloc->waitq &&
 		    vmf->pgoff >= shmem_falloc->start &&
 		    vmf->pgoff < shmem_falloc->next) {
+			struct file *fpin = NULL;
 			wait_queue_head_t *shmem_falloc_waitq;
 			DEFINE_WAIT_FUNC(shmem_fault_wait, synchronous_wake_function);
 
 			ret = VM_FAULT_NOPAGE;
-			if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
-			   !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
-				/* It's polite to up mmap_sem if we can */
-				up_read(&vma->vm_mm->mmap_sem);
+			fpin = maybe_unlock_mmap_for_io(vmf, fpin);
+			if (fpin)
 				ret = VM_FAULT_RETRY;
-			}
 
 			shmem_falloc_waitq = shmem_falloc->waitq;
 			prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
@@ -2049,6 +2047,9 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
 			spin_lock(&inode->i_lock);
 			finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
 			spin_unlock(&inode->i_lock);
+
+			if (fpin)
+				fput(fpin);
 			return ret;
 		}
 		spin_unlock(&inode->i_lock);
-- 
2.21.0

-- 
 Kirill A. Shutemov