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* [patch 025/158] shmem: pin the file in shmem_fault() if mmap_sem is dropped
@ 2019-12-01  1:50 akpm
  0 siblings, 0 replies; only message in thread
From: akpm @ 2019-12-01  1:50 UTC (permalink / raw)
  To: akpm, hannes, hdanton, hughd, josef, kirill.shutemov, linux-mm,
	mm-commits, torvalds, willy

From: "Kirill A. Shutemov" <>
Subject: shmem: pin the file in shmem_fault() if mmap_sem is dropped

syzbot found the following crash:

 BUG: KASAN: use-after-free in perf_trace_lock_acquire+0x401/0x530
 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 <>
Acked-by: Johannes Weiner <>
Reviewed-by: Matthew Wilcox (Oracle) <>
Cc: Hillf Danton <>
Cc: Hugh Dickins <>
Cc: Josef Bacik <>
Signed-off-by: Andrew Morton <>

 mm/shmem.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/mm/shmem.c~shmem-pin-the-file-in-shmem_fault-if-mmap_sem-is-dropped
+++ a/mm/shmem.c
@@ -2022,16 +2022,14 @@ static vm_fault_t shmem_fault(struct vm_
 		    shmem_falloc->waitq &&
 		    vmf->pgoff >= shmem_falloc->start &&
 		    vmf->pgoff < shmem_falloc->next) {
+			struct file *fpin;
 			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, NULL);
+			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_
 			finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
+			if (fpin)
+				fput(fpin);
 			return ret;

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2019-12-01  1:50 [patch 025/158] shmem: pin the file in shmem_fault() if mmap_sem is dropped akpm

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