Re: KASAN: use-after-free Read in shmem_fault (2)

[Kirill A. Shutemov]

On Mon, Sep 09, 2019 at 06:04:12PM +0300, Kirill A. Shutemov wrote:
> On Mon, Sep 09, 2019 at 06:55:21AM -0700, Matthew Wilcox wrote:
> > On Mon, Sep 02, 2019 at 05:20:30PM +0300, Kirill A. Shutemov wrote:
> > > On Mon, Sep 02, 2019 at 06:52:54AM -0700, Matthew Wilcox wrote:
> > > > On Sat, Aug 31, 2019 at 12:58:26PM +0800, Hillf Danton wrote:
> > > > > On Fri, 30 Aug 2019 12:40:06 -0700
> > > > > > syzbot found the following crash on:
> > > > > > 
> > > > > > HEAD commit:    a55aa89a Linux 5.3-rc6
> > > > > > git tree:       upstream
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=12f4beb6600000
> > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=2a6a2b9826fdadf9
> > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=03ee87124ee05af991bd
> > > > > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > > > > 
> > > > > > ==================================================================
> > > > > > 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
> > > > > 
> > > > > --- a/mm/shmem.c
> > > > > +++ b/mm/shmem.c
> > > > > @@ -2021,6 +2021,12 @@ static vm_fault_t shmem_fault(struct vm_
> > > > >  			shmem_falloc_waitq = shmem_falloc->waitq;
> > > > >  			prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
> > > > >  					TASK_UNINTERRUPTIBLE);
> > > > > +			/*
> > > > > +			 * it is not trivial to see what will take place after
> > > > > +			 * releasing i_lock and taking a nap, so hold inode to
> > > > > +			 * be on the safe side.
> > > > 
> > > > I think the comment could be improved.  How about:
> > > > 
> > > > 			 * The file could be unmapped by another thread after
> > > > 			 * releasing i_lock, and the inode then freed.  Hold
> > > > 			 * a reference to the inode to prevent this.
> > > 
> > > It only can happen if mmap_sem was released, so it's better to put
> > > __iget() to the branch above next to up_read(). I've got confused at first
> > > how it is possible from ->fault().
> > > 
> > > This way iput() below should only be called for ret == VM_FAULT_RETRY.
> > 
> > Looking at the rather similar construct in filemap.c, should we solve
> > it the same way, where we inc the refcount on the struct file instead
> > of the inode before releasing the mmap_sem?
> 
> Are you talking about maybe_unlock_mmap_for_io()? Yeah, worth moving it to
> mm/internal.h and reuse.
> 
> Care to prepare the patch? :P

Something like this? Untested.

diff --git a/mm/filemap.c b/mm/filemap.c
index d0cf700bf201..a542f72f57cc 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2349,26 +2349,6 @@ EXPORT_SYMBOL(generic_file_read_iter);
 
 #ifdef CONFIG_MMU
 #define MMAP_LOTSAMISS  (100)
-static struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
-					     struct file *fpin)
-{
-	int flags = vmf->flags;
-
-	if (fpin)
-		return fpin;
-
-	/*
-	 * FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or
-	 * anything, so we only pin the file and drop the mmap_sem if only
-	 * FAULT_FLAG_ALLOW_RETRY is set.
-	 */
-	if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) ==
-	    FAULT_FLAG_ALLOW_RETRY) {
-		fpin = get_file(vmf->vma->vm_file);
-		up_read(&vmf->vma->vm_mm->mmap_sem);
-	}
-	return fpin;
-}
 
 /*
  * lock_page_maybe_drop_mmap - lock the page, possibly dropping the mmap_sem
diff --git a/mm/internal.h b/mm/internal.h
index e32390802fd3..75ffa646de82 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -362,6 +362,27 @@ vma_address(struct page *page, struct vm_area_struct *vma)
 	return max(start, vma->vm_start);
 }
 
+static inline struct file *maybe_unlock_mmap_for_io(struct vm_fault *vmf,
+					     struct file *fpin)
+{
+	int flags = vmf->flags;
+
+	if (fpin)
+		return fpin;
+
+	/*
+	 * FAULT_FLAG_RETRY_NOWAIT means we don't want to wait on page locks or
+	 * anything, so we only pin the file and drop the mmap_sem if only
+	 * FAULT_FLAG_ALLOW_RETRY is set.
+	 */
+	if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) ==
+	    FAULT_FLAG_ALLOW_RETRY) {
+		fpin = get_file(vmf->vma->vm_file);
+		up_read(&vmf->vma->vm_mm->mmap_sem);
+	}
+	return fpin;
+}
+
 #else /* !CONFIG_MMU */
 static inline void clear_page_mlock(struct page *page) { }
 static inline void mlock_vma_page(struct page *page) { }
diff --git a/mm/shmem.c b/mm/shmem.c
index 2bed4761f279..551fa49eb7f6 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2007,16 +2007,14 @@ 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,
@@ -2034,6 +2032,9 @@ 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);
-- 
 Kirill A. Shutemov