Re: [PATCH] mm: drop mmap_sem before calling balance_dirty_pages() in write fault

[Johannes Weiner]

On Thu, Sep 26, 2019 at 04:49:23PM +0300, Kirill A. Shutemov wrote:
> On Tue, Sep 24, 2019 at 05:43:37PM -0400, Johannes Weiner wrote:
> > On Tue, Sep 24, 2019 at 01:46:08PM -0700, Matthew Wilcox wrote:
> > > On Tue, Sep 24, 2019 at 03:42:38PM -0400, Johannes Weiner wrote:
> > > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > 
> > > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Thanks!

> From bdf96fe9e3c1a319e9fd131efbe0118ea41a41b1 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Date: Thu, 26 Sep 2019 16:34:26 +0300
> Subject: [PATCH] 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
>  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>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

I have just one nitpick:

> @@ -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;

That initialization seems unnecessary, as the fpin assignment below is
unconditional in the variable's scope.

The second argument to maybe_unlock_mmap_for_io() for tracking state
when the function is called multiple times in the filemap fault goto
maze, we shouldn't need that here for a simple invocation.

>  			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);

I.e. this:

			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_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);