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SubjectRe: [PATCH] mm: drop mmap_sem before calling balance_dirty_pages() in write fault
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@cmpxchg.org>
> > >
> > > Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Thanks!

> From bdf96fe9e3c1a319e9fd131efbe0118ea41a41b1 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 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@linux.intel.com>
> Reported-by: syzbot+03ee87124ee05af991bd@syzkaller.appspotmail.com
> Cc: Hillf Danton <hdanton@sina.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Hugh Dickins <hughd@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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);

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