On 2019/11/14 10:26, Matthew Wilcox wrote:

On Wed, Nov 13, 2019 at 12:00:22PM +0800, zhong jiang wrote:

...

Hi, Matthew

When appling the following patch, I hit the an warning.

*WARNING: suspicious RCU usage in memfd_wait_for_pins*

It is because we remove the rcu_read_lock/read_read_unlock. but We still use radix_tree_deref_slot checking rcu_read_lock_held().

Ooh. We should be using radix_tree_deref_slot_protected() instead. Can I trouble you to send that patch?

Will do . Thanks,

Sincerely, zhong jiang

...

A simple fix as follows without test.

diff --git a/mm/memfd.c b/mm/memfd.c index 5859705dafe1..af244c9c8b6f 100644 --- a/mm/memfd.c +++ b/mm/memfd.c @@ -40,6 +40,7 @@ static void memfd_tag_pins(struct address_space *mapping) start = 0;

    xa_lock_irq(&mapping->i_pages);

•   rcu_read_lock();
    radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) {
            page = radix_tree_deref_slot(slot);
            if (!page || radix_tree_exception(page)) {
  

@@ -60,6 +61,7 @@ static void memfd_tag_pins(struct address_space *mapping) cond_resched(); xa_lock_irq(&mapping->i_pages); }

•   rcu_read_unlock();
    xa_unlock_irq(&mapping->i_pages);
  

}

Thanks, zhong jiang

On 2019/10/26 0:58, Matthew Wilcox wrote:

...

From: "Matthew Wilcox (Oracle)" willy@infradead.org

The RCU lock is insufficient to protect the radix tree iteration as a deletion from the tree can occur before we take the spinlock to tag the entry. In 4.19, this has manifested as a bug with the following trace:

kernel BUG at lib/radix-tree.c:1429! invalid opcode: 0000 [#1] SMP KASAN PTI CPU: 7 PID: 6935 Comm: syz-executor.2 Not tainted 4.19.36 #25 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014 RIP: 0010:radix_tree_tag_set+0x200/0x2f0 lib/radix-tree.c:1429 Code: 00 00 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 89 44 24 10 e8 a3 29 7e fe 48 8b 44 24 10 48 0f ab 03 e9 d2 fe ff ff e8 90 29 7e fe <0f> 0b 48 c7 c7 e0 5a 87 84 e8 f0 e7 08 ff 4c 89 ef e8 4a ff ac fe RSP: 0018:ffff88837b13fb60 EFLAGS: 00010016 RAX: 0000000000040000 RBX: ffff8883c5515d58 RCX: ffffffff82cb2ef0 RDX: 0000000000000b72 RSI: ffffc90004cf2000 RDI: ffff8883c5515d98 RBP: ffff88837b13fb98 R08: ffffed106f627f7e R09: ffffed106f627f7e R10: 0000000000000001 R11: ffffed106f627f7d R12: 0000000000000004 R13: ffffea000d7fea80 R14: 1ffff1106f627f6f R15: 0000000000000002 FS: 00007fa1b8df2700(0000) GS:ffff8883e2fc0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fa1b8df1db8 CR3: 000000037d4d2001 CR4: 0000000000160ee0 Call Trace: memfd_tag_pins mm/memfd.c:51 [inline] memfd_wait_for_pins+0x2c5/0x12d0 mm/memfd.c:81 memfd_add_seals mm/memfd.c:215 [inline] memfd_fcntl+0x33d/0x4a0 mm/memfd.c:247 do_fcntl+0x589/0xeb0 fs/fcntl.c:421 __do_sys_fcntl fs/fcntl.c:463 [inline] __se_sys_fcntl fs/fcntl.c:448 [inline] __x64_sys_fcntl+0x12d/0x180 fs/fcntl.c:448 do_syscall_64+0xc8/0x580 arch/x86/entry/common.c:293

The problem does not occur in mainline due to the XArray rewrite which changed the locking to exclude modification of the tree during iteration. At the time, nobody realised this was a bugfix. Backport the locking changes to stable.

Cc: stable@vger.kernel.org Reported-by: zhong jiang zhongjiang@huawei.com Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org

---

mm/memfd.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/memfd.c b/mm/memfd.c index 2bb5e257080e..5859705dafe1 100644 --- a/mm/memfd.c +++ b/mm/memfd.c @@ -34,11 +34,12 @@ static void memfd_tag_pins(struct address_space *mapping) void __rcu **slot; pgoff_t start; struct page *page;

• unsigned int tagged = 0;

lru_add_drain(); start = 0;

• rcu_read_lock();

• xa_lock_irq(&mapping->i_pages); radix_tree_for_each_slot(slot, &mapping->i_pages, &iter, start) { page = radix_tree_deref_slot(slot); if (!page || radix_tree_exception(page)) {

@@ -47,18 +48,19 @@ static void memfd_tag_pins(struct address_space *mapping) continue; } } else if (page_count(page) - page_mapcount(page) > 1) {

• 	xa_lock_irq(&mapping->i_pages);
  radix_tree_tag_set(&mapping->i_pages, iter.index,
  		   MEMFD_TAG_PINNED);
  

• 	xa_unlock_irq(&mapping->i_pages);
  

  }

• if (need_resched()) {
  

• 	slot = radix_tree_iter_resume(slot, &iter);
  

• 	cond_resched_rcu();
  

• }
  

• if (++tagged % 1024)
  

• 	continue;
  

• slot = radix_tree_iter_resume(slot, &iter);
  

• xa_unlock_irq(&mapping->i_pages);
  

• cond_resched();
  

• xa_lock_irq(&mapping->i_pages);
  

  }

• rcu_read_unlock();

• xa_unlock_irq(&mapping->i_pages);

} /*

.