Re: [PATCH 4.19] memfd: Fix locking when tagging pins

[zhong jiang <zhongjiang@huawei.com>]

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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().

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);
>  }
>  
>  /*


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