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@xxxxxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxxxxxx
>>> Reported-by: zhong jiang <zhongjiang@xxxxxxxxxx>
>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>>> ---
>>> 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);
>>> }
>>>
>>> /*
> .
>
