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?
> 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);
> > }
> >
> > /*
>
