Messages in this thread | | | Subject | Re: [PATCH] shmem: fix possible deadlocks on shmlock_user_lock | From | Yang Shi <> | Date | Thu, 16 Apr 2020 20:00:05 -0700 |
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On 4/16/20 5:11 PM, Hugh Dickins wrote: > Recent commit 71725ed10c40 ("mm: huge tmpfs: try to split_huge_page() > when punching hole") has allowed syzkaller to probe deeper, uncovering > a long-standing lockdep issue between the irq-unsafe shmlock_user_lock, > the irq-safe xa_lock on mapping->i_pages, and shmem inode's info->lock > which nests inside xa_lock (or tree_lock) since 4.8's shmem_uncharge(). > > user_shm_lock(), servicing SysV shmctl(SHM_LOCK), wants shmlock_user_lock > while its caller shmem_lock() holds info->lock with interrupts disabled; > but hugetlbfs_file_setup() calls user_shm_lock() with interrupts enabled, > and might be interrupted by a writeback endio wanting xa_lock on i_pages. > This may not risk an actual deadlock, since shmem inodes do not take part > in writeback accounting, but there are several easy ways to avoid it. > > Requiring interrupts disabled for shmlock_user_lock would be easy, > but it's a high-level global lock for which that seems inappropriate. > Instead, recall that the use of info->lock to guard info->flags in > shmem_lock() dates from pre-3.1 days, when races with SHMEM_PAGEIN and > SHMEM_TRUNCATE could occur: nowadays it serves no purpose, the only flag > added or removed is VM_LOCKED itself, and calls to shmem_lock() an inode > are already serialized by the caller. Take info->lock out of the chain > and the possibility of deadlock or lockdep warning goes away. > > Reported-by: syzbot+c8a8197c8852f566b9d9@syzkaller.appspotmail.com > Link: https://lore.kernel.org/lkml/000000000000e5838c05a3152f53@google.com/ > Reported-by: syzbot+40b71e145e73f78f81ad@syzkaller.appspotmail.com > Link: https://lore.kernel.org/lkml/0000000000003712b305a331d3b1@google.com/ > Fixes: 4595ef88d136 ("shmem: make shmem_inode_info::lock irq-safe") > Signed-off-by: Hugh Dickins <hughd@google.com> > Cc: Yang Shi <yang.shi@linux.alibaba.com> > --- > > mm/shmem.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-)
Acked-by: Yang Shi <yang.shi@linux.alibaba.com>
> > --- 5.7-rc1/mm/shmem.c 2020-04-11 12:58:26.415524805 -0700 > +++ linux/mm/shmem.c 2020-04-16 11:04:06.729738730 -0700 > @@ -2179,7 +2179,11 @@ int shmem_lock(struct file *file, int lo > struct shmem_inode_info *info = SHMEM_I(inode); > int retval = -ENOMEM; > > - spin_lock_irq(&info->lock); > + /* > + * What serializes the accesses to info->flags? > + * ipc_lock_object() when called from shmctl_do_lock(), > + * no serialization needed when called from shm_destroy(). > + */ > if (lock && !(info->flags & VM_LOCKED)) { > if (!user_shm_lock(inode->i_size, user)) > goto out_nomem; > @@ -2194,7 +2198,6 @@ int shmem_lock(struct file *file, int lo > retval = 0; > > out_nomem: > - spin_unlock_irq(&info->lock); > return retval; > } >
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