From: Hugh Dickins hughd@google.com
[ Upstream commit ea0dfeb4209b4eab954d6e00ed136bc6b48b380d ]
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.
Fixes: 4595ef88d136 ("shmem: make shmem_inode_info::lock irq-safe") Reported-by: syzbot+c8a8197c8852f566b9d9@syzkaller.appspotmail.com Reported-by: syzbot+40b71e145e73f78f81ad@syzkaller.appspotmail.com Signed-off-by: Hugh Dickins hughd@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Acked-by: Yang Shi yang.shi@linux.alibaba.com Cc: Yang Shi yang.shi@linux.alibaba.com Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2004161707410.16322@eggly.anvils Link: https://lore.kernel.org/lkml/000000000000e5838c05a3152f53@google.com/ Link: https://lore.kernel.org/lkml/0000000000003712b305a331d3b1@google.com/ Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- mm/shmem.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c index e71b15da19854..98802ca76a5c3 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2183,7 +2183,11 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user) 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; @@ -2198,7 +2202,6 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user) retval = 0;
out_nomem: - spin_unlock_irq(&info->lock); return retval; }