Re: [PATCH] shmem: fix possible deadlocks on shmlock_user_lock

[Yang Shi]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/lkml/000000000000e5838c05a3152f53@xxxxxxxxxx/
> Reported-by: syzbot+40b71e145e73f78f81ad@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/lkml/0000000000003712b305a331d3b1@xxxxxxxxxx/
> Fixes: 4595ef88d136 ("shmem: make shmem_inode_info::lock irq-safe")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
>
>   mm/shmem.c |    7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)

Acked-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

> --- 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;
>   }
>