Re: [PATCH v2] mm/swap: fix swap_info_struct race between swapoff and get_swap_pages()

[Aaron Lu]

On Tue, Apr 04, 2023 at 11:47:16PM +0800, Rongwei Wang wrote:
> The si->lock must be held when deleting the si from
> the available list.  Otherwise, another thread can
> re-add the si to the available list, which can lead
> to memory corruption. The only place we have found
> where this happens is in the swapoff path. This case
> can be described as below:
> 
> core 0                       core 1
> swapoff
> 
> del_from_avail_list(si)      waiting
> 
> try lock si->lock            acquire swap_avail_lock
>                              and re-add si into
>                              swap_avail_head

                               confused here.

If del_from_avail_list(si) finished in swaoff path, then this si should
not exist in any of the per-node avail list and core 1 should not be
able to re-add it.

I stared at the code for a while and couldn't figure out how this
happened, will continue to look at this tomorrow.

Thanks,
Aaron

> 
> acquire si->lock but
> missing si already be
> added again, and continuing
> to clear SWP_WRITEOK, etc.
> 
> It can be easily found a massive warning messages can
> be triggered inside get_swap_pages() by some special
> cases, for example, we call madvise(MADV_PAGEOUT) on
> blocks of touched memory concurrently, meanwhile, run
> much swapon-swapoff operations (e.g. stress-ng-swap).
> 
> However, in the worst case, panic can be caused by the
> above scene. In swapoff(), the memory used by si could
> be kept in swap_info[] after turning off a swap. This
> means memory corruption will not be caused immediately
> until allocated and reset for a new swap in the swapon
> path. A panic message caused:
> (with CONFIG_PLIST_DEBUG enabled)
> 
> ------------[ cut here ]------------
> top: 00000000e58a3003, n: 0000000013e75cda, p: 000000008cd4451a
> prev: 0000000035b1e58a, n: 000000008cd4451a, p: 000000002150ee8d
> next: 000000008cd4451a, n: 000000008cd4451a, p: 000000008cd4451a
> WARNING: CPU: 21 PID: 1843 at lib/plist.c:60 plist_check_prev_next_node+0x50/0x70
> Modules linked in: rfkill(E) crct10dif_ce(E)...
> CPU: 21 PID: 1843 Comm: stress-ng Kdump: ... 5.10.134+
> Hardware name: Alibaba Cloud ECS, BIOS 0.0.0 02/06/2015
> pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
> pc : plist_check_prev_next_node+0x50/0x70
> lr : plist_check_prev_next_node+0x50/0x70
> sp : ffff0018009d3c30
> x29: ffff0018009d3c40 x28: ffff800011b32a98
> x27: 0000000000000000 x26: ffff001803908000
> x25: ffff8000128ea088 x24: ffff800011b32a48
> x23: 0000000000000028 x22: ffff001800875c00
> x21: ffff800010f9e520 x20: ffff001800875c00
> x19: ffff001800fdc6e0 x18: 0000000000000030
> x17: 0000000000000000 x16: 0000000000000000
> x15: 0736076307640766 x14: 0730073007380731
> x13: 0736076307640766 x12: 0730073007380731
> x11: 000000000004058d x10: 0000000085a85b76
> x9 : ffff8000101436e4 x8 : ffff800011c8ce08
> x7 : 0000000000000000 x6 : 0000000000000001
> x5 : ffff0017df9ed338 x4 : 0000000000000001
> x3 : ffff8017ce62a000 x2 : ffff0017df9ed340
> x1 : 0000000000000000 x0 : 0000000000000000
> Call trace:
>  plist_check_prev_next_node+0x50/0x70
>  plist_check_head+0x80/0xf0
>  plist_add+0x28/0x140
>  add_to_avail_list+0x9c/0xf0
>  _enable_swap_info+0x78/0xb4
>  __do_sys_swapon+0x918/0xa10
>  __arm64_sys_swapon+0x20/0x30
>  el0_svc_common+0x8c/0x220
>  do_el0_svc+0x2c/0x90
>  el0_svc+0x1c/0x30
>  el0_sync_handler+0xa8/0xb0
>  el0_sync+0x148/0x180
> irq event stamp: 2082270
> 
> Now, si->lock locked before calling 'del_from_avail_list()'
> to make sure other thread see the si had been deleted
> and SWP_WRITEOK cleared together, will not reinsert again.
> 
> This problem exists in versions after stable 5.10.y.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Tested-by: Yongchen Yin <wb-yyc939293@xxxxxxxxxxxxxxx>
> Signed-off-by: Rongwei Wang <rongwei.wang@xxxxxxxxxxxxxxxxx>
> ---
>  mm/swapfile.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 62ba2bf577d7..2c718f45745f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -679,6 +679,7 @@ static void __del_from_avail_list(struct swap_info_struct *p)
>  {
>  	int nid;
>  
> +	assert_spin_locked(&p->lock);
>  	for_each_node(nid)
>  		plist_del(&p->avail_lists[nid], &swap_avail_heads[nid]);
>  }
> @@ -2434,8 +2435,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		spin_unlock(&swap_lock);
>  		goto out_dput;
>  	}
> -	del_from_avail_list(p);
>  	spin_lock(&p->lock);
> +	del_from_avail_list(p);
>  	if (p->prio < 0) {
>  		struct swap_info_struct *si = p;
>  		int nid;
> -- 
> 2.27.0
>