Re: 【BUG】NULL pointer dereference at __lookup_swap_cgroup

[Huang, Ying]

Hi,

xialonglong <xialonglong1@xxxxxxxxxx> writes:

> A panic occur in the linux 5.10we meet it only onceit seems that
> there is no special changes between 5.10 and upsteam about swap_cgroup.
>
> The test is based on QEMU with 64GB memory, one 2GB zram device as
> swap area.
> The test steps:
> 1.swapoff -a
> 2.add some memory pressure by stress-ng
> 3.while (2 minutes) {
>  swapoff /dev/zram0
>  swapon /dev/zram0
>  sleep 3
> }
> 4. swapon -a
>
> Preliminary analysis showed that the swap entry point to a swap area
> which have already been swapoff, and no other obvious clues, still
> trying to reproduce it.

We have a patch as follows to fix a similar issue,

2799e77529c2a25492a4395db93996e3dacd762d
Author:     Miaohe Lin <linmiaohe@xxxxxxxxxx>
AuthorDate: Mon Jun 28 19:36:50 2021 -0700
Commit:     Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CommitDate: Tue Jun 29 10:53:49 2021 -0700

swap: fix do_swap_page() race with swapoff

When I was investigating the swap code, I found the below possible race
window:

CPU 1                                   	CPU 2
-----                                   	-----
do_swap_page
  if (data_race(si->flags & SWP_SYNCHRONOUS_IO)
  swap_readpage
    if (data_race(sis->flags & SWP_FS_OPS)) {
                                        	swapoff
					  	  ..
					  	  p->swap_file = NULL;
					  	  ..
    struct file *swap_file = sis->swap_file;
    struct address_space *mapping = swap_file->f_mapping;[oops!]

Note that for the pages that are swapped in through swap cache, this isn't
an issue. Because the page is locked, and the swap entry will be marked
with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been
unlocked.

Fix this race by using get/put_swap_device() to guard against concurrent
swapoff.

Can you check whether that can fix your issue?

Best Regards,
Huang, Ying

> Any known issue about this feature, or any advise will be appreciated.
>
> Here are the panic log,
>
> Unable to handle kernel NULL pointer dereference at virtual address
> 0000000000000740
> Mem abort info:
> ESR = 0x96000004
> EC = 0x25: DABT (current EL), IL = 32 bits SET = 0, FnV = 0 EA = 0,
> S1PTW = 0 Data abort info:
> ISV = 0, ISS = 0x00000004
> CM = 0, WnR = 0
> user pgtable: 4k pages, 48-bit VAs, pgdp=000000010ae6e000
> pgd=0000000000000000, p4d=0000000000000000 Internal error: Oops:
> 96000004 [#1] SMP Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0
> 02/06/2015
> pstate: 00000005 (nzcv daif -PAN -UAO -TCO BTYPE=--)
> pc : lookup_swap_cgroup_id+0x38/0x50
> lr : mem_cgroup_charge+0x9c/0x424
> sp : ffff800102f63bc0
> x29: ffff800102f63bc0 x28: ffff0000d0d64d00
> x27: 0000000000000000 x26: 0000000000000007
> x25: ffff0000018c86a8 x24: ffff0000018c8640
> x23: 0000000000000cc0 x22: 0000000000000001
> x21: 0000000000000001 x20: ffff800102f63d28
> x19: fffffe000373cb40 x18: 0000000000000000
> x17: 0000000000000000 x16: ffff8001004715a4
> x15: 00000000ffffffff x14: 0000000000003000
> x13: 00000000ffffffff x12: 0000000000000040
> x11: ffff0000c0403478 x10: ffff0000c040347a
> x9 : ffff8001003e957c x8 : 000000000009dddd
> x7 : 0000000000000600 x6 : 00000000000000e8
> x5 : 0000020000200000 x4 : ffff000000000000
> x3 : ffff800101f4c030 x2 : 0000000000000000
> x1 : 00000000000001e4 x0 : 0000000000000000
>
> Call trace:
> lookup_swap_cgroup_id+0x38/0x50
> do_swap_page+0xa64/0xc04
> handle_pte_fault+0x1c8/0x214
> __handle_mm_fault+0x1b0/0x380
> handle_mm_fault+0xf4/0x284
> do_page_fault+0x188/0x474
> do_translation_fault+0xb8/0xe4
> do_mem_abort+0x48/0xb0
> el0_da+0x44/0x80
> el0_sync_handler+0x88/0xb4
> el0_sync+0x160/0x180
>
> <lookup_swap_cgroup_id>:       mov   x9, x30
> <lookup_swap_cgroup_id+0x4>:     nop
> <lookup_swap_cgroup_id+0x8>:    
> lsr   x2, x0, #58 SWP_TYPE_SHIFT == 58  x2 =
> swp_type
> <lookup_swap_cgroup_id+0xc>:    
> adrp  x1, 0xffff800101f4c000
> <memcg_sockets_enabled_key+0x8>
> <lookup_swap_cgroup_id+0x10>:   
> add   x3, x1, #0x30    
> x3 == swap_cgroup_ctrl
> <lookup_swap_cgroup_id+0x14>:    ubfx  x6, x0, #11, #47
> <lookup_swap_cgroup_id+0x18>:    add   x2, x2, x2, lsl #1
> <lookup_swap_cgroup_id+0x1c>:    ubfiz  x1, x0, #1, #11
> <lookup_swap_cgroup_id+0x20>:   
> mov   x5,
> #0x200000         
> // #2097152
> <lookup_swap_cgroup_id+0x24>:   
> mov   x4,
> #0xffff000000000000     //
> #-281474976710656
> <lookup_swap_cgroup_id+0x28>:    movk  x5, #0x200, lsl #32
> <lookup_swap_cgroup_id+0x2c>:    hint  #0x19
> <lookup_swap_cgroup_id+0x30>:   
> ldr   x0, [x3,x2,lsl #3] x3=ffff800101f4c030, x0 = 0
> <lookup_swap_cgroup_id+0x34>:    hint  #0x1d
> <lookup_swap_cgroup_id+0x38>:   
> ldr   x0, [x0,x6,lsl #3] x0 = 0 + 0xe8 * 8 == 0x740
> <lookup_swap_cgroup_id+0x3c>:    add   x0, x0, x5
> <lookup_swap_cgroup_id+0x40>:    lsr   x0, x0, #6
> <lookup_swap_cgroup_id+0x44>:    add   x0, x1, x0, lsl #12
> <lookup_swap_cgroup_id+0x48>:    ldrh  w0, [x0,x4]
> <lookup_swap_cgroup_id+0x4c>:    ret