Re: erofs pointer corruption and kernel crash

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi,

On 2026/4/10 16:13, Arseniy Krasnov wrote:
> Hi,
> 
> We found unexpected behaviour of erofs:
> 
> There is function in erofs - 'erofs_onlinefolio_end()'. It has pointer to
> 'struct folio' as first argument, and there is loop inside this function,
> which updates 'private' field of provided folio:
> 
>    do {
>            orig = atomic_read((atomic_t *)&folio->private);
>            DBG_BUGON(orig <= 0);
>            v = dirty << EROFS_ONLINEFOLIO_DIRTY;
>            v |= (orig - 1) | (!!err << EROFS_ONLINEFOLIO_EIO);
>    } while (atomic_cmpxchg((atomic_t *)&folio->private, orig, v) != orig);
> 
> Now, we see that in some rare case, this function processes folio, where
> 'private' is pointer, and thus this loop will update some bits in this
> pointer. Then later kernel dereferences such pointer and crashes.
> 
> To catch this, the following small debug patch was used (e.g. we check that 'private' field is pointer):
> 
> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index 33cb0a7330d2..b1d8deffec4d 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -238,6 +238,11 @@ void erofs_onlinefolio_end(struct folio *folio, int err, bool dirty)
>   {
>       int orig, v;
>   
> +    if (((uintptr_t)folio->private) & 0xffff000000000000) {

No, if erofs_onlinefolio_end() is called, `folio->private`
shouldn't be a pointer, it's just a counter inside, and
storing a pointer is unexpected.

And since the folio is locked, it shouldn't call into
try_to_free_buffers().

Is it easy to reproduce? if yes, can you print other
values like `folio->mapping` and `folio->index` as
well?

I need more informations to find some clues.

Thanks,
Gao Xiang

> +        pr_emerg("\n[foliodbg] %s:%d EROFS FOLIO %px PRIVATE BEFORE %px\n", __func__, __LINE__, folio, folio->private);
> +        dump_stack();
> +    }
> +
>       do {
>           orig = atomic_read((atomic_t *)&folio->private);
>           DBG_BUGON(orig <= 0);
> @@ -245,6 +250,9 @@ void erofs_onlinefolio_end(struct folio *folio, int err, bool dirty)
>           v |= (orig - 1) | (!!err << EROFS_ONLINEFOLIO_EIO);
>       } while (atomic_cmpxchg((atomic_t *)&folio->private, orig, v) != orig);
>   
> +    if (((uintptr_t)folio->private) & 0xffff000000000000)
> +        pr_emerg("\n[foliodbg] %s:%d EROFS FOLIO %px PRIVATE SET %px\n", __func__, __LINE__, folio, folio->private);
> +
>       if (v & (BIT(EROFS_ONLINEFOLIO_DIRTY) - 1))
>           return;
>       folio->private = 0;
> 
> 
> And it gives result:
> 
> [][  T639] [foliodbg] erofs_onlinefolio_end:242 EROFS FOLIO fffffdffc0030440 PRIVATE BEFORE ffff000002b32468
> [][  T639] CPU: 0 UID: 0 PID: 639 Comm: kworker/0:6H Tainted: G O 6.15.11-sdkernel #1 PREEMPT
> [][  T639] Tainted: [O]=OOT_MODULE
> [][  T639] Workqueue: kverityd verity_work
> [][  T639] Call trace:
> [][  T639]  show_stack+0x18/0x30 (C)
> [][  T639]  dump_stack_lvl+0x60/0x80
> [][  T639]  dump_stack+0x18/0x24
> [][  T639]  erofs_onlinefolio_end+0x124/0x130
> [][  T639]  z_erofs_decompress_queue+0x4b0/0x8c0
> [][  T639]  z_erofs_decompress_kickoff+0x88/0x150
> [][  T639]  z_erofs_endio+0x144/0x250
> [][  T639]  bio_endio+0x138/0x150
> [][  T639]  __dm_io_complete+0x1e0/0x2b0
> [][  T639]  clone_endio+0xd0/0x270
> [][  T639]  bio_endio+0x138/0x150
> [][  T639]  verity_finish_io+0x64/0xf0
> [][  T639]  verity_work+0x30/0x40
> [][  T639]  process_one_work+0x180/0x2e0
> [][  T639]  worker_thread+0x2c4/0x3f0
> [][  T639]  kthread+0x12c/0x210
> [][  T639]  ret_from_fork+0x10/0x20
> [][  T639]
> [][  T639] [foliodbg] erofs_onlinefolio_end:254 EROFS FOLIO fffffdffc0030440 PRIVATE SET ffff000022b32467
> [][   T39] Unable to handle kernel paging request at virtual address ffff000022b32467
> [][   T39] Mem abort info:
> [][   T39]   ESR = 0x0000000096000006
> [][   T39]   EC = 0x25: DABT (current EL), IL = 32 bits
> [][   T39]   SET = 0, FnV = 0
> [][   T39]   EA = 0, S1PTW = 0
> [][   T39]   FSC = 0x06: level 2 translation fault
> [][   T39] Data abort info:
> [][   T39]   ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000
> [][   T39]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
> [][   T39]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
> [][   T39] swapper pgtable: 4k pages, 48-bit VAs, pgdp=0000000001e36000
> [][   T39] [ffff000022b32467] pgd=1800000007fff403, p4d=1800000007fff403, pud=1800000007ffe403, pmd=0000000000000000
> [][   T39] Internal error: Oops: 0000000096000006 [#1]  SMP
> [][   T39] Modules linked in: vlsicomm(O)
> [][   T39] CPU: 1 UID: 0 PID: 39 Comm: kswapd0 Tainted: G O 6.15.11-sdkernel #1 PREEMPT
> [][   T39] Tainted: [O]=OOT_MODULE
> [][   T39] pstate: 00400005 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [][   T39] pc : drop_buffers.constprop.0+0x34/0x120
> [][   T39] lr : try_to_free_buffers+0xd0/0x100
> [][   T39] sp : ffff80008105b780
> [][   T39] x29: ffff80008105b780 x28: 0000000000000000 x27: fffffdffc0030448
> [][   T39] x26: ffff80008105b8a0 x25: ffff80008105b868 x24: 0000000000000001
> [][   T39] x23: fffffdffc0030440 x22: ffff80008105b7b0 x21: fffffdffc0030440
> [][   T39] x20: ffff000022b32467 x19: ffff000022b32467 x18: 0000000000000000
> [][   T39] x17: 0000000000000000 x16: 0000000000000000 x15: 00000000d69f4cc0
> [][   T39] x14: ffff0000000c5dc0 x13: 0000000000000000 x12: ffff800080d59b58
> [][   T39] x11: 00000000000000c0 x10: 0000000000000000 x9 : 0000000000000000
> [][   T39] x8 : ffff80008105b7d0 x7 : 0000000000000000 x6 : 000000000000003f
> [][   T39] x5 : 0000000000000000 x4 : fffffdffc0030440 x3 : 1ff0000000004001
> [][   T39] x2 : 1ff0000000004001 x1 : ffff80008105b7b0 x0 : fffffdffc0030440
> [][   T39] Call trace:
> [][   T39]  drop_buffers.constprop.0+0x34/0x120 (P)
> [][   T39]  try_to_free_buffers+0xd0/0x100
> [][   T39]  filemap_release_folio+0x94/0xc0
> [][   T39]  shrink_folio_list+0x8c8/0xc40
> [][   T39]  shrink_lruvec+0x740/0xb80
> [][   T39]  shrink_node+0x2b8/0x9a0
> [][   T39]  balance_pgdat+0x3b8/0x760
> [][   T39]  kswapd+0x220/0x3b0
> [][   T39]  kthread+0x12c/0x210
> [][   T39]  ret_from_fork+0x10/0x20
> [][   T39] Code: 14000004 f9400673 eb13029f 54000180 (f9400262)
> [][   T39] ---[ end trace 0000000000000000 ]---
> [][   T39] Kernel panic - not syncing: Oops: Fatal exception
> [][   T39] SMP: stopping secondary CPUs
> [][   T39] Kernel Offset: disabled
> [][   T39] CPU features: 0x0000,00000000,01000000,0200420b
> [][   T39] Memory Limit: none
> [][   T39] Rebooting in 5 seconds..
> 
> So 'erofs_onlinefolio_end()' takes some folio with 'private' field contains
> some pointer (0xffff000002b32468), "corrupts" this pointer (result will be
> 0xffff000022b32467 - at least we see that 0x20000000 was ORed to original
> pointer and this is (1 << EROFS_ONLINEFOLIO_DIRTY)), and then kernel crashes.
> We guess it is not valid case when such folio is passed as argument to
> 'erofs_onlinefolio_end()'.
> 
> We have the following erofs configuration in buildroot:
> 
> BR2_TARGET_ROOTFS_EROFS=y
> BR2_TARGET_ROOTFS_EROFS_CUSTOM_COMPRESSION=y
> BR2_TARGET_ROOTFS_EROFS_COMPRESSION_ALGORITHMS="zstd,22 --max-extent-bytes 65536 -E48bit"
> BR2_TARGET_ROOTFS_EROFS_FRAGMENTS=y
> BR2_TARGET_ROOTFS_EROFS_PCLUSTERSIZE=65536
> 
> 
> 
> May be You know how to fix it or some ideas? Because we are new at erofs and need to discover and
> learn its source code.
> 
> Thanks