Re: [PATCH] mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios

[Muchun Song <muchun.song@linux.dev>]

> On May 22, 2025, at 11:22, yangge1116@126.com wrote:
> 
> From: Ge Yang <yangge1116@126.com>
> 
> A kernel crash was observed when replacing free hugetlb folios:
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000028
> PGD 0 P4D 0
> Oops: Oops: 0000 [#1] SMP NOPTI
> CPU: 28 UID: 0 PID: 29639 Comm: test_cma.sh Tainted 6.15.0-rc6-zp #41 PREEMPT(voluntary)
> RIP: 0010:alloc_and_dissolve_hugetlb_folio+0x1d/0x1f0
> RSP: 0018:ffffc9000b30fa90 EFLAGS: 00010286
> RAX: 0000000000000000 RBX: 0000000000342cca RCX: ffffea0043000000
> RDX: ffffc9000b30fb08 RSI: ffffea0043000000 RDI: 0000000000000000
> RBP: ffffc9000b30fb20 R08: 0000000000001000 R09: 0000000000000000
> R10: ffff88886f92eb00 R11: 0000000000000000 R12: ffffea0043000000
> R13: 0000000000000000 R14: 00000000010c0200 R15: 0000000000000004
> FS:  00007fcda5f14740(0000) GS:ffff8888ec1d8000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000028 CR3: 0000000391402000 CR4: 0000000000350ef0
> Call Trace:
> <TASK>
> replace_free_hugepage_folios+0xb6/0x100
> alloc_contig_range_noprof+0x18a/0x590
> ? srso_return_thunk+0x5/0x5f
> ? down_read+0x12/0xa0
> ? srso_return_thunk+0x5/0x5f
> cma_range_alloc.constprop.0+0x131/0x290
> __cma_alloc+0xcf/0x2c0
> cma_alloc_write+0x43/0xb0
> simple_attr_write_xsigned.constprop.0.isra.0+0xb2/0x110
> debugfs_attr_write+0x46/0x70
> full_proxy_write+0x62/0xa0
> vfs_write+0xf8/0x420
> ? srso_return_thunk+0x5/0x5f
> ? filp_flush+0x86/0xa0
> ? srso_return_thunk+0x5/0x5f
> ? filp_close+0x1f/0x30
> ? srso_return_thunk+0x5/0x5f
> ? do_dup2+0xaf/0x160
> ? srso_return_thunk+0x5/0x5f
> ksys_write+0x65/0xe0
> do_syscall_64+0x64/0x170
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> There is a potential race between __update_and_free_hugetlb_folio()
> and replace_free_hugepage_folios():
> 
> CPU1                              CPU2
> __update_and_free_hugetlb_folio   replace_free_hugepage_folios
>                                    folio_test_hugetlb(folio)
>                                    -- It's still hugetlb folio.
> 
>  __folio_clear_hugetlb(folio)
>  hugetlb_free_folio(folio)
>                                    h = folio_hstate(folio)
>                                    -- Here, h is NULL pointer
> 
> When the above race condition occurs, folio_hstate(folio) returns
> NULL, and subsequent access to this NULL pointer will cause the
> system to crash. To resolve this issue, execute folio_hstate(folio)
> under the protection of the hugetlb_lock lock, ensuring that
> folio_hstate(folio) does not return NULL.
> 
> Fixes: 04f13d241b8b ("mm: replace free hugepage folios after migration")
> Signed-off-by: Ge Yang <yangge1116@126.com>
> Cc: <stable@vger.kernel.org>

Thanks for fixing this problem. BTW, in order to catch future similar problem,
it is better to add WARN_ON into folio_hstate() to assert if hugetlb_lock
is not held when folio's reference count is zero. For this fix, LGTM.

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.