Re: [PATCH] nfsd: fix race between cache_clean and cache_purge

Chuck Lever
2020-03-24T17:47:12Z

> On Mar 24, 2020, at 11:24 AM, Trond Myklebust <trondmy@hammerspace.com> wrote:
>
> On Tue, 2020-03-24 at 09:38 -0400, Chuck Lever wrote:
>>> On Mar 24, 2020, at 5:49 AM, Yihao Wu <wuyihao@linux.alibaba.com>
>>> wrote:
>>>
>>> cache_purge should hold cache_list_lock as cache_clean does.
>>> Otherwise a cache
>>> can be cache_put twice, which leads to a use-after-free bug.
>>>
>>> To reproduce, run ltp. It happens rarely. /opt/ltp/runltp run -f
>>> net.nfs
>>>
>>> [14454.137661]
>>> ==================================================================
>>> [14454.138863] BUG: KASAN: use-after-free in cache_purge+0xce/0x160
>>> [sunrpc]
>>> [14454.139822] Read of size 4 at addr ffff8883d484d560 by task
>>> nfsd/31993
>>> [14454.140746]
>>> [14454.140995] CPU: 1 PID: 31993 Comm: nfsd Kdump: loaded Not
>>> tainted 4.19.91-0.229.git.87bac30.al7.x86_64.debug #1
>>> [14454.141002] Call Trace:
>>> [14454.141014] dump_stack+0xaf/0xfb[14454.141027] print_address_d
>>> escription+0x6a/0x2a0
>>> [14454.141037] kasan_report+0x166/0x2b0[14454.141057] ?
>>> cache_purge+0xce/0x160 [sunrpc]
>>> [14454.141079] cache_purge+0xce/0x160 [sunrpc]
>>> [14454.141099] nfsd_last_thread+0x267/0x270
>>> [nfsd][14454.141109] ? nfsd_last_thread+0x5/0x270 [nfsd]
>>> [14454.141130] nfsd_destroy+0xcb/0x180 [nfsd]
>>> [14454.141140] ? nfsd_destroy+0x5/0x180 [nfsd]
>>> [14454.141153] nfsd+0x1e4/0x2b0 [nfsd]
>>> [14454.141163] ? nfsd+0x5/0x2b0 [nfsd]
>>> [14454.141173] kthread+0x114/0x150
>>> [14454.141183] ? nfsd_destroy+0x180/0x180 [nfsd]
>>> [14454.141187] ? kthread_park+0xb0/0xb0
>>> [14454.141197] ret_from_fork+0x3a/0x50
>>> [14454.141224]
>>> [14454.141475] Allocated by task 20918:
>>> [14454.142011] kmem_cache_alloc_trace+0x9f/0x2e0
>>> [14454.142027] sunrpc_cache_lookup+0xca/0x2f0 [sunrpc]
>>> [14454.142037] svc_export_parse+0x1e7/0x930 [nfsd]
>>> [14454.142051] cache_do_downcall+0x5a/0x80 [sunrpc]
>>> [14454.142064] cache_downcall+0x78/0x180 [sunrpc]
>>> [14454.142078] cache_write_procfs+0x57/0x80 [sunrpc]
>>> [14454.142083] proc_reg_write+0x90/0xd0
>>> [14454.142088] vfs_write+0xc2/0x1c0
>>> [14454.142092] ksys_write+0x4d/0xd0
>>> [14454.142098] do_syscall_64+0x60/0x250
>>> [14454.142103] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>> [14454.142106]
>>> [14454.142344] Freed by task 19165:
>>> [14454.142804] kfree+0x114/0x300
>>> [14454.142819] cache_clean+0x2a4/0x2e0 [sunrpc]
>>> [14454.142833] cache_flush+0x24/0x60 [sunrpc]
>>> [14454.142845] write_flush.isra.19+0xbe/0x100 [sunrpc]
>>> [14454.142849] proc_reg_write+0x90/0xd0
>>> [14454.142853] vfs_write+0xc2/0x1c0
>>> [14454.142856] ksys_write+0x4d/0xd0
>>> [14454.142860] do_syscall_64+0x60/0x250
>>> [14454.142865] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>> [14454.142867]
>>> [14454.143095] The buggy address belongs to the object at
>>> ffff8883d484d540 which belongs to the cache kmalloc-256 of size 256
>>> [14454.144842] The buggy address is located 32 bytes inside
>>> of 256-byte region [ffff8883d484d540, ffff8883d484d640)
>>> [14454.146463] The buggy address belongs to the page:
>>> [14454.147155] page:ffffea000f521300 count:1 mapcount:0
>>> mapping:ffff888107c02e00 index:0xffff8883d484da40 compound_map
>>> count: 0
>>> [14454.148712] flags: 0x17fffc00010200(slab|head)
>>> [14454.149356] raw: 0017fffc00010200 ffffea000f4baf00
>>> 0000000200000002 ffff888107c02e00
>>> [14454.150453] raw: ffff8883d484da40 0000000080190001
>>> 00000001ffffffff 0000000000000000
>>> [14454.151557] page dumped because: kasan: bad access detected
>>> [14454.152364]
>>> [14454.152606] Memory state around the buggy address:
>>> [14454.153300] ffff8883d484d400: fb fb fb fb fb fb fb fb fb fb fb
>>> fb fb fb fb fb
>>> [14454.154319] ffff8883d484d480: fb fb fb fb fb fb fb fb fb fb fb
>>> fb fb fb fb fb
>>> [14454.155324] >ffff8883d484d500: fc fc fc fc fc fc fc fc fb fb fb
>>> fb fb fb fb fb
>>> [14454.156334]
>>> ^
>>> [14454.157237] ffff8883d484d580: fb fb fb fb fb fb fb fb fb fb fb
>>> fb fb fb fb fb
>>> [14454.158262] ffff8883d484d600: fb fb fb fb fb fb fb fb fc fc fc
>>> fc fc fc fc fc
>>> [14454.159282]
>>> ==================================================================
>>> [14454.160224] Disabling lock debugging due to kernel taint
>>>
>>> Fixes: 471a930ad7d1(SUNRPC: Drop all entries from cache_detail when
>>> cache_purge())
>>> Cc: stable@vger.kernel.org #v4.11+
>>> Signed-off-by: Yihao Wu <wuyihao@linux.alibaba.com>
>>
>> Mechanically this looks OK, but I would feel more comfortable
>> if a domain expert could review this. Neil, Trond, Bruce?
>>
>>
>>> ---
>>> net/sunrpc/cache.c | 3 +++
>>> 1 file changed, 3 insertions(+)
>>>
>>> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
>>> index bd843a81afa0..3e523eefc47f 100644
>>> --- a/net/sunrpc/cache.c
>>> +++ b/net/sunrpc/cache.c
>>> @@ -524,9 +524,11 @@ void cache_purge(struct cache_detail *detail)
>>> struct hlist_node *tmp = NULL;
>>> int i = 0;
>>>
>>> + spin_lock(&cache_list_lock);
>>> spin_lock(&detail->hash_lock);
>>> if (!detail->entries) {
>>> spin_unlock(&detail->hash_lock);
>>> + spin_unlock(&cache_list_lock);
>>> return;
>>> }
>>>
>>> @@ -541,6 +543,7 @@ void cache_purge(struct cache_detail *detail)
>>> }
>>> }
>>> spin_unlock(&detail->hash_lock);
>>> + spin_unlock(&cache_list_lock);
>>> }
>>> EXPORT_SYMBOL_GPL(cache_purge);
>
>
> Hmm... Shouldn't this patch be dropping cache_list_lock() when we call
> sunrpc_end_cache_remove_entry()? The latter does call both
> cache_revisit_request() and cache_put(), and while they do not
> explicitly call anything that holds cache_list_lock, some of those cd-
>> cache_put callbacks do look as if there is potential for deadlock.

I see svc_export_put calling dput, eventually, which might_sleep().

--
Chuck Lever