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

Trond Myklebust
2020-03-24T15:24:28Z

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.

--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com