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Subject: Re: [PATCH] ovl: fix dentry leak in ovl_get_redirect
To:     Liangyan <liangyan.peng@linux.alibaba.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20201218161751.234759-1-liangyan.peng@linux.alibaba.com>
From:   Joseph Qi <joseph.qi@linux.alibaba.com>
Message-ID: <ec73c709-656c-7e53-7a59-de9603a5bba0@linux.alibaba.com>
Date:   Sun, 20 Dec 2020 19:25:56 +0800
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On 12/19/20 12:17 AM, Liangyan wrote:
> We need to lock d_parent->d_lock before dget_dlock, or this may
> have d_lockref updated parallelly like calltrace below which will
> cause dentry->d_lockref leak and risk a crash.
> 
> npm-20576 [028] .... 5705749.040094:
> [28] ovl_set_redirect+0x11c/0x310 //tmp = dget_dlock(d->d_parent);
> [28]?  ovl_set_redirect+0x5/0x310
> [28] ovl_rename+0x4db/0x790 [overlay]
> [28] vfs_rename+0x6e8/0x920
> [28] do_renameat2+0x4d6/0x560
> [28] __x64_sys_rename+0x1c/0x20
> [28] do_syscall_64+0x55/0x1a0
> [28] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> npm-20574 [036] .... 5705749.040094:
> [36] __d_lookup+0x107/0x140 //dentry->d_lockref.count++;
> [36] lookup_fast+0xe0/0x2d0
> [36] walk_component+0x48/0x350
> [36] link_path_walk+0x1bf/0x650
> [36]?  path_init+0x1f6/0x2f0
> [36] path_lookupat+0x82/0x210
> [36] filename_lookup+0xb8/0x1a0
> [36]?  __audit_getname+0xa2/0xb0
> [36]?  getname_flags+0xb9/0x1e0
> [36]?  vfs_statx+0x73/0xe0
> [36] vfs_statx+0x73/0xe0
> [36] __do_sys_statx+0x3b/0x80
> [36]?  syscall_trace_enter+0x1ae/0x2c0
> [36] do_syscall_64+0x55/0x1a0
> [36] entry_SYSCALL_64_
> 
> [   49.799059] PGD 800000061fed7067 P4D 800000061fed7067 PUD 61fec5067 PMD 0
> [   49.799689] Oops: 0002 [#1] SMP PTI
> [   49.800019] CPU: 2 PID: 2332 Comm: node Not tainted 4.19.24-7.20.al7.x86_64 #1
> [   49.800678] Hardware name: Alibaba Cloud Alibaba Cloud ECS, BIOS 8a46cfe 04/01/2014
> [   49.801380] RIP: 0010:_raw_spin_lock+0xc/0x20
> [   49.803470] RSP: 0018:ffffac6fc5417e98 EFLAGS: 00010246
> [   49.803949] RAX: 0000000000000000 RBX: ffff93b8da3446c0 RCX: 0000000a00000000
> [   49.804600] RDX: 0000000000000001 RSI: 000000000000000a RDI: 0000000000000088
> [   49.805252] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffff993cf040
> [   49.805898] R10: ffff93b92292e580 R11: ffffd27f188a4b80 R12: 0000000000000000
> [   49.806548] R13: 00000000ffffff9c R14: 00000000fffffffe R15: ffff93b8da3446c0
> [   49.807200] FS:  00007ffbedffb700(0000) GS:ffff93b927880000(0000) knlGS:0000000000000000
> [   49.807935] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   49.808461] CR2: 0000000000000088 CR3: 00000005e3f74006 CR4: 00000000003606a0
> [   49.809113] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   49.809758] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [   49.810410] Call Trace:
> [   49.810653]  d_delete+0x2c/0xb0
> [   49.810951]  vfs_rmdir+0xfd/0x120
> [   49.811264]  do_rmdir+0x14f/0x1a0
> [   49.811573]  do_syscall_64+0x5b/0x190
> [   49.811917]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [   49.812385] RIP: 0033:0x7ffbf505ffd7
> [   49.814404] RSP: 002b:00007ffbedffada8 EFLAGS: 00000297 ORIG_RAX: 0000000000000054
> [   49.815098] RAX: ffffffffffffffda RBX: 00007ffbedffb640 RCX: 00007ffbf505ffd7
> [   49.815744] RDX: 0000000004449700 RSI: 0000000000000000 RDI: 0000000006c8cd50
> [   49.816394] RBP: 00007ffbedffaea0 R08: 0000000000000000 R09: 0000000000017d0b
> [   49.817038] R10: 0000000000000000 R11: 0000000000000297 R12: 0000000000000012
> [   49.817687] R13: 00000000072823d8 R14: 00007ffbedffb700 R15: 00000000072823d8
> [   49.818338] Modules linked in: pvpanic cirrusfb button qemu_fw_cfg atkbd libps2 i8042
> [   49.819052] CR2: 0000000000000088
> [   49.819368] ---[ end trace 4e652b8aa299aa2d ]---
> [   49.819796] RIP: 0010:_raw_spin_lock+0xc/0x20
> [   49.821880] RSP: 0018:ffffac6fc5417e98 EFLAGS: 00010246
> [   49.822363] RAX: 0000000000000000 RBX: ffff93b8da3446c0 RCX: 0000000a00000000
> [   49.823008] RDX: 0000000000000001 RSI: 000000000000000a RDI: 0000000000000088
> [   49.823658] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffff993cf040
> [   49.825404] R10: ffff93b92292e580 R11: ffffd27f188a4b80 R12: 0000000000000000
> [   49.827147] R13: 00000000ffffff9c R14: 00000000fffffffe R15: ffff93b8da3446c0
> [   49.828890] FS:  00007ffbedffb700(0000) GS:ffff93b927880000(0000) knlGS:0000000000000000
> [   49.830725] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   49.832359] CR2: 0000000000000088 CR3: 00000005e3f74006 CR4: 00000000003606a0
> [   49.834085] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   49.835792] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 
> Fixes: a6c606551141 ("ovl: redirect on rename-dir")
> Signed-off-by: Liangyan <liangyan.peng@linux.alibaba.com>
> Suggested-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> ---
>  fs/overlayfs/dir.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 28a075b5f5b2..9831e7046038 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>  	for (d = dget(dentry); !IS_ROOT(d);) {
>  		const char *name;
>  		int thislen;
> +		struct dentry *parent = NULL;
>  
>  		spin_lock(&d->d_lock);
>  		name = ovl_dentry_get_redirect(d);
> @@ -992,7 +993,26 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>  
>  		buflen -= thislen;
>  		memcpy(&buf[buflen], name, thislen);
> +		parent = d->d_parent;
> +		if (unlikely(!spin_trylock(&parent->d_lock))) {
> +			rcu_read_lock();
> +			spin_unlock(&d->d_lock);
> +again:
> +			parent = READ_ONCE(d->d_parent);
> +			spin_lock(&parent->d_lock);
> +			if (unlikely(parent != dentry->d_parent)) {
> +				spin_unlock(&parent->d_lock);
> +				goto again;
> +			}
> +			rcu_read_unlock();
> +			if (parent != d)
> +				spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
> +			else
> +				parent = NULL;

It seems that parent can't be NULL since d is not root.
So the above logic can be simplified with:
spin_lock(&d->d_lock);

> +		}
>  		tmp = dget_dlock(d->d_parent);

Use parent directly.

Joseph

> +		if (parent)
> +			spin_unlock(&parent->d_lock);
>  		spin_unlock(&d->d_lock);
>  
>  		dput(d);
>