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Subject: Re: [PATCH v3] ovl: fix dentry leak in ovl_get_redirect
To:     Liangyan <liangyan.peng@linux.alibaba.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20201221183327.134077-1-liangyan.peng@linux.alibaba.com>
From:   Joseph Qi <joseph.qi@linux.alibaba.com>
Message-ID: <5a12fda7-8153-6191-416b-d510c993ce63@linux.alibaba.com>
Date:   Tue, 22 Dec 2020 09:59:10 +0800
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On 12/22/20 2:33 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.
> 
>      CPU 0                                CPU 1
> ovl_set_redirect                       lookup_fast
>   ovl_get_redirect                       __d_lookup
>     dget_dlock
>       //no lock protection here            spin_lock(&dentry->d_lock)
>       dentry->d_lockref.count++            dentry->d_lockref.count++
> 
> [   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>
> Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
> Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/overlayfs/dir.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 28a075b5f5b2..e9aa4a12ad82 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;

No need to initialize 'parent' here.

>  
>  		spin_lock(&d->d_lock);
>  		name = ovl_dentry_get_redirect(d);
> @@ -992,11 +993,10 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>  
>  		buflen -= thislen;
>  		memcpy(&buf[buflen], name, thislen);
> -		tmp = dget_dlock(d->d_parent);

'tmp' won't be used any more, so you'd better clean it up.

Thanks,
Joseph

>  		spin_unlock(&d->d_lock);
> -
> +		parent = dget_parent(d);
>  		dput(d);
> -		d = tmp;
> +		d = parent;
>  
>  		/* Absolute redirect: finished */
>  		if (buf[buflen] == '/')
>