[Devel] Re: [PATCH] mounstats NULL pointer dereference

[Serge E. Hallyn]

Quoting Vasily Tarasov (vtaras at openvz.org):
> OpenVZ developers team has encountered the following problem in 2.6.19-rc6
> kernel. After some seconds of running script
> 
> while [[ 1 ]]
> do
> 	find  /proc -name mountstats | xargs cat
> done
> 
> this Oops appears:
> 
> BUG: unable to handle kernel NULL pointer dereference at virtual address
> 00000010
>  printing eip:
> c01a6b70
> *pde = 00000000
> Oops: 0000 [#1]
> SMP
> Modules linked in: xt_length ipt_ttl xt_tcpmss ipt_TCPMSS iptable_mangle
> iptable_filter xt_multiport xt_limit ipt_tos ipt_REJECT ip_tables x_tables
> parport_pc lp parport sunrpc af_packet thermal processor fan button battery
> asus_acpi ac ohci_hcd ehci_hcd usbcore i2c_nforce2 i2c_core tg3 floppy
> pata_amd
> ide_cd cdrom sata_nv libata
> CPU:    1
> EIP:    0060:[<c01a6b70>]    Not tainted VLI
> EFLAGS: 00010246   (2.6.19-rc6 #2)
> EIP is at mountstats_open+0x70/0xf0
> eax: 00000000   ebx: e6247030   ecx: e62470f8   edx: 00000000
> esi: 00000000   edi: c01a6b00   ebp: c33b83c0   esp: f4105eb4
> ds: 007b   es: 007b   ss: 0068
> Process cat (pid: 6044, ti=f4105000 task=f4104a70 task.ti=f4105000)
> Stack: c33b83c0 c04ee940 f46a4a80 c33b83c0 e4df31b4 c01a6b00 f4105000 c0169231
>        e4df31b4 c33b83c0 c33b83c0 f4105f20 00000003 f4105000 c0169445 f2503cf0
>        f7f8c4c0 00008000 c33b83c0 00000000 00008000 c0169350 f4105f20 00008000
> Call Trace:
>  [<c01a6b00>] mountstats_open+0x0/0xf0
>  [<c0169231>] __dentry_open+0x181/0x250
>  [<c0169445>] nameidata_to_filp+0x35/0x50
>  [<c0169350>] do_filp_open+0x50/0x60
>  [<c01873d6>] seq_read+0xc6/0x300
>  [<c0169511>] get_unused_fd+0x31/0xc0
>  [<c01696d3>] do_sys_open+0x63/0x110
>  [<c01697a7>] sys_open+0x27/0x30
>  [<c01030bd>] sysenter_past_esp+0x56/0x79
>  =======================
> Code: 45 74 8b 54 24 20 89 44 24 08 8b 42 f0 31 d2 e8 47 cb f8 ff 85 c0 89 c3
> 74 51 8d 80 a0 04 00 00 e8 46 06 2c 00 8b 83 48 04 00 00 <8b> 78 10 85 ff 74
> 03
> f0 ff 07 b0 01 86 83 a0 04 00 00 f0 ff 4b
> EIP: [<c01a6b70>] mountstats_open+0x70/0xf0 SS:ESP 0068:f4105eb4
> 
> The problem is that task->nsproxy can be equal NULL for some time during
> task exit. This patch fixes the BUG.
> 
> Signed-off-by: Vasily Tarasov <vtaras at openvz.org>

Thanks, Vasily.  I couldn't reproduce the bug, but I see it must be
there.  Fix looks good, and boots and tests fine here.

Acked-by: Serge E. Hallyn <serue at us.ibm.com>

> --
> 
> --- ./fs/proc/base.c.mountstatsfix	2006-11-18 02:43:48.000000000 +0300
> +++ ./fs/proc/base.c	2006-11-18 03:11:41.000000000 +0300
> @@ -442,7 +442,8 @@
> 
>  		if (task) {
>  			task_lock(task);
> -			namespace = task->nsproxy->namespace;
> +			if (task->nsproxy)
> +				namespace = task->nsproxy->namespace;
>  			if (namespace)
>  				get_namespace(namespace);
>  			task_unlock(task);
> --- ./include/linux/nsproxy.h.mountstatsfix	2006-11-18 02:43:51.000000000 +0300
> +++ ./include/linux/nsproxy.h	2006-11-18 03:15:25.000000000 +0300
> @@ -45,8 +45,10 @@
>  {
>  	struct nsproxy *ns = p->nsproxy;
>  	if (ns) {
> -		put_nsproxy(ns);
> +		task_lock(p);
>  		p->nsproxy = NULL;
> +		task_unlock(p);
> +		put_nsproxy(ns);
>  	}
>  }
>  #endif
> -
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