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Date:	Sat, 13 Sep 2014 20:54:32 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	shakil A Khan <shakilk1729@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	edumazet@...gle.com, davem@...emloft.net
Subject: Re: [PATCH] Freeing dst when the reference count <0 causes general
 protection fault, it could be a major security flaw as rogue app can modify
 dst to crash kernel.

On Sat, 2014-09-13 at 11:35 -0700, shakil A Khan wrote:
> On Saturday, September 13, 2014 04:50:22 AM Eric Dumazet wrote:
> > On Sat, 2014-09-13 at 01:27 -0700, Shakil A Khan wrote:
> > > Signed-off-by: Shakil A Khan <shakilk1729@...il.com>
> > > ---
> > > 
> > >  net/core/dst.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/net/core/dst.c b/net/core/dst.c
> > > index a028409..6a848b0 100644
> > > --- a/net/core/dst.c
> > > +++ b/net/core/dst.c
> > > @@ -284,7 +284,10 @@ void dst_release(struct dst_entry *dst)
> > > 
> > >  		int newrefcnt;
> > >  		
> > >  		newrefcnt = atomic_dec_return(&dst->__refcnt);
> > > 
> > > -		WARN_ON(newrefcnt < 0);
> > > +
> > > +		if (WARN(newrefcnt < 0, "dst reference count less than zero"))
> > > +			return;
> > > +
> > > 
> > >  		if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)
> > >  		
> > >  			call_rcu(&dst->rcu_head, dst_destroy_rcu);
> > >  	
> > >  	}
> > 
> > A rogue application can not do trigger this, unless a major bug in the
> > kernel exists.
> 
> Please check this kernel trace. It is able to crash the kernel.
> 
> general protection fault: 0000 [#1] PREEMPT SMP
> Modules linked in: nfsv3 nfs_acl rpcsec_gss_krb5 auth_rpcgss oid_registry 
> nfsv4 nfs fscache lockd sunrpc tun nbd ipmi_si ipmi_watchdog ipmi_devintf 
> ipmi_msghandler xt_mark xt_owner ipt_MASQUERADE xt_physdev xt_state xt_LOG 
> iptable_mangle iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat 
> nf_conntrack iptable_filter ip_tables xen_acpi_processor xen_pciback 
> xen_netback xen_blkback xen_gntalloc xen_gntdev xenfs xen_privcmd bridge stp 
> llc ipv6 ext4 jbd2 freq_table mperf coretemp crc32c_intel ghash_clmulni_intel 
> microcode pcspkr sb_edac edac_core lpc_ich mfd_core i2c_i801 sg ioatdma igb 
> dca i2c_algo_bit i2c_core ptp pps_core ext3 jbd mbcache sd_mod crc_t10dif 
> aesni_intel ablk_helper cryptd lrw gf128mul glue_helper aes_x86_64 ahci 
> libahci isci libsas scsi_transport_sas megaraid_sas wmi dm_mirror 
> dm_region_hash dm_log dm_mod [last unloaded: iTCO_vendor_support]
> CPU: 6 PID: 15324 Comm: XXXX Not tainted 3.10.45-xen.322.17.41238 #1
> Hardware name: McAfee, Inc. ATD-6000/S4600LH...., BIOS 
> SE5C600.86B.02.01.0002.082220131453 08/22/2013
> task: ffff882bc6255000 ti: ffff882bc61aa000 task.ti: ffff882bc61aa000
> RIP: e030:[<ffffffff8148473f>]  [<ffffffff8148473f>] ipv4_dst_destroy+0x3b/0x77
> RSP: e02b:ffff882bc61abb48  EFLAGS: 00010296
> RAX: dead000000200200 RBX: ffff882bc625bc80 RCX: 0001338a9b7110db
> RDX: dead000000100100 RSI: ffffffff82267e30 RDI: 00000000000003f4
> RBP: ffff882bc61abb58 R08: 00000000d5d6df8b R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: ffffffff820e5880 R14: ffff88070e584b80 R15: 0000000000000000
> FS:  00007f8d3fff2700(0000) GS:ffff88081e6c0000(0000) knlGS:0000000000000000
> CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000031d0e36ac0 CR3: 0000002db0165000 CR4: 0000000000042660
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> Stack:
>  ffff882bc61abb58 ffff882bc625bc80 ffff882bc61abb88 ffffffff8145bfc5
>  ffff882bc61abba8 ffff882bc625bc80 0000000000000000 ffff882bc625bc80
>  ffff882bc61abba8 ffffffff8145c2c5 ffff88070e584b80 ffff882b991c2300
> Call Trace:
>  [<ffffffff8145bfc5>] dst_destroy+0x29/0xbd
>  [<ffffffff8145c2c5>] dst_release+0x58/0x67
>  [<ffffffff814ad539>] tcp_v4_do_rcv+0x11b/0x287
>  [<ffffffff81443e24>] __release_sock+0x7c/0xe7
>  [<ffffffff81443ebd>] release_sock+0x2e/0x7c
>  [<ffffffff81499cdf>] tcp_sendmsg+0xe0/0xd41
>  [<ffffffff814bf369>] inet_sendmsg+0x7d/0xa0
>  [<ffffffff8143d794>] sock_aio_write+0x159/0x17d
>  [<ffffffff81005859>] ? __raw_callee_save_xen_pmd_val+0x11/0x1e
>  [<ffffffff8116d12b>] do_sync_write+0x7f/0xa7
>  [<ffffffff8116d392>] ? rw_verify_area+0x56/0xd5
>  [<ffffffff8116d555>] vfs_write+0x144/0x170
>  [<ffffffff8116d977>] SyS_write+0x54/0x8f
>  [<ffffffff810d078f>] ? __audit_syscall_exit+0x20c/0x29c
>  [<ffffffff8151e959>] system_call_fastpath+0x16/0x1b
> Code: fb 48 8d 87 b0 00 00 00 48 39 87 b0 00 00 00 74 4f 48 c7 c7 04 8f 3c 82 
> e8 32 23 09 00 48 8b 93 b0 00 00 00 48 8b 83 b8 00 00 00 <48> 89 42 08 48 89 
> 10 48 b9 00 01 10 00 00 00 ad de 48 89 8b b0
> RIP  [<ffffffff8148473f>] ipv4_dst_destroy+0x3b/0x77
>  RSP <ffff882bc61abb48>
> ---[ end trace d56f90482c47af91 ]---
> Kernel panic - not syncing: Fatal exception in interrupt
> 
>  
> > 
> > Instead of trying to hide the kernel bug, we need to fix it.
>     There are two problems with this. First the list has somehow got out of 
> sync in terms of number of delete and allocate, so we need to fix that.
> But at the same time refcount if <0 signifies its been already freed so we need 
> not free up.
> We need fix for both and my patch targets later as my system works fine with 
> this patch.
> > 
> > Can you describe how this could trigger with a pristine kernel ?
> This is triggered with custom software to imitate malware traffic(Kernel was not 
> having any custom patch whatsoever, it was a pristine kernel 3.10.45).
> Point is if an application can crash this, then it would be big security flaw 
> not exactly similar but like SSL love bug, which can be exploited to bring 
> down systems.

3.10.45 is old, you are a bit late, as we already spent a good amount of
time fixing all this about 3 months ago.

3.10.45 misses this fix :

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=7f502361531e9eecb396cf99bdc9e9a59f7ebd7f

And this one :

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f88649721268999bdff09777847080a52004f691

And also :

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=9709674e68646cee5a24e3000b3558d25412203a

All these should already be in 3.10.54

I suggest you try a 3.10.54 kernel. If it still crashes, then try a 3.16
kernel. 

Then, we'll investigate, _if_ needed.

Thanks.



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