[PATCH] skb_array: fix NULL-pointer exception

[George Cherian]

While running a multiple VM testscase with each VM running iperf
traffic between others the following kernel NULL pointer exception
was seen.

Race appears when the tun driver instance of one VM calls skb_array_produce
(from tun_net_xmit) and the the destined VM's skb_array_consume
(from tun_ring_recv), which could run concurrently on another core. Due to
which the sock_wfree gets called again from the tun_ring_recv context.

The fix is to add write/read barrier calls to be sure that we get proper
values in the tun_ring_recv context.

Crash log
[35321.580227] Unable to handle kernel NULL pointer dereference at virtual address 00000060
[35321.596720] pgd = ffff809ee552f000
[35321.603723] [00000060] *pgd=0000009f514ac003, *pud=0000009f54f7c003, *pmd=0000000000000000
[35321.620588] Internal error: Oops: 96000006 1 SMP
[35321.630461] Modules linked in: xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_4
[35321.728501] CPU: 114 PID: 5560 Comm: qemu-system-aar Not tainted 4.11.8-4k-vhe-lse+ #3
[35321.744510] Hardware name: Cavium Inc. Unknown/Unknown, BIOS 1.0 07/24/2017
[35321.758602] task: ffff80bed7fca880 task.stack: ffff80beb5128000
[35321.770600] PC is at sock_wfree+0x24/0x80
[35321.778746] LR is at skb_release_head_state+0x68/0xf8
[35321.788979] pc : [<ffff000008a772fc>] lr : [<ffff000008a79238>] pstate: 40400149
[35321.803930] sp : ffff80beb512bc30
[35321.810648] x29: ffff80beb512bc30 x28: ffff80bed7fca880
[35321.821400] x27: 000000000000004e x26: 0000000000000000
[35321.832156] x25: 000000000000000c x24: 0000000000000000
[35321.842947] x23: ffff809ece3e4900 x22: ffff80beb512be00
[35321.853729] x21: ffff000009138000 x20: 0000000000000144
[35321.864507] x19: 0000000000000000 x18: 0000000000000014
[35321.875276] x17: 0000ffff9d9689a0 x16: ffff00000828b3f8
[35321.886048] x15: 0000504d7b000000 x14: e90ab50c48680a08
[35321.896824] x13: 0101000017773f52 x12: 1080d422c00e5db6
[35321.907595] x11: 68c322bd3930cf7a x10: a8c0d07aa8c0ad16
[35321.918352] x9 : 000000001da4ed90 x8 : b50c48680a080101
[35321.929099] x7 : 000017770c521080 x6 : 000000001d6c13f2
[35321.939865] x5 : 000000001d6c13f2 x4 : 000000000000000e
[35321.950619] x3 : 000000085ff97d82 x2 : 0000000000000000
[35321.961376] x1 : ffff000008a772d8 x0 : 0000000000000500
[35321.975193] Process qemu-system-aar (pid: 5560, stack limit = 0xffff80beb5128000)
[35321.990347] Stack: (0xffff80beb512bc30 to 0xffff80beb512c000)
[35322.001982] bc20: ffff80beb512bc50 ffff000008a79238
[35322.017817] bc40: ffff809e8fd7be00 000000000000004e ffff80beb512bc70 ffff000008a79488
[35322.033651] bc60: ffff809e8fd7be00 ffff00000881307c ffff80beb512bc90 ffff000008a79678
[35322.049489] bc80: ffff809e8fd7be00 ffff80beb512be00 ffff80beb512bcb0 ffff000008812f24
[35322.065321] bca0: ffff809e8fd7be00 000000000000004e ffff80beb512bd50 ffff0000088133f0
[35322.081165] bcc0: ffff809ece3e4900 0000000000011000 ffff80beb512bdd8 ffff80beb512be00
[35322.097001] bce0: 000000001d6c13a4 0000000000000015 0000000000000124 000000000000003f
[35322.112866] bd00: ffff000008bc2000 ffff00000847b5ac 0000000000020000 ffff80be00080000
[35322.128701] bd20: 0022000000000001 ffff80bed7fc0010 ffff000008100c38 0000000000000000
[35322.144539] bd40: 0000000000000000 0000000000040b08 ffff80beb512bd80 ffff000008288f80
[35322.160395] bd60: ffff000009138000 ffff809ee7cd3500 0000000000011000 ffff80beb512beb0
[35322.176255] bd80: ffff80beb512be30 ffff00000828a224 0000000000011000 ffff809ee7cd3500
[35322.192109] bda0: 000000001d6c13a4 ffff80beb512beb0 0000000000011000 0000000000000000
[35322.207974] bdc0: 0000000000000000 000000001d6c13a4 0000000000011000 ffff809ee7cd3500
[35322.223822] bde0: 000000000000004e 0000000000000000 0000000000000000 0000000000000000
[35322.239661] be00: ffff80be00000000 000000000000004e 0000000000010fb2 ffff80beb512bdc8
[35322.255519] be20: 0000000000000001 0000000000040b08 ffff80beb512be70 ffff00000828b464
[35322.271392] be40: ffff000009138000 ffff809ee7cd3501 ffff809ee7cd3500 000000001d6c13a4
[35322.287255] be60: 0000000000011000 0000000000000015 0000000000000000 ffff0000080833f0
[35322.303090] be80: 0000000000000000 000080bef0071000 ffffffffffffffff 0000ffff9d9689cc
[35322.318951] bea0: 0000000080000000 000080bef0071000 000000000000004e 0000000000040b08
[35322.334771] bec0: 000000000000000e 000000001d6c13a4 0000000000011000 0000ffff9cc89108
[35322.350640] bee0: 0000000000000002 0000ffff9cc89000 0000ffff9cc896f0 0000000000000000
[35322.366500] bf00: 000000000000003f 000000001da4ed90 a8c0d07aa8c0ad16 68c322bd3930cf7a
[35322.382358] bf20: 1080d422c00e5db6 0101000017773f52 e90ab50c48680a08 0000504d7b000000
[35322.398209] bf40: 0000000000000000 0000ffff9d9689a0 0000000000000014 0000000000000030
[35322.414063] bf60: 000000001d6c13a4 000000001d6c0db0 000000001d6d1db0 00000000006fbbc8
[35322.429901] bf80: 0000000000011000 000000001d6ab3e8 000000001d6ab3a4 00000000007ee4c8
[35322.445751] bfa0: 0000000000000000 0000fffff363ab70 0000ffff9d9689b8 0000fffff363ab30
[35322.461588] bfc0: 0000ffff9d9689cc 0000000080000000 000000000000000e 000000000000003f
[35322.477445] bfe0: 0000000000000000 0000000000000000 ffff809ed043d758 0000000000000000
[35322.493283] Call trace:
[35322.498275] Exception stack(0xffff80beb512ba40 to 0xffff80beb512bb70)
[35322.511303] ba40: 0000000000000000 0001000000000000 ffff80beb512bc30 ffff000008a772fc
[35322.527152] ba60: 0000000040400149 0000000000000049 ffff000008bc2000 ffff80bed7fca880
[35322.542992] ba80: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[35322.558863] baa0: 0000000000000000 000000001d895758 ffff80beb512bb78 0000000000000000
[35322.574702] bac0: 000000050000000b 0000000800000001 0000000a00000001 0000000b00000001
[35322.590550] bae0: 0000000e00000001 0000001800010001 ffff80beb512bbf0 0000000000040b08
[35322.606416] bb00: 0000000000000500 ffff000008a772d8 0000000000000000 000000085ff97d82
[35322.622287] bb20: 000000000000000e 000000001d6c13f2 000000001d6c13f2 000017770c521080
[35322.638144] bb40: b50c48680a080101 000000001da4ed90 a8c0d07aa8c0ad16 68c322bd3930cf7a
[35322.653992] bb60: 1080d422c00e5db6 0101000017773f52
[35322.663908] [<ffff000008a772fc>] sock_wfree+0x24/0x80
[35322.674168] [<ffff000008a79238>] skb_release_head_state+0x68/0xf8
[35322.686535] [<ffff000008a79488>] skb_release_all+0x20/0x40
[35322.697663] [<ffff000008a79678>] consume_skb+0x38/0xd8
[35322.708120] [<ffff000008812f24>] tun_do_read.part.9+0x20c/0x4f0
[35322.720149] [<ffff0000088133f0>] tun_chr_read_iter+0xc0/0xe0
[35322.731638] [<ffff000008288f80>] __vfs_read+0x100/0x160
[35322.742249] [<ffff00000828a224>] vfs_read+0x8c/0x148
[35322.752344] [<ffff00000828b464>] SyS_read+0x6c/0xd8
[35322.762263] [<ffff0000080833f0>] el0_svc_naked+0x24/0x28
[35322.773042] Code: d503201f f9400e93 b940e280 91051274 (f9403261)

Reported-by: Joseph DeVincentis <Joseph.DeVincentis@xxxxxxxxxx>
Signed-off-by: George Cherian <george.cherian@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 include/linux/skb_array.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/skb_array.h b/include/linux/skb_array.h
index 8621ffd..1b9c0f8 100644
--- a/include/linux/skb_array.h
+++ b/include/linux/skb_array.h
@@ -45,6 +45,13 @@ static inline bool skb_array_full(struct skb_array *a)
 
 static inline int skb_array_produce(struct skb_array *a, struct sk_buff *skb)
 {
+	/*
+	 * This barrier is necessary in order to prevent race condition
+	 * with skb_array_consume(). This issue was seen in VM testcases
+	 * with multiple instances of iperf pumping traffic over tun
+	 * interface.
+	 */
+	wmb();
 	return ptr_ring_produce(&a->ring, skb);
 }
 
@@ -94,6 +101,8 @@ static inline bool skb_array_empty_any(struct skb_array *a)
 
 static inline struct sk_buff *skb_array_consume(struct skb_array *a)
 {
+	/* Refer to skb_array_produce() for details */
+	rmb();
 	return ptr_ring_consume(&a->ring);
 }
 
-- 
2.1.4