[PATCH V2] arm64: fix oops in concurrently setting insn_emulation sysctls

["haibinzhang(张海斌)" <haibinzhang@tencent.com>]

How to reproduce:
    launch two shell executions:
       #!/bin/bash
       while [ 1 ];
       do
           echo 1 > /proc/sys/abi/swp
       done

Oops info:
    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
    Internal error: Oops: 96000006 [#1] SMP
    Call trace:
    update_insn_emulation_mode+0xc0/0x148
    emulation_proc_handler+0x64/0xb8
    proc_sys_call_handler+0x9c/0xf8
    proc_sys_write+0x18/0x20
    __vfs_write+0x20/0x48
    vfs_write+0xe4/0x1d0
    ksys_write+0x70/0xf8
    __arm64_sys_write+0x20/0x28
    el0_svc_common.constprop.0+0x7c/0x1c0
    el0_svc_handler+0x2c/0xa0
    el0_svc+0x8/0x200

emulation_proc_handler changes table->data for proc_dointvec_minmax
and so it isn't allowed to reenter before restoring table->data,
which isn't right now.
To fix this issue, keep the table->data as &insn->current_mode and
use container_of() to retrieve the insn pointer. Another mutex is
used to protect against the current_mode update but not for retrieving
insn_emulation as table->data is no longer changing.

Signed-off-by: hewenliang <hewenliang4@huawei.com>
Signed-off-by: Haibin Zhang <haibinzhang@tencent.com>
---
 arch/arm64/kernel/armv8_deprecated.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
index 6875a16b09d2..fb0e7c7b2e20 100644
--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -59,6 +59,7 @@ struct insn_emulation {
 static LIST_HEAD(insn_emulation);
 static int nr_insn_emulated __initdata;
 static DEFINE_RAW_SPINLOCK(insn_emulation_lock);
+static DEFINE_MUTEX(insn_emulation_mutex);
 
 static void register_emulation_hooks(struct insn_emulation_ops *ops)
 {
@@ -207,10 +208,10 @@ static int emulation_proc_handler(struct ctl_table *table, int write,
 				  loff_t *ppos)
 {
 	int ret = 0;
-	struct insn_emulation *insn = (struct insn_emulation *) table->data;
+	struct insn_emulation *insn = container_of(table->data, struct insn_emulation, current_mode);
 	enum insn_emulation_mode prev_mode = insn->current_mode;
 
-	table->data = &insn->current_mode;
+	mutex_lock(&insn_emulation_mutex);
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 
 	if (ret || !write || prev_mode == insn->current_mode)
@@ -223,7 +224,7 @@ static int emulation_proc_handler(struct ctl_table *table, int write,
 		update_insn_emulation_mode(insn, INSN_UNDEF);
 	}
 ret:
-	table->data = insn;
+	mutex_unlock(&insn_emulation_mutex);
 	return ret;
 }
 
@@ -247,7 +248,7 @@ static void __init register_insn_emulation_sysctl(void)
 		sysctl->maxlen = sizeof(int);
 
 		sysctl->procname = insn->ops->name;
-		sysctl->data = insn;
+		sysctl->data = &insn->current_mode;
 		sysctl->extra1 = &insn->min;
 		sysctl->extra2 = &insn->max;
 		sysctl->proc_handler = emulation_proc_handler;
-- 
2.34.1