[PATCH v5.10] arm64: fix a concurrency issue in emulation_proc_handler()

[ruanjinjie]

This patch is addressing an issue in stable linux-5.10 only.

In linux-6.1, the related code is refactored in 124c49b1b("arm64:
armv8_deprecated: rework deprected instruction handling") and
this issue was incidentally fixed. However, the patch changes a lot and
is not specific to this issue.

This patch is designed to solve the problem of repeated addition of linked
lists described below with few changes.

In emulation_proc_handler(), read and write operations are performed on
insn->current_mode. In the concurrency scenario, mutex only protects
writing insn->current_mode, and not protects the read. Suppose there are
two concurrent tasks, task1 updates insn->current_mode to INSN_EMULATE
in the critical section, the prev_mode of task2 is still the old data
INSN_UNDEF of insn->current_mode. As a result, two tasks call
update_insn_emulation_mode twice with prev_mode = INSN_UNDEF and
current_mode = INSN_EMULATE, then call register_emulation_hooks twice,
resulting in a list_add double problem.

Call trace:
 __list_add_valid+0xd8/0xe4
 register_undef_hook+0x94/0x13c
 update_insn_emulation_mode+0xd0/0x12c
 emulation_proc_handler+0xd8/0xf4
 proc_sys_call_handler+0x140/0x250
 proc_sys_write+0x1c/0x2c
 new_sync_write+0xec/0x18c
 vfs_write+0x214/0x2ac
 ksys_write+0x70/0xfc
 __arm64_sys_write+0x24/0x30
 el0_svc_common.constprop.0+0x7c/0x1bc
 do_el0_svc+0x2c/0x94
 el0_svc+0x20/0x30
 el0_sync_handler+0xb0/0xb4
 el0_sync+0x160/0x180

Fixes: af483947d472 ("arm64: fix oops in concurrently setting insn_emulation sysctls")
Cc: stable at vger.kernel.org#5.10.x
Cc: gregkh at linuxfoundation.org
Signed-off-by: ruanjinjie <ruanjinjie at huawei.com>
Acked-by: Mark Rutland <mark.rutland at arm.com>
---
 arch/arm64/kernel/armv8_deprecated.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
index 91b8a8378ba3..5f8da0d7b832 100644
--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -208,10 +208,12 @@ static int emulation_proc_handler(struct ctl_table *table, int write,
 				  loff_t *ppos)
 {
 	int ret = 0;
-	struct insn_emulation *insn = container_of(table->data, struct insn_emulation, current_mode);
-	enum insn_emulation_mode prev_mode = insn->current_mode;
+	struct insn_emulation *insn;
+	enum insn_emulation_mode prev_mode;
 
 	mutex_lock(&insn_emulation_mutex);
+	insn = container_of(table->data, struct insn_emulation, current_mode);
+	prev_mode = insn->current_mode;
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 
 	if (ret || !write || prev_mode == insn->current_mode)
-- 
2.25.1