[PATCH] stackleak: Fix a race between stack erasing sysctl handlers

[Muchun Song]

There is a race between the assignment of `table->data` and write value
to the pointer of `table->data` in the __do_proc_doulongvec_minmax().
Fix this by duplicating the `table`, and only update the duplicate of
it.

Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack 
erasing")
Signed-off-by: Muchun Song <songmuc...@bytedance.com>
---
 kernel/stackleak.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index a8fc9ae1d03d..fd95b87478ff 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c
@@ -25,10 +25,15 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
        int ret = 0;
        int state = !static_branch_unlikely(&stack_erasing_bypass);
        int prev_state = state;
+       struct ctl_table dup_table = *table;
 
-       table->data = &state;
-       table->maxlen = sizeof(int);
-       ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+       /*
+        * In order to avoid races with __do_proc_doulongvec_minmax(), we
+        * can duplicate the @table and alter the duplicate of it.
+        */
+       dup_table.data = &state;
+       dup_table.maxlen = sizeof(int);
+       ret = proc_dointvec_minmax(&dup_table, write, buffer, lenp, ppos);
        state = !!state;
        if (ret || !write || state == prev_state)
                return ret;
-- 
2.11.0