[PATCH 4.9 274/310] random: use lockless method of accessing and updating f->reg_idx

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <ty...@mit.edu>

commit 92e75428ffc90e2a0321062379f883f3671cfebe upstream.

Linus pointed out that there is a much more efficient way of avoiding
the problem that we were trying to address in commit 9dfa7bba35ac0:
"fix race in drivers/char/random.c:get_reg()".

Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Cc: Michael Schmitz <schmitz...@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 drivers/char/random.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1115,15 +1115,15 @@ static void add_interrupt_bench(cycles_t
 static __u32 get_reg(struct fast_pool *f, struct pt_regs *regs)
 {
        __u32 *ptr = (__u32 *) regs;
-       unsigned long flags;
+       unsigned int idx;
 
        if (regs == NULL)
                return 0;
-       local_irq_save(flags);
-       if (f->reg_idx >= sizeof(struct pt_regs) / sizeof(__u32))
-               f->reg_idx = 0;
-       ptr += f->reg_idx++;
-       local_irq_restore(flags);
+       idx = READ_ONCE(f->reg_idx);
+       if (idx >= sizeof(struct pt_regs) / sizeof(__u32))
+               idx = 0;
+       ptr += idx++;
+       WRITE_ONCE(f->reg_idx, idx);
        return *ptr;
 }