Patch "random: use lockless method of accessing and updating f->reg_idx" has been added to the 3.18-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    random: use lockless method of accessing and updating f->reg_idx

to the 3.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     random-use-lockless-method-of-accessing-and-updating-f-reg_idx.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 92e75428ffc90e2a0321062379f883f3671cfebe Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Wed, 7 Jun 2017 19:01:32 -0400
Subject: random: use lockless method of accessing and updating f->reg_idx

From: Theodore Ts'o <tytso@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 <tytso@mit.edu>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -863,15 +863,15 @@ 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;
 }
 


Patches currently in stable-queue which might be from tytso@mit.edu are

queue-3.18/fix-race-in-drivers-char-random.c-get_reg.patch
queue-3.18/ext4-fix-off-by-one-on-max-nr_pages-in-ext4_find_unwritten_pgoff.patch
queue-3.18/random-use-lockless-method-of-accessing-and-updating-f-reg_idx.patch