[PATCH] md/raid5: set conf->mddev before the first setup_conf() error path

[Yogesh Gaur <yogeshgaur.83@gmail.com>]

free_conf() starts with log_exit(), which falls through to
raid5_has_ppl() when conf->log is NULL:

	static inline void log_exit(struct r5conf *conf)
	{
		if (conf->log)
			r5l_exit_log(conf);
		else if (raid5_has_ppl(conf))
			ppl_exit_log(conf);
	}

raid5_has_ppl() reads conf->mddev->flags. setup_conf() only assigns
conf->mddev after bioset_init(), but five of its error paths -- the
pending_data, alloc_thread_groups(), conf->disks, extra_page and
bioset_init() failures -- jump to "abort:" before that, and abort: calls
free_conf(). conf comes from kzalloc, so conf->mddev is still NULL and
free_conf() dereferences it:

	BUG: KASAN: null-ptr-deref in raid5_has_ppl drivers/md/raid5-log.h:54 [inline]
	BUG: KASAN: null-ptr-deref in log_exit drivers/md/raid5-log.h:128 [inline]
	BUG: KASAN: null-ptr-deref in free_conf+0x81/0x5d0 drivers/md/raid5.c:7549
	Read of size 8 at addr 0000000000000028 by task syz.3.20/5681
	Call Trace:
	 <TASK>
	 free_conf+0x81/0x5d0 drivers/md/raid5.c:7549
	 setup_conf+0x1720/0x2ad0 drivers/md/raid5.c:7885
	 raid5_run+0x8cc/0x2560 drivers/md/raid5.c:8129
	 md_run+0xc3d/0x1cd0 drivers/md/md.c:6779
	 do_md_run+0x35/0x720 drivers/md/md.c:6880
	 array_state_store+0x958/0xe90 drivers/md/md.c:-1
	 </TASK>

The faulting address is offsetof(struct mddev, flags).

conf->mddev is a back pointer that is constant for the lifetime of the
conf and does not depend on anything computed in between, so assign it
as soon as the conf is allocated. Nothing between the allocation and the
old assignment reads conf->mddev -- alloc_thread_groups() takes the conf
but never looks at its mddev, and the rdev_for_each() loop walks the
mddev argument directly.

All five paths are allocation failures, so this needs memory pressure or
fault injection to hit, which is how syzbot found it.

Reported-by: syzbot+270624bb31d478afe62e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=270624bb31d478afe62e
Fixes: ff875738edd4 ("raid5: separate header for log functions")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>
---
 drivers/md/raid5.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index b91545ce090d..f98d3bdd3484 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -7675,6 +7675,9 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	if (conf == NULL)
 		goto abort;
 
+	/* free_conf() dereferences this, so set it before the first goto */
+	conf->mddev = mddev;
+
 #if PAGE_SIZE != DEFAULT_STRIPE_SIZE
 	conf->stripe_size = DEFAULT_STRIPE_SIZE;
 	conf->stripe_shift = ilog2(DEFAULT_STRIPE_SIZE) - 9;
@@ -7743,7 +7746,6 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	ret = bioset_init(&conf->bio_split, BIO_POOL_SIZE, 0, 0);
 	if (ret)
 		goto abort;
-	conf->mddev = mddev;
 
 	ret = -ENOMEM;
 	conf->stripe_hashtbl = kzalloc(PAGE_SIZE, GFP_KERNEL);
-- 
2.55.0.windows.5