[PATCH] md: add max_disks bound check in older version

[Aaditya Kansal <aadityakansal390@gmail.com>]

#syz test

syzbot reported array-index-out-of-bounds in super_90_sync()
    UBSAN: array-index-out-of-bounds in drivers/md/md.c:1697:17
    index 124 is out of range for type 'mdp_disk_t [27]'
    CPU: 1 UID: 0 PID: 10197 Comm: syz.3.1362 Tainted: G    L
    syzkaller #0 PREEMPT(full)
    Tainted: [L]=SOFTLOCKUP
    Hardware name: Google Google Compute Engine/Google Compute Engine,
    BIOS Google 08/05/2026
    Call Trace:
     <TASK>
     __dump_stack lib/dump_stack.c:94 [inline]
     dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
     ubsan_epilogue+0xa/0x30 lib/ubsan.c:233
     __ubsan_handle_out_of_bounds+0xcc/0xf0 lib/ubsan.c:455
     super_90_sync+0x1eed/0x1ff0 drivers/md/md.c:1697
 ...

super_90_sync() exists for older version 0.90, which supports up to 27
maximum disks (MD_SB_DISKS). This function doesn't contain any bound checks for
raid disks. The repro wrote higher value (124) in raid_disks sysfs attribute
to trigger this bug.

Add bound checks in super_90_sync() to prevent index-out-of-bounds error. This
bound check hardens super_90_sync() function, such that it can catch max disk
index-out-of-bounds coming from any path, including raid_disks
attribute.

Signed-off-by: Aaditya Kansal <aadityakansal390@gmail.com>
---
 drivers/md/md.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 680b34a63cb3..f5fce41637f1 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -1693,6 +1693,12 @@ static void super_90_sync(struct mddev *mddev, struct md_rdev *rdev)
 			desc_nr = rdev2->raid_disk;
 		else
 			desc_nr = next_spare++;
+
+		if (desc_nr >= MD_SB_DISKS) {
+			pr_warn("md: raid_disks = %d exceeds max_disks = %d for version 0.90\n",
+					desc_nr, MD_SB_DISKS);
+			continue;
+		}
 		rdev2->desc_nr = desc_nr;
 		d = &sb->disks[rdev2->desc_nr];
 		nr_disks++;
@@ -1722,7 +1728,7 @@ static void super_90_sync(struct mddev *mddev, struct md_rdev *rdev)
 			d->state |= (1<<MD_DISK_FAILFAST);
 	}
 	/* now set the "removed" and "faulty" bits on any missing devices */
-	for (i=0 ; i < mddev->raid_disks ; i++) {
+	for (i = 0; i < mddev->raid_disks && i < MD_SB_DISKS; i++) {
 		mdp_disk_t *d = &sb->disks[i];
 		if (d->state == 0 && d->number == 0) {
 			d->number = i;
--
2.55.0