UBI: Fastmap: Fix race in ubi_eba_atomic_leb_change()

This function a) requests a new PEB, b) writes data to it,
c) returns the old PEB and d) registers the new PEB in the EBA table.

For the non-fastmap case this works perfectly fine and is powercut safe.
Is fastmap enabled this can lead to issues.
If a new fastmap is written between a) and c) the freshly requested PEB
is no longer in a pool and will not be scanned upon attaching.
If now a powercut happens between c) and d) the freshly requested PEB
will not be scanned and the old one got already scheduled for erase.
After attaching the EBA table will point to a erased PEB.

Fix this issue by swapping steps c) and d).
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/eba.c

@@ -900,7 +900,7 @@ int ubi_eba_write_leb_st(struct ubi_device *ubi, struct ubi_volume *vol,
 int ubi_eba_atomic_leb_change(struct ubi_device *ubi, struct ubi_volume *vol,
 			      int lnum, const void *buf, int len)
 {
-	int err, pnum, tries = 0, vol_id = vol->vol_id;
+	int err, pnum, old_pnum, tries = 0, vol_id = vol->vol_id;
 	struct ubi_vid_hdr *vid_hdr;
 	uint32_t crc;
 
@@ -963,16 +963,17 @@ int ubi_eba_atomic_leb_change(struct ubi_device *ubi, struct ubi_volume *vol,
 		goto write_error;
 	}
 
-	if (vol->eba_tbl[lnum] >= 0) {
-		err = ubi_wl_put_peb(ubi, vol_id, lnum, vol->eba_tbl[lnum], 0);
-		if (err)
-			goto out_leb_unlock;
-	}
-
 	down_read(&ubi->fm_sem);
+	old_pnum = vol->eba_tbl[lnum];
 	vol->eba_tbl[lnum] = pnum;
 	up_read(&ubi->fm_sem);
 
+	if (old_pnum >= 0) {
+		err = ubi_wl_put_peb(ubi, vol_id, lnum, old_pnum, 0);
+		if (err)
+			goto out_leb_unlock;
+	}
+
 out_leb_unlock:
 	leb_write_unlock(ubi, vol_id, lnum);
 out_mutex: