drbd: fix access of unallocated pages and kernel panic
BUG: unable to handle kernel NULL pointer dereference at (null)
...
[<d1e17561>] ? _drbd_bm_set_bits+0x151/0x240 [drbd]
[<d1e236f8>] ? receive_bitmap+0x4f8/0xbc0 [drbd]
This fixes an off-by-one error in the receive_bitmap() path,
if run-length encoded bitmap transfer is enabled.
If the bitmap is an exact multiple of PAGE_SIZE, which means the visible
capacity of the drbd device is an exact multiple of 128 MiB (for 4k page
size), and bitmap compression (use-rle) is enabled (which became default
with 8.4), and the very last bit is dirty and reported in an rle
comressed bitmap packet, we ended up trying to kmap_atomic a page pointer
that does not exist (bitmap->bm_pages[last index + 1]).
bug introduced by:
Date: Fri Jul 24 15:33:24 2009 +0200
set bits: optimize for complete last word, fix off-by-one-word corner case
made effective by:
Date: Thu Dec 16 00:32:38 2010 +0100
drbd: get rid of unused debug code
Long time ago, we had paranoia code in the bitmap that allocated one
extra word, assigned a magic value, and checked on every occasion that
the magic value was still unchanged.
That debug code is unused, the extra long word complicates code a bit.
Get rid of it.
No-one triggered this bug in the last few years, because a large subset
of our userbase is unaffected:
* typically the last few blocks of a device are not modified
frequently, and remain unset
* use-rle was disabled by default in drbd < 8.4
* those with slightly "odd" device sizes, or
* drbd internal meta data (which will skew the device size slightly,
thus makes it harder to have a bug relevant device size)
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
diff --git a/drivers/block/drbd/drbd_bitmap.c b/drivers/block/drbd/drbd_bitmap.c
index b5c5ff5..fcb956b 100644
--- a/drivers/block/drbd/drbd_bitmap.c
+++ b/drivers/block/drbd/drbd_bitmap.c
@@ -1475,10 +1475,17 @@
first_word = 0;
spin_lock_irq(&b->bm_lock);
}
-
/* last page (respectively only page, for first page == last page) */
last_word = MLPP(el >> LN2_BPL);
- bm_set_full_words_within_one_page(mdev->bitmap, last_page, first_word, last_word);
+
+ /* consider bitmap->bm_bits = 32768, bitmap->bm_number_of_pages = 1. (or multiples).
+ * ==> e = 32767, el = 32768, last_page = 2,
+ * and now last_word = 0.
+ * We do not want to touch last_page in this case,
+ * as we did not allocate it, it is not present in bitmap->bm_pages.
+ */
+ if (last_word)
+ bm_set_full_words_within_one_page(mdev->bitmap, last_page, first_word, last_word);
/* possibly trailing bits.
* example: (e & 63) == 63, el will be e+1.