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drbd: fix access of unallocated pages and kernel panic

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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>
This commit is contained in:
Lars Ellenberg 2012-06-08 13:18:51 +02:00 committed by Philipp Reisner
parent 4fd1ffaa12
commit 4eccc57979
1 changed files with 9 additions and 2 deletions
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11
drivers/block/drbd/drbd_bitmap.c
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@ -1475,10 +1475,17 @@ void _drbd_bm_set_bits(struct drbd_conf *mdev, const unsigned long s, const unsi
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.