From: Lars Ellenberg Date: Fri, 8 Jun 2012 11:18:51 +0000 (+0200) Subject: drbd: fix access of unallocated pages and kernel panic X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=4eccc579795290a58e2262fa4e9d083d7672e699;p=linux-beck.git drbd: fix access of unallocated pages and kernel panic BUG: unable to handle kernel NULL pointer dereference at (null) ... [] ? _drbd_bm_set_bits+0x151/0x240 [drbd] [] ? 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 Signed-off-by: Lars Ellenberg --- diff --git a/drivers/block/drbd/drbd_bitmap.c b/drivers/block/drbd/drbd_bitmap.c index b5c5ff53cb57..fcb956bb4b4c 100644 --- a/drivers/block/drbd/drbd_bitmap.c +++ b/drivers/block/drbd/drbd_bitmap.c @@ -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.