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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>
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1 changed files with 9 additions and 2 deletions
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@ -1475,9 +1475,16 @@ void _drbd_bm_set_bits(struct drbd_conf *mdev, const unsigned long s, const unsi
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first_word = 0;
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spin_lock_irq(&b->bm_lock);
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}
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/* last page (respectively only page, for first page == last page) */
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last_word = MLPP(el >> LN2_BPL);
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/* consider bitmap->bm_bits = 32768, bitmap->bm_number_of_pages = 1. (or multiples).
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* ==> e = 32767, el = 32768, last_page = 2,
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* and now last_word = 0.
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* We do not want to touch last_page in this case,
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* as we did not allocate it, it is not present in bitmap->bm_pages.
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*/
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if (last_word)
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bm_set_full_words_within_one_page(mdev->bitmap, last_page, first_word, last_word);
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/* possibly trailing bits.
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