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[PATCH 4.5 095/128] ext4: address UBSAN warning in mb_find_order_for_block()

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4.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nicolai Stange <nicstange@gmail.com>

commit b5cb316cdf3a3f5f6125412b0f6065185240cfdc upstream.

Currently, in mb_find_order_for_block(), there's a loop like the following:

  while (order <= e4b->bd_blkbits + 1) {
    ...
    bb += 1 << (e4b->bd_blkbits - order);
  }

Note that the updated bb is used in the loop's next iteration only.

However, at the last iteration, that is at order == e4b->bd_blkbits + 1,
the shift count becomes negative (c.f. C99 6.5.7(3)) and UBSAN reports

  UBSAN: Undefined behaviour in fs/ext4/mballoc.c:1281:11
  shift exponent -1 is negative
  [...]
  Call Trace:
   [<ffffffff818c4d35>] dump_stack+0xbc/0x117
   [<ffffffff818c4c79>] ? _atomic_dec_and_lock+0x169/0x169
   [<ffffffff819411bb>] ubsan_epilogue+0xd/0x4e
   [<ffffffff81941cbc>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
   [<ffffffff81941ac1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
   [<ffffffff816e93a0>] ? ext4_mb_generate_from_pa+0x590/0x590
   [<ffffffff816502c8>] ? ext4_read_block_bitmap_nowait+0x598/0xe80
   [<ffffffff816e7b7e>] mb_find_order_for_block+0x1ce/0x240
   [...]

Unless compilers start to do some fancy transformations (which at least
GCC 6.0.0 doesn't currently do), the issue is of cosmetic nature only: the
such calculated value of bb is never used again.

Silence UBSAN by introducing another variable, bb_incr, holding the next
increment to apply to bb and adjust that one by right shifting it by one
position per loop iteration.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=114701
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=112161

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/ext4/mballoc.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1259,6 +1259,7 @@ static void ext4_mb_unload_buddy(struct
 static int mb_find_order_for_block(struct ext4_buddy *e4b, int block)
 {
 	int order = 1;
+	int bb_incr = 1 << (e4b->bd_blkbits - 1);
 	void *bb;
 
 	BUG_ON(e4b->bd_bitmap == e4b->bd_buddy);
@@ -1271,7 +1272,8 @@ static int mb_find_order_for_block(struc
 			/* this block is part of buddy of order 'order' */
 			return order;
 		}
-		bb += 1 << (e4b->bd_blkbits - order);
+		bb += bb_incr;
+		bb_incr >>= 1;
 		order++;
 	}
 	return 0;

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[PATCH 4.5 000/128] 4.5.7-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 01:00 +0200
  [PATCH 4.5 126/128] Btrfs: fix unexpected return value of fiemap Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 127/128] btrfs: scrub: Set bbio to NULL before calling btrfs_map_block Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 098/128] PM / sleep: Handle failures in device_suspend_late() consistently Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 093/128] ext4: fix check of dqget() return value in ext4_ioctl_setproject() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 110/128] btrfs: allow balancing to dup with multi-device Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 128/128] btrfs: make state preallocation more speculative in __set_extent_bit Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 120/128] btrfs: fix int32 overflow in shrink_delalloc(). Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 095/128] ext4: address UBSAN warning in mb_find_order_for_block() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 102/128] gcov: disable tree-loop-im to reduce stack usage Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 097/128] nfs: avoid race that crashes nfs_init_commit Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 100/128] scripts/package/Makefile: rpmbuild add support of RPMOPTS Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 115/128] btrfs: add check to sysfs handler of label Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 096/128] ext4: silence UBSAN in ext4_mb_init() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 116/128] Btrfs: fix divide error upon chunks stripe_len Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 123/128] btrfs: pass the right error code to the btrfs_std_error Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 113/128] btrfs: fix lock dep warning, move scratch dev out of device_list_mutex and uuid_mutex Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 122/128] btrfs: fix memory leak during RAID 5/6 device replacement Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 109/128] Btrfs: do not create empty block group if we have allocated data Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 114/128] btrfs: add read-only check to sysfs handler of features Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 119/128] btrfs: add write protection to SET_FEATURES ioctl Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 117/128] Btrfs: remove BUG_ON()s in btrfs_map_block Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 111/128] btrfs: fix mixed block count of available space Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 118/128] btrfs: fix lock dep warning move scratch super outside of chunk_mutex Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 125/128] Btrfs: fix empty symlink after creating symlink and fsync parent dir Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 101/128] mm: thp: avoid false positive VM_BUG_ON_PAGE in page_move_anon_rmap() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:40 +0200
  [PATCH 4.5 112/128] btrfs: avoid overflowing f_bfree Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-06-06 16:50 +0200
  Re: [PATCH 4.5 000/128] 4.5.7-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2016-06-06 19:30 +0200
  Re: [PATCH 4.5 000/128] 4.5.7-stable review Guenter Roeck <linux@roeck-us.net> - 2016-06-07 15:40 +0200

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