Patch "ext4: address UBSAN warning in mb_find_order_for_block()" has been added to the 4.6-stable tree

Patch "ext4: address UBSAN warning in mb_find_order_for_block()" has been added to the 4.6-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    ext4: address UBSAN warning in mb_find_order_for_block()

to the 4.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ext4-address-ubsan-warning-in-mb_find_order_for_block.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From b5cb316cdf3a3f5f6125412b0f6065185240cfdc Mon Sep 17 00:00:00 2001
From: Nicolai Stange <nicstange@gmail.com>
Date: Thu, 5 May 2016 17:38:03 -0400
Subject: ext4: address UBSAN warning in mb_find_order_for_block()

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
@@ -1266,6 +1266,7 @@ 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);
@@ -1278,7 +1279,8 @@ 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;


Patches currently in stable-queue which might be from nicstange@gmail.com are

queue-4.6/ext4-address-ubsan-warning-in-mb_find_order_for_block.patch
queue-4.6/ext4-silence-ubsan-in-ext4_mb_init.patch