[PATCH v2 0/2] fs/ext4: mballoc.c: silence two UBSAN reports

[PATCH v2 0/2] fs/ext4: mballoc.c: silence two UBSAN reports

[Nicolai Stange]

v1 can be found here:

  http://lkml.kernel.org/g/1458417247-3164-1-git-send-email-nicstange@gmail.com


Unfortunately, I failed to recognize that the very same issue fixed by v1
appears again at another place in fs/ext4/mballoc.c.

Hence a v2 with the [2/2] addressing that second occurence.


Applicable to linux-next-20160318.


Changes to v1:

 [1/2] ("fs/ext4: mb_find_order_for_block(): silence UBSAN")
  - corrected GCC version from 4.6.0 to 6.0.0 in commit message 

 [2/2] ("fs/ext4: ext4_mb_init(): silence UBSAN")
  - new.


Nicolai Stange (2):
  fs/ext4: mb_find_order_for_block(): silence UBSAN
  fs/ext4: ext4_mb_init(): silence UBSAN

 fs/ext4/mballoc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

-- 
2.7.3

[PATCH v2 1/2] fs/ext4: mb_find_order_for_block(): silence UBSAN

[Nicolai Stange]

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.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 fs/ext4/mballoc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 50e05df..4bc89fe 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1266,6 +1266,7 @@ static void ext4_mb_unload_buddy(struct ext4_buddy *e4b)
 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(struct ext4_buddy *e4b, int block)
 			/* 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;
-- 
2.7.3

[PATCH v2 2/2] fs/ext4: ext4_mb_init(): silence UBSAN

[Nicolai Stange]

Currently, in ext4_mb_init(), there's a loop like the following:

  do {
    ...
    offset += 1 << (sb->s_blocksize_bits - i);
    i++;
  } while (i <= sb->s_blocksize_bits + 1);

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

However, at the last iteration, that is at i == sb->s_blocksize_bits + 1,
the shift count becomes equal to (unsigned)-1 > 31 (c.f. C99 6.5.7(3))
and UBSAN reports

  UBSAN: Undefined behaviour in fs/ext4/mballoc.c:2621:15
  shift exponent 4294967295 is too large for 32-bit type 'int'
  [...]
  Call Trace:
   [<ffffffff818c4d25>] dump_stack+0xbc/0x117
   [<ffffffff818c4c69>] ? _atomic_dec_and_lock+0x169/0x169
   [<ffffffff819411ab>] ubsan_epilogue+0xd/0x4e
   [<ffffffff81941cac>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
   [<ffffffff81941ab1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
   [<ffffffff814b6dc1>] ? kmem_cache_alloc+0x101/0x390
   [<ffffffff816fc13b>] ? ext4_mb_init+0x13b/0xfd0
   [<ffffffff814293c7>] ? create_cache+0x57/0x1f0
   [<ffffffff8142948a>] ? create_cache+0x11a/0x1f0
   [<ffffffff821c2168>] ? mutex_lock+0x38/0x60
   [<ffffffff821c23ab>] ? mutex_unlock+0x1b/0x50
   [<ffffffff814c26ab>] ? put_online_mems+0x5b/0xc0
   [<ffffffff81429677>] ? kmem_cache_create+0x117/0x2c0
   [<ffffffff816fcc49>] ext4_mb_init+0xc49/0xfd0
   [...]

Observe that the mentioned shift exponent, 4294967295, equals (unsigned)-1.

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 offset is never used again.

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

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 fs/ext4/mballoc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4bc89fe..8dc0d9b 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2585,7 +2585,7 @@ int ext4_mb_init(struct super_block *sb)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	unsigned i, j;
-	unsigned offset;
+	unsigned offset, offset_incr;
 	unsigned max;
 	int ret;
 
@@ -2614,11 +2614,13 @@ int ext4_mb_init(struct super_block *sb)
 
 	i = 1;
 	offset = 0;
+	offset_incr = 1 << (sb->s_blocksize_bits - 1);
 	max = sb->s_blocksize << 2;
 	do {
 		sbi->s_mb_offsets[i] = offset;
 		sbi->s_mb_maxs[i] = max;
-		offset += 1 << (sb->s_blocksize_bits - i);
+		offset += offset_incr;
+		offset_incr = offset_incr >> 1;
 		max = max >> 1;
 		i++;
 	} while (i <= sb->s_blocksize_bits + 1);
-- 
2.7.3

Re: [PATCH v2 1/2] fs/ext4: mb_find_order_for_block(): silence UBSAN

[Theodore Ts'o]

On Sat, Mar 19, 2016 at 10:12:04PM +0100, Nicolai Stange wrote:
> 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.
> 
> Signed-off-by: Nicolai Stange <nicstange@gmail.com>

Thanks, applied.

					- Ted

Re: [PATCH v2 2/2] fs/ext4: ext4_mb_init(): silence UBSAN

[Theodore Ts'o]

On Sat, Mar 19, 2016 at 10:12:05PM +0100, Nicolai Stange wrote:
> Currently, in ext4_mb_init(), there's a loop like the following:
> 
>   do {
>     ...
>     offset += 1 << (sb->s_blocksize_bits - i);
>     i++;
>   } while (i <= sb->s_blocksize_bits + 1);
> 
> Note that the updated offset is used in the loop's next iteration only.
> 
> However, at the last iteration, that is at i == sb->s_blocksize_bits + 1,
> the shift count becomes equal to (unsigned)-1 > 31 (c.f. C99 6.5.7(3))
> and UBSAN reports
> 
>   UBSAN: Undefined behaviour in fs/ext4/mballoc.c:2621:15
>   shift exponent 4294967295 is too large for 32-bit type 'int'
>   [...]
>   Call Trace:
>    [<ffffffff818c4d25>] dump_stack+0xbc/0x117
>    [<ffffffff818c4c69>] ? _atomic_dec_and_lock+0x169/0x169
>    [<ffffffff819411ab>] ubsan_epilogue+0xd/0x4e
>    [<ffffffff81941cac>] __ubsan_handle_shift_out_of_bounds+0x1fb/0x254
>    [<ffffffff81941ab1>] ? __ubsan_handle_load_invalid_value+0x158/0x158
>    [<ffffffff814b6dc1>] ? kmem_cache_alloc+0x101/0x390
>    [<ffffffff816fc13b>] ? ext4_mb_init+0x13b/0xfd0
>    [<ffffffff814293c7>] ? create_cache+0x57/0x1f0
>    [<ffffffff8142948a>] ? create_cache+0x11a/0x1f0
>    [<ffffffff821c2168>] ? mutex_lock+0x38/0x60
>    [<ffffffff821c23ab>] ? mutex_unlock+0x1b/0x50
>    [<ffffffff814c26ab>] ? put_online_mems+0x5b/0xc0
>    [<ffffffff81429677>] ? kmem_cache_create+0x117/0x2c0
>    [<ffffffff816fcc49>] ext4_mb_init+0xc49/0xfd0
>    [...]
> 
> Observe that the mentioned shift exponent, 4294967295, equals (unsigned)-1.
> 
> 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 offset is never used again.
> 
> Silence UBSAN by introducing another variable, offset_incr, holding the
> next increment to apply to offset and adjust that one by right shifting it
> by one position per loop iteration.
> 
> Signed-off-by: Nicolai Stange <nicstange@gmail.com>

Applied, thanks.

						- Ted