2020-05-09 05:39:11

by Jia-Ju Bai

Subject: [PATCH 4/4] fs: btrfs: fix a data race in btrfs_block_rsv_release()

The functions btrfs_block_rsv_release() and
btrfs_update_delayed_refs_rsv() are concurrently executed at runtime in
the following call contexts:

Thread 1:
btrfs_file_write_iter()
btrfs_buffered_write()
btrfs_delalloc_release_extents()
btrfs_inode_rsv_release()
__btrfs_block_rsv_release()

Thread 2:
finish_ordered_fn()
btrfs_finish_ordered_io()
insert_reserved_file_extent()
__btrfs_drop_extents()
btrfs_free_extent()
btrfs_add_delayed_data_ref()
btrfs_update_delayed_refs_rsv()

In __btrfs_block_rsv_release():
else if (... && !delayed_rsv->full)

In btrfs_update_delayed_refs_rsv():
spin_lock(&delayed_rsv->lock);
delayed_rsv->size += num_bytes;
delayed_rsv->full = 0;
spin_unlock(&delayed_rsv->lock);

Thus a data race for delayed_rsv->full can occur.
This race was found and actually reproduced by our conccurency fuzzer.

To fix this race, the spinlock delayed_rsv->lock is used to
protect the access to delayed_rsv->full in btrfs_block_rsv_release().

Signed-off-by: Jia-Ju Bai <[email protected]>
---
fs/btrfs/block-rsv.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
index 27efec8f7c5b..89c53a7137b4 100644
--- a/fs/btrfs/block-rsv.c
+++ b/fs/btrfs/block-rsv.c
@@ -277,6 +277,11 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
struct btrfs_block_rsv *delayed_rsv = &fs_info->delayed_refs_rsv;
struct btrfs_block_rsv *target = NULL;
+ unsigned short full = 0;
+
+ spin_lock(&delayed_rsv->lock);
+ full = delayed_rsv->full;
+ spin_unlock(&delayed_rsv->lock);

/*
* If we are the delayed_rsv then push to the global rsv, otherwise dump
@@ -284,7 +289,7 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
*/
if (block_rsv == delayed_rsv)
target = global_rsv;
- else if (block_rsv != global_rsv && !delayed_rsv->full)
+ else if (block_rsv != global_rsv && !full)
target = delayed_rsv;

if (target && block_rsv->space_info != target->space_info)
--
2.17.1

2020-05-12 22:23:14

by David Sterba

Subject: Re: [PATCH 4/4] fs: btrfs: fix a data race in btrfs_block_rsv_release()

On Sat, May 09, 2020 at 01:34:31PM +0800, Jia-Ju Bai wrote:
> The functions btrfs_block_rsv_release() and
> btrfs_update_delayed_refs_rsv() are concurrently executed at runtime in
> the following call contexts:
>
> Thread 1:
> btrfs_file_write_iter()
> btrfs_buffered_write()
> btrfs_delalloc_release_extents()
> btrfs_inode_rsv_release()
> __btrfs_block_rsv_release()
>
> Thread 2:
> finish_ordered_fn()
> btrfs_finish_ordered_io()
> insert_reserved_file_extent()
> __btrfs_drop_extents()
> btrfs_free_extent()
> btrfs_add_delayed_data_ref()
> btrfs_update_delayed_refs_rsv()
>
> In __btrfs_block_rsv_release():
> else if (... && !delayed_rsv->full)
>
> In btrfs_update_delayed_refs_rsv():
> spin_lock(&delayed_rsv->lock);
> delayed_rsv->size += num_bytes;
> delayed_rsv->full = 0;
> spin_unlock(&delayed_rsv->lock);
>
> Thus a data race for delayed_rsv->full can occur.
> This race was found and actually reproduced by our conccurency fuzzer.
>
> To fix this race, the spinlock delayed_rsv->lock is used to
> protect the access to delayed_rsv->full in btrfs_block_rsv_release().
>
> Signed-off-by: Jia-Ju Bai <[email protected]>
> ---
> fs/btrfs/block-rsv.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
> index 27efec8f7c5b..89c53a7137b4 100644
> --- a/fs/btrfs/block-rsv.c
> +++ b/fs/btrfs/block-rsv.c
> @@ -277,6 +277,11 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
> struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
> struct btrfs_block_rsv *delayed_rsv = &fs_info->delayed_refs_rsv;
> struct btrfs_block_rsv *target = NULL;
> + unsigned short full = 0;
> +
> + spin_lock(&delayed_rsv->lock);
> + full = delayed_rsv->full;
> + spin_unlock(&delayed_rsv->lock);
>
> /*
> * If we are the delayed_rsv then push to the global rsv, otherwise dump
> @@ -284,7 +289,7 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
> */
> if (block_rsv == delayed_rsv)
> target = global_rsv;
> - else if (block_rsv != global_rsv && !delayed_rsv->full)
> + else if (block_rsv != global_rsv && !full)

This has been reported as suspicous
https://lore.kernel.org/linux-btrfs/CAAwBoOJDjei5Hnem155N_cJwiEkVwJYvgN-tQrwWbZQGhFU=cA@mail.gmail.com/

and there's an answer that this is racy but does not cause any
unexpected behaviour.

2020-05-13 02:20:09

by Jia-Ju Bai

Subject: Re: [PATCH 4/4] fs: btrfs: fix a data race in btrfs_block_rsv_release()

On 2020/5/13 6:18, David Sterba wrote:
> On Sat, May 09, 2020 at 01:34:31PM +0800, Jia-Ju Bai wrote:
>> The functions btrfs_block_rsv_release() and
>> btrfs_update_delayed_refs_rsv() are concurrently executed at runtime in
>> the following call contexts:
>>
>> Thread 1:
>> btrfs_file_write_iter()
>> btrfs_buffered_write()
>> btrfs_delalloc_release_extents()
>> btrfs_inode_rsv_release()
>> __btrfs_block_rsv_release()
>>
>> Thread 2:
>> finish_ordered_fn()
>> btrfs_finish_ordered_io()
>> insert_reserved_file_extent()
>> __btrfs_drop_extents()
>> btrfs_free_extent()
>> btrfs_add_delayed_data_ref()
>> btrfs_update_delayed_refs_rsv()
>>
>> In __btrfs_block_rsv_release():
>> else if (... && !delayed_rsv->full)
>>
>> In btrfs_update_delayed_refs_rsv():
>> spin_lock(&delayed_rsv->lock);
>> delayed_rsv->size += num_bytes;
>> delayed_rsv->full = 0;
>> spin_unlock(&delayed_rsv->lock);
>>
>> Thus a data race for delayed_rsv->full can occur.
>> This race was found and actually reproduced by our conccurency fuzzer.
>>
>> To fix this race, the spinlock delayed_rsv->lock is used to
>> protect the access to delayed_rsv->full in btrfs_block_rsv_release().
>>
>> Signed-off-by: Jia-Ju Bai <[email protected]>
>> ---
>> fs/btrfs/block-rsv.c | 7 ++++++-
>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c
>> index 27efec8f7c5b..89c53a7137b4 100644
>> --- a/fs/btrfs/block-rsv.c
>> +++ b/fs/btrfs/block-rsv.c
>> @@ -277,6 +277,11 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
>> struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
>> struct btrfs_block_rsv *delayed_rsv = &fs_info->delayed_refs_rsv;
>> struct btrfs_block_rsv *target = NULL;
>> + unsigned short full = 0;
>> +
>> + spin_lock(&delayed_rsv->lock);
>> + full = delayed_rsv->full;
>> + spin_unlock(&delayed_rsv->lock);
>>
>> /*
>> * If we are the delayed_rsv then push to the global rsv, otherwise dump
>> @@ -284,7 +289,7 @@ u64 btrfs_block_rsv_release(struct btrfs_fs_info *fs_info,
>> */
>> if (block_rsv == delayed_rsv)
>> target = global_rsv;
>> - else if (block_rsv != global_rsv && !delayed_rsv->full)
>> + else if (block_rsv != global_rsv && !full)
> This has been reported as suspicous
> https://lore.kernel.org/linux-btrfs/CAAwBoOJDjei5Hnem155N_cJwiEkVwJYvgN-tQrwWbZQGhFU=cA@mail.gmail.com/
>
> and there's an answer that this is racy but does not cause any
> unexpected behaviour.

Okay, thanks :)


Best wishes,
Jia-Ju Bai