linux-kernel - Re: [PATCH] fs: btrfs: fix a possible use-after-free bug caused by incorrect error handling in prepare_to

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Date:   Thu, 21 Jul 2022 10:47:05 -0400
From:   Sweet Tea Dorminy <sweettea-kernel@...miny.me>
To:     Zixuan Fu <r33s3n6@...il.com>, clm@...com, josef@...icpanda.com,
        dsterba@...e.com
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        baijiaju1990@...il.com, TOTE Robot <oslab@...nghua.edu.cn>
Subject: Re: [PATCH] fs: btrfs: fix a possible use-after-free bug caused by
 incorrect error handling in prepare_to_relocate()


On 7/21/22 03:48, Zixuan Fu wrote:
> In btrfs_relocate_block_group(), the structure variable rc is allocated.
> Then btrfs_relocate_block_group() calls relocate_block_group() ->
> prepare_to_relocate() -> set_reloc_control(), and assigns rc to the
> variable fs_info->reloc_ctl. When prepare_to_relocate() returns, it
> calls btrfs_commit_transaction() -> btrfs_start_dirty_block_groups()
> -> btrfs_alloc_path() -> kmem_cache_zalloc(), which may fail. When the
> failure occurs, btrfs_relocate_block_group() detects the error and frees
> rc and doesn't set fs_info->reloc_ctl to NULL. After that, in
> btrfs_init_reloc_root(), rc is retrieved from fs_info->reloc_ctl and
> then used, which may cause a use-after-free bug.
> 
> This possible bug can be triggered by calling btrfs_ioctl_balance()
> before calling btrfs_ioctl_defrag().
> 
> To fix this possible bug, in prepare_to_relocate(), an if statement
> is added to check whether btrfs_commit_transaction() fails. If the
> failure occurs, unset_reloc_control() is called to set
> fs_info->reloc_ctl to NULL.
> 
> The error log in our fault-injection testing is shown as follows:
> 
> [   58.751070] BUG: KASAN: use-after-free in btrfs_init_reloc_root+0x7ca/0x920 [btrfs]
> ...
> [   58.753577] Call Trace:
> ...
> [   58.755800]  kasan_report+0x45/0x60
> [   58.756066]  btrfs_init_reloc_root+0x7ca/0x920 [btrfs]
> [   58.757304]  record_root_in_trans+0x792/0xa10 [btrfs]
> [   58.757748]  btrfs_record_root_in_trans+0x463/0x4f0 [btrfs]
> [   58.758231]  start_transaction+0x896/0x2950 [btrfs]
> [   58.758661]  btrfs_defrag_root+0x250/0xc00 [btrfs]
> [   58.759083]  btrfs_ioctl_defrag+0x467/0xa00 [btrfs]
> [   58.759513]  btrfs_ioctl+0x3c95/0x114e0 [btrfs]
> ...
> [   58.768510] Allocated by task 23683:
> [   58.768777]  ____kasan_kmalloc+0xb5/0xf0
> [   58.769069]  __kmalloc+0x227/0x3d0
> [   58.769325]  alloc_reloc_control+0x10a/0x3d0 [btrfs]
> [   58.769755]  btrfs_relocate_block_group+0x7aa/0x1e20 [btrfs]
> [   58.770228]  btrfs_relocate_chunk+0xf1/0x760 [btrfs]
> [   58.770655]  __btrfs_balance+0x1326/0x1f10 [btrfs]
> [   58.771071]  btrfs_balance+0x3150/0x3d30 [btrfs]
> [   58.771472]  btrfs_ioctl_balance+0xd84/0x1410 [btrfs]
> [   58.771902]  btrfs_ioctl+0x4caa/0x114e0 [btrfs]
> ...
> [   58.773337] Freed by task 23683:
> ...
> [   58.774815]  kfree+0xda/0x2b0
> [   58.775038]  free_reloc_control+0x1d6/0x220 [btrfs]
> [   58.775465]  btrfs_relocate_block_group+0x115c/0x1e20 [btrfs]
> [   58.775944]  btrfs_relocate_chunk+0xf1/0x760 [btrfs]
> [   58.776369]  __btrfs_balance+0x1326/0x1f10 [btrfs]
> [   58.776784]  btrfs_balance+0x3150/0x3d30 [btrfs]
> [   58.777185]  btrfs_ioctl_balance+0xd84/0x1410 [btrfs]
> [   58.777621]  btrfs_ioctl+0x4caa/0x114e0 [btrfs]
> ...
> 
> Reported-by: TOTE Robot <oslab@...nghua.edu.cn>
> Signed-off-by: Zixuan Fu <r33s3n6@...il.com>
Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@...miny.me>
> ---
>   fs/btrfs/relocation.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index a6dc827e75af..342ade83d062 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -3573,7 +3573,12 @@ int prepare_to_relocate(struct reloc_control *rc)
>   		 */
>   		return PTR_ERR(trans);
>   	}
> -	return btrfs_commit_transaction(trans);
> +
> +	ret = btrfs_commit_transaction(trans);
> +	if (ret)
> +		unset_reloc_control(rc);
> +
> +	return ret;
>   }
>   
>   static noinline_for_stack int relocate_block_group(struct reloc_control *rc)