linux-kernel - Re: [PATCH] btrfs: fix a potential double put bug and some related use-after-free bugs

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Message-ID: <1dea8c28-8ca6-83d4-8eb0-84fe1ebb7cc9@suse.com>
Date:   Wed, 18 Aug 2021 15:11:45 +0300
From:   Nikolay Borisov <nborisov@...e.com>
To:     Wentao_Liang <Wentao_Liang_g@....com>, clm@...com
Cc:     josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: fix a potential double put bug and some related
 use-after-free bugs



On 18.08.21 г. 12:15, Wentao_Liang wrote:
> In line 2955 (#1), "btrfs_put_block_group(cache);" drops the reference to
> cache and may cause the cache to be released. However, in line 3014, the
> cache is dropped again by the same put function (#4). Double putting the
> cache can lead to an incorrect reference count.
> 
> Furthermore, according to the definition of btrfs_put_block_group() in fs/
> btrfs/block-group.c, if the reference count of the cache is one at the
> first put, it will be freed by kfree(). Using it again may result in the
> use-after-free flaw. In fact, after the first put (line 2955), the cache
> is also accessed in a few places (#2, #3), e.g., lines 2967, 2973, 2974,
> ….
> 
> We believe that the first put of the cache is unnecessary (#1).
> We can fix the above bugs by removing the redundant
> "btrfs_put_block_group(cache);" in line 2955 (#1).
> 
> 2951         if (!list_empty(&cache->io_list)) {
> ...
> 2955             btrfs_put_block_group(cache);
> 				 //#1 the first drop to cache (unnecessary)
> ...
> 2957         }
> ...
> 2967         cache_save_setup(cache, trans, path); //#2 use the cache
> ...
> 2972          //#3 use the cache several times
> 
> 2973         if (!ret && cache->disk_cache_state == BTRFS_DC_SETUP) {
> 2974             cache->io_ctl.inode = NULL;
> 2975             ret = btrfs_write_out_cache(trans, cache, path);
> 2976             if (ret == 0 && cache->io_ctl.inode) {
> 2977                 num_started++;
> 2978                 should_put = 0;
> 2979                 list_add_tail(&cache->io_list, io);
> 2980             } else {
> ...
> 2985                 ret = 0;
> 2986             }
> 2987         }
> 2988         if (!ret) {
> 2989             ret = update_block_group_item(trans, path, cache);
> ...
> 3003             if (ret == -ENOENT) {
> ...
> 3006                 ret = update_block_group_item(trans, path, cache);
> 3007             }
> ...
> 3010         }
> 3011
> ...
> 3013         if (should_put)
> 3014             btrfs_put_block_group(cache);
> 				//#4 the second drop to cache
> 
> Signed-off-by: Wentao_Liang <Wentao_Liang_g@....com>


Apart form the patch being buggy you have not demonstrated why doing 2
put block groups is actually given that there are invariant that
guarantee bg will have at least 2 refs held. So it seems you have
produced the patch without considering the big picture of how btrfs'
block group state machine works.