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Date:   Wed, 19 Feb 2020 20:20:30 +0000
From:   "Singh, Balbir" <sblbir@...zon.com>
To:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "Jitindar SIngh, Suraj" <surajjs@...zon.com>
CC:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "tytso@....edu" <tytso@....edu>,
        "sjitindarsingh@...il.com" <sjitindarsingh@...il.com>
Subject: Re: [PATCH 2/3] ext4: fix potential race between s_group_info online
 resizing and access

On Tue, 2020-02-18 at 19:08 -0800, Suraj Jitindar Singh wrote:
> During an online resize an array of pointers to s_group_info gets replaced
> so it can get enlarged. If there is a concurrent access to the array in
> ext4_get_group_info() and this memory has been reused then this can lead to
> an invalid memory access.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206443
> Signed-off-by: Suraj Jitindar Singh <surajjs@...zon.com>
> Cc: stable@...r.kernel.org
> ---
>  fs/ext4/ext4.h    |  6 +++---
>  fs/ext4/mballoc.c | 10 ++++++----
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 236fc6500340..3f4aaaae7da6 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -2994,13 +2994,13 @@ static inline
>  struct ext4_group_info *ext4_get_group_info(struct super_block *sb,
>  					    ext4_group_t group)
>  {
> -	 struct ext4_group_info ***grp_info;
> +	 struct ext4_group_info **grp_info;
>  	 long indexv, indexh;
>  	 BUG_ON(group >= EXT4_SB(sb)->s_groups_count);
> -	 grp_info = EXT4_SB(sb)->s_group_info;
>  	 indexv = group >> (EXT4_DESC_PER_BLOCK_BITS(sb));
>  	 indexh = group & ((EXT4_DESC_PER_BLOCK(sb)) - 1);
> -	 return grp_info[indexv][indexh];
> +	 grp_info = sbi_array_rcu_deref(EXT4_SB(sb), s_group_info, indexv);
> +	 return grp_info[indexh];
>  }
>  
>  /*
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index f64838187559..0d9b17afc85f 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2356,7 +2356,7 @@ int ext4_mb_alloc_groupinfo(struct super_block *sb,
> ext4_group_t ngroups)
>  {
>  	struct ext4_sb_info *sbi = EXT4_SB(sb);
>  	unsigned size;
> -	struct ext4_group_info ***new_groupinfo;
> +	struct ext4_group_info ***old_groupinfo, ***new_groupinfo;
>  
>  	size = (ngroups + EXT4_DESC_PER_BLOCK(sb) - 1) >>
>  		EXT4_DESC_PER_BLOCK_BITS(sb);
> @@ -2369,13 +2369,15 @@ int ext4_mb_alloc_groupinfo(struct super_block *sb,
> ext4_group_t ngroups)
>  		ext4_msg(sb, KERN_ERR, "can't allocate buddy meta group");
>  		return -ENOMEM;
>  	}
> -	if (sbi->s_group_info) {
> +	old_groupinfo = sbi->s_group_info;
> +	if (sbi->s_group_info)
>  		memcpy(new_groupinfo, sbi->s_group_info,
>  		       sbi->s_group_info_size * sizeof(*sbi->s_group_info));
> -		kvfree(sbi->s_group_info);
> -	}
>  	sbi->s_group_info = new_groupinfo;
> +	rcu_assign_pointer(sbi->s_group_info, new_groupinfo);
>  	sbi->s_group_info_size = size / sizeof(*sbi->s_group_info);
> +	if (old_groupinfo)
> +		ext4_kvfree_array_rcu(old_groupinfo);
>  	ext4_debug("allocated s_groupinfo array for %d meta_bg's\n", 
>  		   sbi->s_group_info_size);
>  	return 0;

Reviewed-by: Balbir Singh <sblbir@...zon.com>

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