Re: [PATCH] ext4: potential crash on allocation error in ext4_alloc_flex_bg_array()

["Jitindar SIngh, Suraj" <surajjs@amazon.com>]

On Fri, 2020-02-28 at 12:22 +0300, Dan Carpenter wrote:
> If sbi->s_flex_groups_allocated is zero and the first allocation
> fails
> then this code will crash.  The problem is that "i--" will set "i" to
> -1 but when we compare "i >= sbi->s_flex_groups_allocated" then the
> -1
> is type promoted to unsigned and becomes UINT_MAX.  Since UINT_MAX
> is more than zero, the condition is true so we call
> kvfree(new_groups[-1]).
> The loop will carry on freeing invalid memory until it crashes.
> 
> Fixes: 7c990728b99e ("ext4: fix potential race between s_flex_groups
> online resizing and access")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

Reviewed-by: Suraj Jitindar Singh <surajjs@amazon.com>
Cc: <stable@kernel.org>

> ---
> I changed this from a -- loop to a ++ loop because I knew it would
> make
> Walter Harms happy.  He hates -- loops and I don't when his birthday
> so
> I'm celebrating it today.  :)
> 
>  fs/ext4/super.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index ff1b764b0c0e..0c7c4adb664e 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2391,7 +2391,7 @@ int ext4_alloc_flex_bg_array(struct super_block
> *sb, ext4_group_t ngroup)
>  {
>  	struct ext4_sb_info *sbi = EXT4_SB(sb);
>  	struct flex_groups **old_groups, **new_groups;
> -	int size, i;
> +	int size, i, j;
>  
>  	if (!sbi->s_log_groups_per_flex)
>  		return 0;
> @@ -2412,8 +2412,8 @@ int ext4_alloc_flex_bg_array(struct super_block
> *sb, ext4_group_t ngroup)
>  					 sizeof(struct flex_groups)),
>  					 GFP_KERNEL);
>  		if (!new_groups[i]) {
> -			for (i--; i >= sbi->s_flex_groups_allocated; i-
> -)
> -				kvfree(new_groups[i]);
> +			for (j = sbi->s_flex_groups_allocated; j < i;
> j++)
> +				kvfree(new_groups[j]);
>  			kvfree(new_groups);
>  			ext4_msg(sb, KERN_ERR,
>  				 "not enough memory for %d flex
> groups", size);