linux-ext4 - Re: [PATCH v3] ext4: fix possible overflow in ext4_trim

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Date:	Tue, 27 Sep 2011 13:19:24 +0200 (CEST)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
cc:	linux-ext4@...r.kernel.org, tm@....ma, tytso@....edu
Subject: Re: [PATCH v3] ext4: fix possible overflow in ext4_trim_fs()

On Wed, 7 Sep 2011, Lukas Czerner wrote:

> The overflow can happen when we are calling get_group_no_and_offset()
> which stores the result of do_div() in 32 bit long type. However the
> result might be bigger than that if big blocknr is passed in. This will
> most likely happen when calling FITRIM with the default argument len =
> ULLONG_MAX.
> 
> Fix this by using "end" variable instead of "start+len" as it is easier
> to get right and specifically check that the end is not beyond the end
> of the file system, so we are sure that the result of
> get_group_no_and_offset() will not overflow. Otherwise truncate it to
> the size of the file system.

Any comments on this ? Note that the test case will be in xfstests soon.

http://www.spinics.net/lists/xfs/msg07251.html

Thanks!
-Lukas


> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> ---
> v2 - Change commit description to better describe the problem
> v3 - Use end instead of start+len, rework the fix
> 
>  fs/ext4/mballoc.c |   43 +++++++++++++++++++++++--------------------
>  1 files changed, 23 insertions(+), 20 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 17a5a57..fa8b14b 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4946,37 +4946,36 @@ out:
>  int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
>  {
>  	struct ext4_group_info *grp;
> -	ext4_group_t first_group, last_group;
> -	ext4_group_t group, ngroups = ext4_get_groups_count(sb);
> +	ext4_group_t group, first_group, last_group;
>  	ext4_grpblk_t cnt = 0, first_block, last_block;
> -	uint64_t start, len, minlen, trimmed = 0;
> +	uint64_t start, end, minlen, trimmed = 0;
>  	ext4_fsblk_t first_data_blk =
>  			le32_to_cpu(EXT4_SB(sb)->s_es->s_first_data_block);
> +	ext4_fsblk_t max_blks = ext4_blocks_count(EXT4_SB(sb)->s_es);
>  	int ret = 0;
>  
>  	start = range->start >> sb->s_blocksize_bits;
> -	len = range->len >> sb->s_blocksize_bits;
> +	end = start + (range->len >> sb->s_blocksize_bits) - 1;
>  	minlen = range->minlen >> sb->s_blocksize_bits;
>  
> -	if (unlikely(minlen > EXT4_BLOCKS_PER_GROUP(sb)))
> +	if (unlikely(minlen > EXT4_BLOCKS_PER_GROUP(sb)) ||
> +	    unlikely(start > max_blks))
>  		return -EINVAL;
> -	if (start + len <= first_data_blk)
> +	if (unlikely(end >= max_blks))
> +		end = max_blks - 1;
> +	if (end <= first_data_blk)
>  		goto out;
> -	if (start < first_data_blk) {
> -		len -= first_data_blk - start;
> +	if (start < first_data_blk)
>  		start = first_data_blk;
> -	}
>  
>  	/* Determine first and last group to examine based on start and len */
>  	ext4_get_group_no_and_offset(sb, (ext4_fsblk_t) start,
>  				     &first_group, &first_block);
> -	ext4_get_group_no_and_offset(sb, (ext4_fsblk_t) (start + len),
> +	ext4_get_group_no_and_offset(sb, (ext4_fsblk_t) end,
>  				     &last_group, &last_block);
> -	last_group = (last_group > ngroups - 1) ? ngroups - 1 : last_group;
> -	last_block = EXT4_BLOCKS_PER_GROUP(sb);
>  
> -	if (first_group > last_group)
> -		return -EINVAL;
> +	/* The last block to discard in the group */
> +	end = EXT4_BLOCKS_PER_GROUP(sb);
>  
>  	for (group = first_group; group <= last_group; group++) {
>  		grp = ext4_get_group_info(sb, group);
> @@ -4990,22 +4989,26 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
>  		/*
>  		 * For all the groups except the last one, last block will
>  		 * always be EXT4_BLOCKS_PER_GROUP(sb), so we only need to
> -		 * change it for the last group in which case start +
> -		 * len < EXT4_BLOCKS_PER_GROUP(sb).
> +		 * change it for the last group, note that last_block is
> +		 * already computed earlier by ext4_get_group_no_and_offset()
>  		 */
> -		if (first_block + len < EXT4_BLOCKS_PER_GROUP(sb))
> -			last_block = first_block + len;
> -		len -= last_block - first_block;
> +		if (group == last_group)
> +			end = last_block;
>  
>  		if (grp->bb_free >= minlen) {
>  			cnt = ext4_trim_all_free(sb, group, first_block,
> -						last_block, minlen);
> +						end, minlen);
>  			if (cnt < 0) {
>  				ret = cnt;
>  				break;
>  			}
>  		}
>  		trimmed += cnt;
> +
> +		/*
> +		 * For every group except the first one, we are sure
> +		 * that the first block to discard will be block #0.
> +		 */
>  		first_block = 0;
>  	}
>  	range->len = trimmed * sb->s_blocksize;
> 

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