Re: [PATCH 1/5] fs: ext4: validate log2_block_size from superblock at mount

Ahmad Fatoum
2026-04-10T11:34:12Z

On 4/2/26 12:12 PM, Sascha Hauer wrote:
> The superblock's log2_block_size field is read from disk and used
> throughout the ext4 code without validation. EXT2_BLOCK_SIZE() computes
> 1 << (log2_block_size + 10), so a crafted value of 22 or larger causes
> undefined behavior from shifting an int by >= 32 bits.
>
> This leads to wrong allocation sizes for indirect block buffers and
> enables downstream buffer overflows in every function that reads
> block-sized data.
>
> Validate that log2_block_size does not exceed the maximum defined by
> EXT2_MAX_BLOCK_LOG_SIZE (65536 byte blocks) at mount time, rejecting
> the filesystem early if it does.
>
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

This is analogous to Linux commit 62aeb94433fc ("ext2: Check block size
validity during mount"), which addressed a Syzbot report in 2023.

> ---
> fs/ext4/ext4_common.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/ext4/ext4_common.c b/fs/ext4/ext4_common.c
> index a38593105f..094293a069 100644
> --- a/fs/ext4/ext4_common.c
> +++ b/fs/ext4/ext4_common.c
> @@ -514,6 +514,14 @@ int ext4fs_mount(struct ext_filesystem *fs)
> goto fail;
> }
>
> + if (le32_to_cpu(data->sblock.log2_block_size) >
> + EXT2_MAX_BLOCK_LOG_SIZE - EXT2_MIN_BLOCK_LOG_SIZE) {
> + dev_err(fs->dev, "invalid block size %u\n",> +
le32_to_cpu(data->sblock.log2_block_size));
> + ret = -EINVAL;
> + goto fail;
> + }
> +
> if (le32_to_cpu(data->sblock.revision_level) == 0) {
> fs->inodesz = 128;
> fs->gdsize = 32;
>

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