[PATCH v2 1/3] ext4: Fix integer overflow in ext4fs_read_symlink()

[Heinrich Schuchardt]

On 09.08.24 11:54, Richard Weinberger wrote:
> While zalloc() takes a size_t type, adding 1 to the le32 variable
> will overflow.
> A carefully crafted ext4 filesystem can exhibit an inode size of 0xffffffff
> and as consequence zalloc() will do a zero allocation.
>
> Later in the function the inode size is again used for copying data.
> So an attacker can overwrite memory.
>
> Avoid the overflow by using the __builtin_add_overflow() helper.
>
> Signed-off-by: Richard Weinberger <richard at nod.at>
> ---
>   fs/ext4/ext4_common.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/ext4_common.c b/fs/ext4/ext4_common.c
> index 52152a2295..36999b608f 100644
> --- a/fs/ext4/ext4_common.c
> +++ b/fs/ext4/ext4_common.c
> @@ -2181,13 +2181,18 @@ static char *ext4fs_read_symlink(struct ext2fs_node *node)
>   	struct ext2fs_node *diro = node;
>   	int status;
>   	loff_t actread;
> +	size_t alloc_size;
>
>   	if (!diro->inode_read) {
>   		status = ext4fs_read_inode(diro->data, diro->ino, &diro->inode);
>   		if (status == 0)
>   			return NULL;
>   	}
> -	symlink = zalloc(le32_to_cpu(diro->inode.size) + 1);
> +
> +	if (__builtin_add_overflow(le32_to_cpu(diro->inode.size), 1, &alloc_size))
> +		return NULL;

Thank you for pointing at the problematic code.

You are calling __builtin_add_overflow(int, int, size_t *).

__builtin_add_overflow() is not defined in the C-standard.

Is there any well defined behavior for this on systems where size_t has
more bits than int?

I could imagine implementations just copying an int to the first 32 bits
of alloc_size and leaving the other bits untouched.

Hopefully your C-compiler simply refuses to compile this code.

I would prefer to stick to standard C:

alloc_size = le32_to_cpu(diro->inode.size) + 1UL;
if (!alloc_size)
	return NULL;

Here an overflow can only occur on 32bit systems.

Best regards

Heinrich

> +
> +	symlink = zalloc(alloc_size);
>   	if (!symlink)
>   		return NULL;
>