Re: [PATCH v2] ext4: Annotate struct ext4_xattr_inode_array with __counted_by()

[Kees Cook <kees@kernel.org>]

On Tue, Jul 30, 2024 at 07:23:04PM +0200, Thorsten Blum wrote:
> Add the __counted_by compiler attribute to the flexible array member
> inodes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> CONFIG_FORTIFY_SOURCE.
> 
> Remove the now obsolete comment on the count field.
> 
> Refactor ext4_expand_inode_array() by assigning count before copying any
> data using memcpy(). Copy only the inodes array instead of the whole
> struct because count has been set explicitly.
> 
> Use struct_size() and struct_size_t() instead of offsetof().
> 
> Change the data type of the local variable count to unsigned int to
> match the struct's count data type.
> 
> Compile-tested only.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
> ---
> Changes in v2:
> - Adjust ext4_expand_inode_array() as suggested by Gustavo A. R. Silva
> - Use struct_size() and struct_size_t() instead of offsetof()
> - Link to v1: https://lore.kernel.org/linux-kernel/20240729110454.346918-3-thorsten.blum@toblux.com/
> ---
>  fs/ext4/xattr.c | 20 +++++++++-----------
>  fs/ext4/xattr.h |  4 ++--
>  2 files changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 46ce2f21fef9..b27543587103 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -2879,11 +2879,10 @@ ext4_expand_inode_array(struct ext4_xattr_inode_array **ea_inode_array,
>  	if (*ea_inode_array == NULL) {
>  		/*
>  		 * Start with 15 inodes, so it fits into a power-of-two size.
> -		 * If *ea_inode_array is NULL, this is essentially offsetof()
>  		 */
>  		(*ea_inode_array) =
> -			kmalloc(offsetof(struct ext4_xattr_inode_array,
> -					 inodes[EIA_MASK]),
> +			kmalloc(struct_size_t(struct ext4_xattr_inode_array,

This can be struct_size(*ea_inode_array, ...) too, like you have below.

> +					      inodes, EIA_MASK),
>  				GFP_NOFS);
>  		if (*ea_inode_array == NULL)
>  			return -ENOMEM;
> @@ -2891,17 +2890,16 @@ ext4_expand_inode_array(struct ext4_xattr_inode_array **ea_inode_array,
>  	} else if (((*ea_inode_array)->count & EIA_MASK) == EIA_MASK) {
>  		/* expand the array once all 15 + n * 16 slots are full */
>  		struct ext4_xattr_inode_array *new_array = NULL;
> -		int count = (*ea_inode_array)->count;
> +		unsigned int count = (*ea_inode_array)->count;
>  
> -		/* if new_array is NULL, this is essentially offsetof() */
> -		new_array = kmalloc(
> -				offsetof(struct ext4_xattr_inode_array,
> -					 inodes[count + EIA_INCR]),
> -				GFP_NOFS);
> +		new_array = kmalloc(struct_size(*ea_inode_array, inodes,
> +						count + EIA_INCR),
> +				    GFP_NOFS);
>  		if (new_array == NULL)
>  			return -ENOMEM;
> -		memcpy(new_array, *ea_inode_array,
> -		       offsetof(struct ext4_xattr_inode_array, inodes[count]));
> +		new_array->count = count;
> +		memcpy(new_array->inodes, (*ea_inode_array)->inodes,
> +		       count * sizeof(struct inode *));
>  		kfree(*ea_inode_array);
>  		*ea_inode_array = new_array;
>  	}
> diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
> index bd97c4aa8177..e14fb19dc912 100644
> --- a/fs/ext4/xattr.h
> +++ b/fs/ext4/xattr.h
> @@ -130,8 +130,8 @@ struct ext4_xattr_ibody_find {
>  };
>  
>  struct ext4_xattr_inode_array {
> -	unsigned int count;		/* # of used items in the array */
> -	struct inode *inodes[];
> +	unsigned int count;
> +	struct inode *inodes[] __counted_by(count);
>  };
>  
>  extern const struct xattr_handler ext4_xattr_user_handler;

Thanks for the reworking!

-- 
Kees Cook