linux-hardening - Re: [PATCH] fscrypt: Replace 1-element array with flexible array

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Date:   Mon, 22 May 2023 15:34:55 -0700
From:   Bill Wendling <morbo@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] fscrypt: Replace 1-element array with flexible array

On Mon, May 22, 2023 at 2:39 PM Kees Cook <keescook@...omium.org> wrote:
>
> 1-element arrays are deprecated, and are being replaced with C99
> flexible arrays[1]. In the future, we can add annotations for the
> flexible array member "encrypted_path" to have a size determined
> by the "len" member.
>
> As sizes were being calculated with the extra byte intentionally,
> propagate the difference so there is no change in binary output.
>
> [1] https://github.com/KSPP/linux/issues/79
>
> Cc: Eric Biggers <ebiggers@...nel.org>
> Cc: "Theodore Y. Ts'o" <tytso@....edu>
> Cc: Jaegeuk Kim <jaegeuk@...nel.org>
> Cc: Gustavo A. R. Silva <gustavoars@...nel.org>
> Cc: linux-fscrypt@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-By: Bill Wendling <morbo@...gle.com>

(With a tear in my eye about the original code...)

> ---
>  fs/crypto/fscrypt_private.h |  2 +-
>  fs/crypto/hooks.c           | 10 +++++-----
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
> index 7ab5a7b7eef8..2d63da48635a 100644
> --- a/fs/crypto/fscrypt_private.h
> +++ b/fs/crypto/fscrypt_private.h
> @@ -171,7 +171,7 @@ fscrypt_policy_flags(const union fscrypt_policy *policy)
>   */
>  struct fscrypt_symlink_data {
>         __le16 len;
> -       char encrypted_path[1];
> +       char encrypted_path[];
>  } __packed;
>
>  /**
> diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
> index 9e786ae66a13..6238dbcadcad 100644
> --- a/fs/crypto/hooks.c
> +++ b/fs/crypto/hooks.c
> @@ -255,10 +255,10 @@ int fscrypt_prepare_symlink(struct inode *dir, const char *target,
>          * for now since filesystems will assume it is there and subtract it.
>          */
>         if (!__fscrypt_fname_encrypted_size(policy, len,
> -                                           max_len - sizeof(struct fscrypt_symlink_data),
> +                                           max_len - sizeof(struct fscrypt_symlink_data) - 1,
>                                             &disk_link->len))
>                 return -ENAMETOOLONG;
> -       disk_link->len += sizeof(struct fscrypt_symlink_data);
> +       disk_link->len += sizeof(struct fscrypt_symlink_data) + 1;
>
>         disk_link->name = NULL;
>         return 0;
> @@ -289,7 +289,7 @@ int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
>                 if (!sd)
>                         return -ENOMEM;
>         }
> -       ciphertext_len = disk_link->len - sizeof(*sd);
> +       ciphertext_len = disk_link->len - sizeof(*sd) - 1;
>         sd->len = cpu_to_le16(ciphertext_len);
>
>         err = fscrypt_fname_encrypt(inode, &iname, sd->encrypted_path,
> @@ -367,7 +367,7 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
>          * the ciphertext length, even though this is redundant with i_size.
>          */
>
> -       if (max_size < sizeof(*sd))
> +       if (max_size < sizeof(*sd) + 1)
>                 return ERR_PTR(-EUCLEAN);
>         sd = caddr;
>         cstr.name = (unsigned char *)sd->encrypted_path;
> @@ -376,7 +376,7 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
>         if (cstr.len == 0)
>                 return ERR_PTR(-EUCLEAN);
>
> -       if (cstr.len + sizeof(*sd) - 1 > max_size)
> +       if (cstr.len + sizeof(*sd) > max_size)
>                 return ERR_PTR(-EUCLEAN);
>
>         err = fscrypt_fname_alloc_buffer(cstr.len, &pstr);
> --
> 2.34.1
>