[PATCH] KEYS: trusted: fix writing past end of buffer in trusted_read()

[Mimi Zohar]

On Thu, 2017-10-26 at 13:57 -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers at google.com>
> 
> When calling keyctl_read() on a key of type "trusted", if the
> user-supplied buffer was too small, the kernel ignored the buffer length
> and just wrote past the end of the buffer, potentially corrupting
> userspace memory.  Fix it by instead returning the size required, as per
> the documentation for keyctl_read().
> 
> We also don't even fill the buffer at all in this case, as this is
> slightly easier to implement than doing a short read, and either
> behavior appears to be permitted.  It also makes it match the behavior
> of the "encrypted" key type.
> 
> Fixes: d00a1c72f7f4 ("keys: add new trusted key-type")
> Reported-by: Ben Hutchings <ben at decadent.org.uk>
> Cc: <stable at vger.kernel.org> # v2.6.38+
> Signed-off-by: Eric Biggers <ebiggers at google.com>

Thanks!

Reviewed-by: Mimi Zohar <zohar at linux.vnet.ibm.com>

> ---
>  security/keys/trusted.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/security/keys/trusted.c b/security/keys/trusted.c
> index bd85315cbfeb..98aa89ff7bfd 100644
> --- a/security/keys/trusted.c
> +++ b/security/keys/trusted.c
> @@ -1147,20 +1147,21 @@ static long trusted_read(const struct key *key, char __user *buffer,
>  	p = dereference_key_locked(key);
>  	if (!p)
>  		return -EINVAL;
> -	if (!buffer || buflen <= 0)
> -		return 2 * p->blob_len;
> -	ascii_buf = kmalloc(2 * p->blob_len, GFP_KERNEL);
> -	if (!ascii_buf)
> -		return -ENOMEM;
> 
> -	bufp = ascii_buf;
> -	for (i = 0; i < p->blob_len; i++)
> -		bufp = hex_byte_pack(bufp, p->blob[i]);
> -	if ((copy_to_user(buffer, ascii_buf, 2 * p->blob_len)) != 0) {
> +	if (buffer && buflen >= 2 * p->blob_len) {
> +		ascii_buf = kmalloc(2 * p->blob_len, GFP_KERNEL);
> +		if (!ascii_buf)
> +			return -ENOMEM;
> +
> +		bufp = ascii_buf;
> +		for (i = 0; i < p->blob_len; i++)
> +			bufp = hex_byte_pack(bufp, p->blob[i]);
> +		if (copy_to_user(buffer, ascii_buf, 2 * p->blob_len) != 0) {
> +			kzfree(ascii_buf);
> +			return -EFAULT;
> +		}
>  		kzfree(ascii_buf);
> -		return -EFAULT;
>  	}
> -	kzfree(ascii_buf);
>  	return 2 * p->blob_len;
>  }
> 

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