linux-kernel - Re: [PATCH] security: keys: trusted: Fix memory leaks on allocated blob

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Date:   Mon, 26 Jul 2021 11:50:51 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Colin King <colin.king@...onical.com>
Cc:     James Bottomley <jejb@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] security: keys: trusted: Fix memory leaks on allocated
 blob

On Fri, Jul 23, 2021 at 06:21:21PM +0100, Colin King wrote:
> @@ -441,6 +449,10 @@ static int tpm2_load_cmd(struct tpm_chip *chip,
>  		rc = -EPERM;
>  
>  	return rc;
> +
> +err:
> +	kfree(blob);

This needs to be:

	if (blob != payload->blob)
		kfree(blob);

Otherwise it leads to a use after free.

> +	return rc;
>  }

How this is allocated is pretty scary looking!

security/keys/trusted-keys/trusted_tpm2.c
    96  static int tpm2_key_decode(struct trusted_key_payload *payload,
    97                             struct trusted_key_options *options,
    98                             u8 **buf)
    99  {
   100          int ret;
   101          struct tpm2_key_context ctx;
   102          u8 *blob;
   103  
   104          memset(&ctx, 0, sizeof(ctx));
   105  
   106          ret = asn1_ber_decoder(&tpm2key_decoder, &ctx, payload->blob,
   107                                 payload->blob_len);
   108          if (ret < 0)
   109                  return ret;

Old form?

   110  
   111          if (ctx.priv_len + ctx.pub_len > MAX_BLOB_SIZE)
   112                  return -EINVAL;

It's really scary to me that if the lengths are too large for kmalloc()
then we just use "payload->blob".

   113  
   114          blob = kmalloc(ctx.priv_len + ctx.pub_len + 4, GFP_KERNEL);

blob is allocated here.

   115          if (!blob)
   116                  return -ENOMEM;
   117  
   118          *buf = blob;
   119          options->keyhandle = ctx.parent;
   120  
   121          memcpy(blob, ctx.priv, ctx.priv_len);
   122          blob += ctx.priv_len;
   123  
   124          memcpy(blob, ctx.pub, ctx.pub_len);
   125  
   126          return 0;
   127  }

[ snip ]

   371          u32 attrs;
   372  
   373          rc = tpm2_key_decode(payload, options, &blob);
   374          if (rc) {
   375                  /* old form */
   376                  blob = payload->blob;
                        ^^^^^^^^^^^^^^^^^^^^

   377                  payload->old_format = 1;
   378          }
   379  

regards,
dan carpenter