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

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Date:   Mon, 26 Jul 2021 11:56:36 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Dan Carpenter <dan.carpenter@...cle.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 26/07/2021 09:50, Dan Carpenter wrote:
> 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);

Good spot! Thanks Dan.

> 
> Otherwise it leads to a use after free.
> 
>> +	return rc;
>>  }
> 
> How this is allocated is pretty scary looking!

it is kinda mind bending.

Colin

> 
> 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
>