[bug report] evm: Check also if *tfm is an error pointer in init_desc()

[Dan Carpenter]

On Tue, May 12, 2020 at 11:31:53AM +0000, Roberto Sassu wrote:
> > From: Dan Carpenter [mailto:dan.carpenter at oracle.com]
> > Sent: Tuesday, May 12, 2020 12:48 PM
> > 
> > Hello Roberto Sassu,
> > 
> > The patch 53de3b080d5e: "evm: Check also if *tfm is an error pointer
> > in init_desc()" from Apr 27, 2020, leads to the following static
> > checker warning:
> > 
> > 	security/integrity/evm/evm_crypto.c:119 init_desc()
> > 	error: '*tfm' dereferencing possible ERR_PTR()
> > 
> > security/integrity/evm/evm_crypto.c
> >     89
> >     90                  tfm = &evm_tfm[hash_algo];
> >     91                  algo = hash_algo_name[hash_algo];
> >     92          }
> >     93
> >     94          if (IS_ERR_OR_NULL(*tfm)) {
> > 
> > This used to be a "if (!*tfm)" check.
> > 
> >     95                  mutex_lock(&mutex);
> >     96                  if (*tfm)
> >     97                          goto out;
> > 
> > Then we test again with the lock held.  But in the new code if "*tfm"
> > is an error pointer then we jump directly to the unlock and crash on the
> > next line.  I can't see how the commit would fix anything.
> 
> Hello Dan
> 
> you are right. The fix should be applied in both places.
> 
> if (!IS_ERR_OR_NULL(*tfm))
> 	goto out;

No.  I was wrong.

> 
> >     98                  *tfm = crypto_alloc_shash(algo, 0, CRYPTO_NOLOAD);
> >     99                  if (IS_ERR(*tfm)) {
> >    100                          rc = PTR_ERR(*tfm);
> >    101                          pr_err("Can not allocate %s (reason: %ld)\n", algo, rc);
> >    102                          *tfm = NULL;
> >    103                          mutex_unlock(&mutex);
> >    104                          return ERR_PTR(rc);
> >    105                  }
> >    106                  if (type == EVM_XATTR_HMAC) {
> >    107                          rc = crypto_shash_setkey(*tfm, evmkey, evmkey_len);
> >    108                          if (rc) {
> >    109                                  crypto_free_shash(*tfm);
> >    110                                  *tfm = NULL;
> >    111                                  mutex_unlock(&mutex);
> >    112                                  return ERR_PTR(rc);
> >    113                          }
> >    114                  }
> >    115  out:
> >    116                  mutex_unlock(&mutex);
> >    117          }
> >    118
> >    119          desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(*tfm),
> >                                                                      ^^^^
> > I don't understand how using *tfm outside of a lock is safe at all
> > anyway.
> 
> I think the purpose of the mutex is just to  prevent two concurrent
> allocations. Later, it should not be a problem, as *tfm is never freed.
> 

Actually by the time we take the lock then *tfm is either valid or NULL
so this code works.  It's confusing though.

regards,
dan carpenter