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

[Roberto Sassu]

> From: owner-linux-security-module at vger.kernel.org [mailto:owner-linux-
> security-module at vger.kernel.org] On Behalf Of Dan Carpenter
> Sent: Tuesday, May 12, 2020 3:04 PM
> On Tue, May 12, 2020 at 12:45:06PM +0000, Roberto Sassu wrote:
> > > From: owner-linux-security-module at vger.kernel.org [mailto:owner-
> linux-
> > > security-module at vger.kernel.org] On Behalf Of Dan Carpenter
> > > Sent: Tuesday, May 12, 2020 2:34 PM
> > > 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.
> >
> > static inline bool __must_check IS_ERR_OR_NULL(__force const void *ptr)
> > {
> >         return unlikely(!ptr) || IS_ERR_VALUE((unsigned long)ptr);
> > }
> >
> > CPU#1			CPU#2
> > 			*tfm = crypto_alloc_shash(algo, 0,
> CRYPTO_NOLOAD);
> > unlikely(!ptr)
> > 			*tfm = NULL;
> > IS_ERR_VALUE((unsigned long)ptr);
> >
> > desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(*tfm),
> >
> > Could this happen?
> 
> Yeah.  Huh.  That's true.  Good eyes.
> 
> I feel like this would be more clear as well if we used a temporary
> variable instead of working directly on "*tfm".

Yes, seems better. I will send a new version of the patch.

Thanks

Roberto

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