From: Dan Carpenter <dan.carpenter@oracle.com>
To: Roberto Sassu <roberto.sassu@huawei.com>
Cc: "linux-integrity@vger.kernel.org"
<linux-integrity@vger.kernel.org>,
"linux-security-module@vger.kernel.org"
<linux-security-module@vger.kernel.org>,
Silviu Vlasceanu <Silviu.Vlasceanu@huawei.com>
Subject: Re: [bug report] evm: Check also if *tfm is an error pointer in init_desc()
Date: Tue, 12 May 2020 16:04:11 +0300 [thread overview]
Message-ID: <20200512130410.GB2056@kadam> (raw)
In-Reply-To: <7fbab163eb76459a82ebe94c577a4954@huawei.com>
On Tue, May 12, 2020 at 12:45:06PM +0000, Roberto Sassu wrote:
> > From: owner-linux-security-module@vger.kernel.org [mailto:owner-linux-
> > security-module@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@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".
regards,
dan carpenter
next prev parent reply other threads:[~2020-05-12 13:04 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-12 10:48 [bug report] evm: Check also if *tfm is an error pointer in init_desc() Dan Carpenter
2020-05-12 11:31 ` Roberto Sassu
2020-05-12 12:34 ` Dan Carpenter
2020-05-12 12:45 ` Roberto Sassu
2020-05-12 13:04 ` Dan Carpenter [this message]
2020-05-12 13:08 ` Roberto Sassu
2020-05-12 13:19 ` [PATCH] evm: Fix a small race " Dan Carpenter
2020-05-12 13:43 ` Roberto Sassu
2020-05-12 17:47 ` [PATCH v2] " Dan Carpenter
2020-05-14 6:47 ` Roberto Sassu
2020-05-14 7:11 ` Krzysztof Struczynski
2020-05-14 18:21 ` Mimi Zohar
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