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Subject: Re: [PATCH] EVM: Fix null dereference on xattr when xattr fails to
 allocate
From:   Mimi Zohar <zohar@linux.vnet.ibm.com>
To:     Colin King <colin.king@canonical.com>,
        Matthew Garrett <mjg59@google.com>,
        James Morris <jmorris@namei.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org
Cc:     kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Tue, 29 May 2018 08:30:27 -0400
In-Reply-To: <20180527225510.25612-1-colin.king@canonical.com>
References: <20180527225510.25612-1-colin.king@canonical.com>
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Hi Colin,

On Sun, 2018-05-27 at 23:55 +0100, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> In the case where the allocation of xattr fails and xattr is NULL, the
> error exit return path via label 'out' will dereference xattr when
> kfree'ing xattr-name.  Fix this by only kfree'ing xattr->name and xattr
> when xattr is non-null.
> 
> Detected by CoverityScan, CID#1469366 ("Dereference after null check")
> 
> Fixes: fa516b66a1bf ("EVM: Allow runtime modification of the set of verified xattrs")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  security/integrity/evm/evm_secfs.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/security/integrity/evm/evm_secfs.c b/security/integrity/evm/evm_secfs.c
> index fb8bc950aceb..cf5cd303d7c0 100644
> --- a/security/integrity/evm/evm_secfs.c
> +++ b/security/integrity/evm/evm_secfs.c
> @@ -253,8 +253,10 @@ static ssize_t evm_write_xattrs(struct file *file, const char __user *buf,
>  out:
>  	audit_log_format(ab, " res=%d", err);
>  	audit_log_end(ab);
> -	kfree(xattr->name);
> -	kfree(xattr);
> +	if (xattr) {
> +		kfree(xattr->name);
> +		kfree(xattr);
> +	}
>  	return err;
>  }
> 

Thanks!  To fix this problem, I think more is needed.

Without the xattr, there is nothing to audit except the attempt to
extend the xattr list.  Failure to allocate the xattr or xattr->name
should either result in a different audit message or return
immediately without any audit message.

Mimi