linux-kernel - Re: [GIT PULL] Fix for Integrity subsystem null pointer deref

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Date:	Wed, 29 Oct 2014 08:59:59 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	James Morris <jmorris@...ei.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"security@...nel.org" <security@...nel.org>
Subject: Re: [GIT PULL] Fix for Integrity subsystem null pointer deref

On Tue, 2014-10-28 at 22:08 -0700, Andy Lutomirski wrote: 
> On Tue, Oct 28, 2014 at 8:55 PM, James Morris <jmorris@...ei.org> wrote:
> > These changes fix a bug in xattr handling, where the evm and ima
> > inode_setxattr() functions do not check for empty xattrs being passed from
> > userspace (leading to user-triggerable null pointer dereferences).
> >
> > Please pull.
> >
> >
> > The following changes since commit 9f76628da20f96a179ca62b504886f99ecc29223:
> >
> >   Merge branch 'for-3.18' of git://linux-nfs.org/~bfields/linux (2014-10-28 13:32:06 -0700)
> >
> > are available in the git repository at:
> >
> >   git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git for-linus
> >
> > Dmitry Kasatkin (2):
> >       ima: check xattr value length and type in the ima_inode_setxattr()
> 
> I haven't read this one, but:
> 
> >       evm: check xattr value length and type in evm_inode_setxattr()
> 
> const struct evm_ima_xattr_data *xattr_data = xattr_value;
> - if ((strcmp(xattr_name, XATTR_NAME_EVM) == 0)
> - && (xattr_data->type == EVM_XATTR_HMAC))
> - return -EPERM;
> + if (strcmp(xattr_name, XATTR_NAME_EVM) == 0) {
> + if (!xattr_value_len)
> + return -EINVAL;
> + if (xattr_data->type != EVM_IMA_XATTR_DIGSIG)
> + return -EPERM;
> + }
> 
> Huh?  (Sorry about severe whitespace damage.)
> 
> Shouldn't there be something like if (xattr_value_len < sizeof(struct
> evm_ima_xattr_data)) return -EINVAL?

Prior to commit 2fb1c9a "evm: prohibit userspace writing 'security.evm'
HMAC value", a process with CAP_SYS_ADMIN could write either an HMAC or
signature. As the HMAC key should only be known to the kernel, only
signatures are now allowed.  Instead of "struct evm_ima_xattr_data", the
code should reflect this change and use "struct signature_v2_hdr".
We'll clean up this code for the next release.  For now, this patch
prevents the oops.

thanks,

Mimi

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