Re: vfs_getxattr_alloc() problem

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2020-04-27 at 14:54 -0400, Mimi Zohar wrote:
> On Fri, 2020-04-24 at 14:32 +0000, Roberto Sassu wrote:
> > > Hi Roberto,
> > > 
> > > On Tue, 2020-04-21 at 10:58 +0000, Roberto Sassu wrote:
> > > > Hi Mimi
> > > >
> > > > I found a problem in the calculation of the EVM digest.
> > > >
> > > > If an xattr is in the security domain, vfs_getxattr() calls xattr_getsecurity(),
> > > > which is implemented by LSMs. vfs_getxattr_alloc() instead calls directly
> > > > the filesystem function to read xattrs.
> > > >
> > > > The problem arises for example when you have a file with a portable
> > > > signature on the correct SELinux label (with \0) and you set
> > > security.selinux
> > > > manually:
> > > >
> > > > setfattr -n security.selinux -v "system_u:object_r:bin_t:s0" cat
> > > >
> > > > Although the length passed is 26 bytes (without \0), you get:
> > > >
> > > > # attr -l cat
> > > > Attribute "selinux" has a 27 byte value for cat
> > > >
> > > > which includes \0.
> > > >
> > > > From user space, evmctl does not complain (the signature is ok) because
> > > > it calculates the EVM digest with \0, but EVM verification fails (because it
> > > > calculates the digest without \0).
> > > >
> > > > Should this problem be fixed?
> > > 
> > > I don't seem to be having any problems verifying the EVM immutable &
> > > portable signatures.  To test, I've copied a properly labeled file
> > > twice, once with the "--preserve=xattr" and once without it.  I signed
> > > the properly labeled file with the EVM immutable & portable signature.
> > >  On the other file, I first set the selinux label before signing it.
> > > If there was a problem manually writing the SELinux label, the
> > > security.evm labels would be different, which they aren't.
> > 
> > [root@vm demo]# ls -lZ /bin/cat
> > -rwxr-xr-x. 1 root root system_u:object_r:bin_t:s0 85520 Apr 24 16:20 /bin/cat
> > [root@vm demo]# evmctl sign -o -a sha256 --imahash --key $PWD/signing_key.pem /bin/cat -v -v
> > hash(sha256): 0404d3d78d8249317ed50056ec7d04da382488f36a6127f4e9161792d97f13e10bc6
> > name: security.selinux, size: 27
> > 73797374656d5f753a6f626a6563745f723a62696e5f743a733000
> > no xattr: security.SMACK64
> > no xattr: security.apparmor
> > name: security.ima, size: 34
> > 0404d3d78d8249317ed50056ec7d04da382488f36a6127f4e9161792d97f13e10bc6
> > no xattr: security.capability
> > calc_evm_hash:532 hmac_misc (24): 0000000000000000000000000000000000000000ed810000
> > hash(sha256): 331e36ce1b32374a22e12df28b58d79536c0ee97ba01451bd60343191c073b55
> > calc_keyid_v2:735 keyid: aecec286
> > keyid: aecec286
> > evm/ima signature: 520 bytes
> > ...
> > [root@vm demo]# cat
> > ^C
> > [root@vm demo]# setfattr -n security.selinux -v "system_u:object_r:bin_t:s0" /bin/cat
> 
> In the past, when I looked at writing the same SELinux label, there
> was some performance improvement that only updated the label if the
> label actually changed.  Unless things have changed since, I don't
> think the same selinux label is rewritten.
> 
> > [root@vm demo]# evmctl verify -o -a sha256 --imahash /bin/cat -v -v
> > calc_keyid_v2:735 keyid: aecec286
> > keyid: aecec286
> > key 1: aecec286 /etc/keys/x509_evm.der
> > name: security.selinux, size: 27
> > 73797374656d5f753a6f626a6563745f723a62696e5f743a733000
> > no xattr: security.SMACK64
> > no xattr: security.apparmor
> > name: security.ima, size: 34
> > 0404d3d78d8249317ed50056ec7d04da382488f36a6127f4e9161792d97f13e10bc6
> > no xattr: security.capability
> > calc_evm_hash:532 hmac_misc (24): 0000000000000000000000000000000000000000ed810000
> > hash(sha256): 331e36ce1b32374a22e12df28b58d79536c0ee97ba01451bd60343191c073b55
> > /bin/cat: verification is OK
> > [root@vm demo]# cat
> > -bash: /usr/bin/cat: Permission denied
> > [root@vm demo]#
> > 
> > It fails because the actual xattr in the filesystem is:
> > 
> > name: security.selinux, size: 26
> > 73797374656d5f753a6f626a6563745f723a62696e5f743a7330
> 
> Looking at security/selinux/hooks.c:  I'm seeing a
> comment selinux_inode_setxattr() that says:
> 
> /* We strip a nul only if it is at the end, otherwise the
>  * context contains a nul and we should audit that */

strace shows setxattr is writing 26 bytes:

setxattr("/bin/cat-test", "security.selinux",
"system_u:object_r:bin_t:s0", 26, 0) = 0

Mimi