From: Christian Brauner [mailto:christian.brauner@ubuntu.com]
Sent: Thursday, March 25, 2021 1:14 PM
On Thu, Mar 25, 2021 at 10:53:43AM +0000, Roberto Sassu wrote:

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From: Roberto Sassu
Sent: Friday, March 5, 2021 4:19 PM
With the patch to allow xattr/attr operations if a portable signature
verification fails, cp and tar can copy all xattrs/attrs so that at the
end of the process verification succeeds.

However, it might happen that the xattrs/attrs are already set to the
correct value (taken at signing time) and signature verification succeeds
before the copy has completed. For example, an archive might contains

files

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owned by root and the archive is extracted by root.

Then, since portable signatures are immutable, all subsequent

operations

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fail (e.g. fchown()), even if the operation is legitimate (does not alter
the current value).

This patch avoids this problem by reporting successful operation to user
space when that operation does not alter the current value of

xattrs/attrs.

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Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 security/integrity/evm/evm_main.c | 96
+++++++++++++++++++++++++++++++
 1 file changed, 96 insertions(+)

diff --git a/security/integrity/evm/evm_main.c

b/security/integrity/evm/evm_main.c
index eab536fa260f..a07516dcb920 100644

--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c

@@ -18,6 +18,7 @@
 #include <linux/integrity.h>
 #include <linux/evm.h>
 #include <linux/magic.h>
+#include <linux/posix_acl_xattr.h>

 #include <crypto/hash.h>
 #include <crypto/hash_info.h>

@@ -328,6 +329,79 @@ static enum integrity_status

evm_verify_current_integrity(struct dentry *dentry)
 	return evm_verify_hmac(dentry, NULL, NULL, 0, NULL);
 }

+/*
+ * evm_xattr_acl_change - check if passed ACL changes the inode

mode

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+ * @dentry: pointer to the affected dentry
+ * @xattr_name: requested xattr
+ * @xattr_value: requested xattr value
+ * @xattr_value_len: requested xattr value length
+ *
+ * Check if passed ACL changes the inode mode, which is protected by
EVM.
+ *
+ * Returns 1 if passed ACL causes inode mode change, 0 otherwise.
+ */
+static int evm_xattr_acl_change(struct dentry *dentry, const char
*xattr_name,
+				const void *xattr_value, size_t
xattr_value_len)
+{
+	umode_t mode;
+	struct posix_acl *acl = NULL, *acl_res;
+	struct inode *inode = d_backing_inode(dentry);
+	int rc;
+
+	/* UID/GID in ACL have been already converted from user to init ns
*/
+	acl = posix_acl_from_xattr(&init_user_ns, xattr_value,
xattr_value_len);
+	if (!acl)

Based on Mimi's review, I will change this to:

if (IS_ERR_OR_NULL(acl))

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+		return 1;
+
+	acl_res = acl;
+	rc = posix_acl_update_mode(&init_user_ns, inode, &mode,
&acl_res);

About this part, probably it is not correct.

I'm writing a test for this patch that checks if operations
that don't change the file mode succeed and those that
do fail.

mount-idmapped --map-mount b:3001:0:1 /mnt /mnt-idmapped
pushd /mnt
echo "test" > test-file
chown 3001 test-file
chgrp 3001 test-file
chmod 2644 test-file
<check enabled>
setfacl --set u::rw,g::r,o::r,m:r test-file (expected to succeed, caller has

CAP_FSETID, so S_ISGID is not dropped)

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setfacl --set u::rw,g::r,o::r,m:rw test-file (expected to fail)
pushd /mnt-idmapped
capsh --drop=cap_fsetid -- -c setfacl --set u::rw,g::r,o::r test-file (expected

to succeed, caller is in the owning group of test-file, so S_ISGID is not
dropped)

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After adding a debug line in posix_acl_update_mode():
printk("%s: %d(%d) %d\n", __func__,

in_group_p(i_gid_into_mnt(mnt_userns, inode)),
__kgid_val(i_gid_into_mnt(mnt_userns, inode)),
capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID));

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without passing mnt_userns:
[  748.262582] setfacl --set u::rw,g::r,o::r,m:r test-file
[  748.268021] posix_acl_update_mode: 0(3001) 1
[  748.268035] posix_acl_update_mode: 0(3001) 1
[  748.268570] setfacl --set u::rw,g::r,o::r,m:rw test-file
[  748.274193] posix_acl_update_mode: 0(3001) 1
[  748.279198] capsh --drop=cap_fsetid -- -c setfacl --set u::rw,g::r,o::r test-

file

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[  748.287894] posix_acl_update_mode: 0(3001) 0

passing mnt_userns:
[   81.159766] setfacl --set u::rw,g::r,o::r,m:r test-file
[   81.165207] posix_acl_update_mode: 0(3001) 1
[   81.165226] posix_acl_update_mode: 0(3001) 1
[   81.165732] setfacl --set u::rw,g::r,o::r,m:rw test-file
[   81.170978] posix_acl_update_mode: 0(3001) 1
[   81.176014] capsh --drop=cap_fsetid -- -c setfacl --set u::rw,g::r,o::r test-

file

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[   81.184648] posix_acl_update_mode: 1(0) 0
[   81.184663] posix_acl_update_mode: 1(0) 0

The difference is that, by passing mnt_userns, the caller (root) is
in the owning group of the file (3001 -> 0). Without passing mnt_userns,
it is not (3001 -> 3001).

Christian, Andreas, could you confirm that this is correct?

Hey Robert,

Thanks for the Cc and thanks for testing this with and without idmapped
mounts; very much appreciated.

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If there are no objections, I will send an additional patch to pass
mnt_userns to EVM.

Yes, since you're starting to verify attrs and posix_acl changes that
deal with uids/gids you need to account for the mnt_userns. I've pulled
and applied your patch locally and looked through it. I think you need
to change:

- evm_inode_setxattr()
- evm_inode_removexattr()

to take a mnt_userns. That should be straightforward. I already changed
security_inode_setxattr() to pass down the mnt_userns so you need to
simply pass that further down:

- security_inode_setxattr(mnt_userns, ...)
  -> evm_inode_setxattr(mnt_userns, ...)

- security_inode_removexattr(mnt_userns, ...)
  -> evm_inode_removexattr(mnt_userns, ...)

Hi Christian

yes, I changed both.

The rest looks sane to me.

Fwiw, I'm mainting a large test-suite that I wrote for idmapped mounts
but that aims to cover all vfs operations independent of them. It aims
for:
- test vfs feature x on regular mounts
- test vfs feature on idmapped mounts
- test vfs feature in user namespaces
- test vfs feature on idmapped mount in user namespaces
I'm in the process of upstreaming it for xfstests (cf. [1]). It also
includes tests for xattrs/acls and fscaps. if ima and evm want to add
something to this that'd be great but if you maintain your own testing
that's of course totally ok.
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/brauner/xfstests-
dev.git/log/?h=idmapped_mounts

Yes, I think it would be good to have more tests and compare
IMA/EVM behavior between non and idmapped mounts.

For now, I wanted to be sure that my patch set works well
on top of your patches. Thanks a lot for reviewing this!

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

Thanks!
Christian