linux-integrity.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
* [PATCH] evm: Check xattr size misalignment between kernel and user
@ 2021-06-21  9:39 Roberto Sassu
  2021-06-21 12:22 ` Mimi Zohar
  0 siblings, 1 reply; 2+ messages in thread
From: Roberto Sassu @ 2021-06-21  9:39 UTC (permalink / raw)
  To: zohar, paul, stephen.smalley.work, casey, stefanb
  Cc: linux-integrity, linux-security-module, linux-kernel, selinux,
	Roberto Sassu

The kernel and the user obtain an xattr value in two different ways:

kernel (EVM): uses vfs_getxattr_alloc() which obtains the xattr value from
              the filesystem handler (raw value);

user (ima-evm-utils): uses vfs_getxattr() which obtains the xattr value
                      from the LSMs (normalized value).

Normally, this does not have an impact unless security.selinux is set with
setfattr, with a value not terminated by '\0' (this is not the recommended
way, security.selinux should be set with the appropriate tools such as
chcon and restorecon).

In this case, the kernel and the user see two different xattr values: the
former sees the xattr value without '\0' (raw value), the latter sees the
value with '\0' (value normalized by SELinux).

This could result in two different verification outcomes from EVM and
ima-evm-utils, if a signature was calculated with a security.selinux value
terminated by '\0' and the value set in the filesystem is not terminated by
'\0'. The former would report verification failure due to the missing '\0',
while the latter would report verification success (because it gets the
normalized value with '\0').

This patch mitigates this issue by comparing in evm_calc_hmac_or_hash() the
size of the xattr returned by the two xattr functions and by warning the
user if there is a misalignment.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
---
 security/integrity/evm/evm_crypto.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index 96b22f2ac27a..462c5258322a 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -221,7 +221,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
 	size_t xattr_size = 0;
 	char *xattr_value = NULL;
 	int error;
-	int size;
+	int size, user_space_size;
 	bool ima_present = false;
 
 	if (!(inode->i_opflags & IOP_XATTR) ||
@@ -276,6 +276,12 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
 		if (size < 0)
 			continue;
 
+		user_space_size = vfs_getxattr(&init_user_ns, dentry,
+					       xattr->name, NULL, 0);
+		if (user_space_size != size)
+			pr_debug("file %s: xattr %s size mismatch (kernel: %d, user: %d)\n",
+				 dentry->d_name.name, xattr->name, size,
+				 user_space_size);
 		error = 0;
 		xattr_size = size;
 		crypto_shash_update(desc, (const u8 *)xattr_value, xattr_size);
-- 
2.25.1


^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH] evm: Check xattr size misalignment between kernel and user
  2021-06-21  9:39 [PATCH] evm: Check xattr size misalignment between kernel and user Roberto Sassu
@ 2021-06-21 12:22 ` Mimi Zohar
  0 siblings, 0 replies; 2+ messages in thread
From: Mimi Zohar @ 2021-06-21 12:22 UTC (permalink / raw)
  To: Roberto Sassu, paul, stephen.smalley.work, casey, stefanb
  Cc: linux-integrity, linux-security-module, linux-kernel, selinux

On Mon, 2021-06-21 at 11:39 +0200, Roberto Sassu wrote:
> The kernel and the user obtain an xattr value in two different ways:
> 
> kernel (EVM): uses vfs_getxattr_alloc() which obtains the xattr value from
>               the filesystem handler (raw value);
> 
> user (ima-evm-utils): uses vfs_getxattr() which obtains the xattr value
>                       from the LSMs (normalized value).
> 
> Normally, this does not have an impact unless security.selinux is set with
> setfattr, with a value not terminated by '\0' (this is not the recommended
> way, security.selinux should be set with the appropriate tools such as
> chcon and restorecon).
> 
> In this case, the kernel and the user see two different xattr values: the
> former sees the xattr value without '\0' (raw value), the latter sees the
> value with '\0' (value normalized by SELinux).
> 
> This could result in two different verification outcomes from EVM and
> ima-evm-utils, if a signature was calculated with a security.selinux value
> terminated by '\0' and the value set in the filesystem is not terminated by
> '\0'. The former would report verification failure due to the missing '\0',
> while the latter would report verification success (because it gets the
> normalized value with '\0').
> 
> This patch mitigates this issue by comparing in evm_calc_hmac_or_hash() the
> size of the xattr returned by the two xattr functions and by warning the
> user if there is a misalignment.

Instead of "misalignment" how about using the word "discrepancy" here
and in the Subject line?

> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> Suggested-by: Mimi Zohar <zohar@linux.ibm.com>

Otherwise, 
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>


^ permalink raw reply	[flat|nested] 2+ messages in thread

end of thread, other threads:[~2021-06-21 12:22 UTC | newest]

Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-06-21  9:39 [PATCH] evm: Check xattr size misalignment between kernel and user Roberto Sassu
2021-06-21 12:22 ` Mimi Zohar

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).