> 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
> owned by root and the archive is extracted by root.
>
> Then, since portable signatures are immutable, all subsequent operations
> 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.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> ---
> 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
> + * @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))
> + 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)
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)
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));
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
[ 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
[ 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?
If there are no objections, I will send an additional patch to pass
mnt_userns to EVM.
Thanks
Roberto
HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli
> +
> + posix_acl_release(acl);
> +
> + if (rc)
> + return 1;
> +
> + if (inode->i_mode != mode)
> + return 1;
> +
> + return 0;
> +}
> +
> +/*
> + * evm_xattr_change - check if passed xattr value differs from current
> value
> + * @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 xattr value differs from current value.
> + *
> + * Returns 1 if passed xattr value differs from current value, 0 otherwise.
> + */
> +static int evm_xattr_change(struct dentry *dentry, const char
> *xattr_name,
> + const void *xattr_value, size_t xattr_value_len)
> +{
> + char *xattr_data = NULL;
> + int rc = 0;
> +
> + if (posix_xattr_acl(xattr_name))
> + return evm_xattr_acl_change(dentry, xattr_name,
> xattr_value,
> + xattr_value_len);
> +
> + rc = vfs_getxattr_alloc(&init_user_ns, dentry, xattr_name,
> &xattr_data,
> + 0, GFP_NOFS);
> + if (rc < 0)
> + return 1;
> +
> + if (rc == xattr_value_len)
> + rc = memcmp(xattr_value, xattr_data, rc);
> + else
> + rc = 1;
> +
> + kfree(xattr_data);
> + return rc;
> +}
> +
> /*
> * evm_protect_xattr - protect the EVM extended attribute
> *
> @@ -388,6 +462,10 @@ static int evm_protect_xattr(struct dentry *dentry,
> const char *xattr_name,
> if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> return 0;
>
> + if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> + !evm_xattr_change(dentry, xattr_name, xattr_value,
> xattr_value_len))
> + return 0;
> +
> if (evm_status != INTEGRITY_PASS)
> integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
> dentry->d_name.name,
> "appraise_metadata",
> @@ -527,6 +605,19 @@ void evm_inode_post_removexattr(struct dentry
> *dentry, const char *xattr_name)
> evm_update_evmxattr(dentry, xattr_name, NULL, 0);
> }
>
> +static int evm_attr_change(struct dentry *dentry, struct iattr *attr)
> +{
> + struct inode *inode = d_backing_inode(dentry);
> + unsigned int ia_valid = attr->ia_valid;
> +
> + if ((!(ia_valid & ATTR_UID) || uid_eq(attr->ia_uid, inode->i_uid))
> &&
> + (!(ia_valid & ATTR_GID) || gid_eq(attr->ia_gid, inode->i_gid)) &&
> + (!(ia_valid & ATTR_MODE) || attr->ia_mode == inode->i_mode))
> + return 0;
> +
> + return 1;
> +}
> +
> /**
> * evm_inode_setattr - prevent updating an invalid EVM extended
> attribute
> * @dentry: pointer to the affected dentry
> @@ -557,6 +648,11 @@ int evm_inode_setattr(struct dentry *dentry, struct
> iattr *attr)
> (evm_status == INTEGRITY_FAIL_IMMUTABLE) ||
> (evm_ignore_error_safe(evm_status)))
> return 0;
> +
> + if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> + !evm_attr_change(dentry, attr))
> + return 0;
> +
> integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> d_backing_inode(dentry),
> dentry->d_name.name, "appraise_metadata",
> integrity_status_msg[evm_status], -EPERM, 0);
> --
> 2.26.2
