When EVM_ALLOW_METADATA_WRITES is set, EVM allows any operation on metadata. Its main purpose is to allow users to freely set metadata when they are protected by a portable signature, until the HMAC key is loaded.
However, IMA is not notified about metadata changes and, after the first appraisal, always allows access to the files without checking metadata again.
This patch checks in evm_reset_status() if EVM_ALLOW_METADATA WRITES is enabled and if it is, sets the IMA_CHANGE_XATTR/ATTR bits depending on the operation performed. At the next appraisal, metadata are revalidated.
This patch also adds a call to evm_reset_status() in evm_inode_post_setattr() so that EVM won't return the cached status the next time appraisal is performed.
Cc: stable@vger.kernel.org # 4.16.x Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-protected metadata") Signed-off-by: Roberto Sassu roberto.sassu@huawei.com --- security/integrity/evm/evm_main.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-)
diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c index 41cc6a4aaaab..d4d918183094 100644 --- a/security/integrity/evm/evm_main.c +++ b/security/integrity/evm/evm_main.c @@ -478,13 +478,17 @@ int evm_inode_removexattr(struct dentry *dentry, const char *xattr_name) return evm_protect_xattr(dentry, xattr_name, NULL, 0); }
-static void evm_reset_status(struct inode *inode) +static void evm_reset_status(struct inode *inode, int bit) { struct integrity_iint_cache *iint;
iint = integrity_iint_find(inode); - if (iint) + if (iint) { + if (evm_initialized & EVM_ALLOW_METADATA_WRITES) + set_bit(bit, &iint->atomic_flags); + iint->evm_status = INTEGRITY_UNKNOWN; + } }
/** @@ -507,7 +511,7 @@ void evm_inode_post_setxattr(struct dentry *dentry, const char *xattr_name, && !posix_xattr_acl(xattr_name))) return;
- evm_reset_status(dentry->d_inode); + evm_reset_status(dentry->d_inode, IMA_CHANGE_XATTR);
evm_update_evmxattr(dentry, xattr_name, xattr_value, xattr_value_len); } @@ -527,7 +531,7 @@ void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name) if (!evm_key_loaded() || !evm_protected_xattr(xattr_name)) return;
- evm_reset_status(dentry->d_inode); + evm_reset_status(dentry->d_inode, IMA_CHANGE_XATTR);
evm_update_evmxattr(dentry, xattr_name, NULL, 0); } @@ -600,6 +604,8 @@ void evm_inode_post_setattr(struct dentry *dentry, int ia_valid) if (!evm_key_loaded()) return;
+ evm_reset_status(dentry->d_inode, IMA_CHANGE_ATTR); + if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) evm_update_evmxattr(dentry, NULL, NULL, 0); }
On Thu, 2020-06-18 at 18:04 +0200, Roberto Sassu wrote:
^after the first successful appraisal
EVM modifying IMA bits crosses the boundary between EVM and IMA. There is already an IMA post_setattr hook. IMA could reset its own bit there. If necessary EVM could export as a function it's status info.
Mimi
I wouldn't try to guess in IMA when EVM resets its status. We would have to duplicate the logic to check if an EVM key is loaded, if the passed xattr is a POSIX ACL, ...
I think it is better to set a flag, maybe a new one, directly in EVM, to notify the integrity subsystem that iint->evm_status is no longer valid.
If the EVM flag is set, IMA would reset the appraisal flags, as it uses iint->evm_status for appraisal. We can consider to reset also the measure flags when we have a template that includes file metadata.
Roberto
HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Li Jian, Shi Yanli
On Tue, 2020-09-01 at 09:08 +0000, Roberto Sassu wrote:
Agreed, but IMA could call an EVM function.
When would IMA read the EVM flag? Who would reset the flag? At what point would it be reset? Just as EVM shouldn't be resetting the IMA flag, IMA shouldn't be resetting the EVM flag.
Mimi
linux-stable-mirror@lists.linaro.org
-
Mimi Zohar -
Roberto Sassu