> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx] > Sent: Thursday, September 17, 2020 2:01 PM > [Cc'ing John Johansen] > > Hi Roberto, > > On Fri, 2020-09-04 at 11:26 +0200, Roberto Sassu wrote: > > 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 > > successful appraisal, always allows access to the files without checking > > metadata again. > > > > This patch introduces the new atomic flag EVM_RESET_STATUS in > > integrity_iint_cache that is set in the EVM post hooks and cleared in > > evm_verify_hmac(). IMA checks the new flag in process_measurement() > and if > > it is set, it clears the appraisal flags. > > > > Although the flag could be cleared also by evm_inode_setxattr() and > > evm_inode_setattr() before IMA sees it, this does not happen if > > EVM_ALLOW_METADATA_WRITES is set. Since the only remaining caller is > > evm_verifyxattr(), this ensures that IMA always sees the flag set before it > > is cleared. > > > > 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@xxxxxxxxxxxxxxx # 4.16.x > > Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of > EVM-protected metadata") > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx> > > --- > > security/integrity/evm/evm_main.c | 17 +++++++++++++++-- > > security/integrity/ima/ima_main.c | 8 ++++++-- > > security/integrity/integrity.h | 1 + > > 3 files changed, 22 insertions(+), 4 deletions(-) > > > > diff --git a/security/integrity/evm/evm_main.c > b/security/integrity/evm/evm_main.c > > index 4e9f5e8b21d5..05be1ad3e6f3 100644 > > --- a/security/integrity/evm/evm_main.c > > +++ b/security/integrity/evm/evm_main.c > > @@ -221,8 +221,15 @@ static enum integrity_status > evm_verify_hmac(struct dentry *dentry, > > evm_status = (rc == -ENODATA) ? > > INTEGRITY_NOXATTRS : INTEGRITY_FAIL; > > out: > > - if (iint) > > + if (iint) { > > + /* > > + * EVM_RESET_STATUS can be cleared only by > evm_verifyxattr() > > + * when EVM_ALLOW_METADATA_WRITES is set. This > guarantees that > > + * IMA sees the EVM_RESET_STATUS flag set before it is > cleared. > > + */ > > + clear_bit(EVM_RESET_STATUS, &iint->atomic_flags); > > iint->evm_status = evm_status; > > True IMA is currently the only caller of evm_verifyxattr() in the > upstreamed kernel, but it is an exported function, which may be called > from elsewhere. The previous version crossed the boundary between EVM > & IMA with EVM modifying the IMA flag directly. This version assumes > that IMA will be the only caller. Otherwise, I like this version. Ok, I think it is better, as you suggested, to export a new EVM function that tells if evm_reset_status() will be executed in the EVM post hooks, and to call this function from IMA. IMA would then call ima_reset_appraise_flags() also depending on the result of the new EVM function. ima_reset_appraise_flags() should be called in a post hook in IMA. Should I introduce it? Thanks Roberto HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Li Jian, Shi Yanli > Mimi > > > + } > > kfree(xattr_data); > > return evm_status; > > } > > @@ -418,8 +425,12 @@ static void evm_reset_status(struct inode *inode) > > struct integrity_iint_cache *iint; > > > > iint = integrity_iint_find(inode); > > - if (iint) > > + if (iint) { > > + if (evm_initialized & EVM_ALLOW_METADATA_WRITES) > > + set_bit(EVM_RESET_STATUS, &iint->atomic_flags); > > + > > iint->evm_status = INTEGRITY_UNKNOWN; > > + } > > } > > > > /** > > @@ -513,6 +524,8 @@ void evm_inode_post_setattr(struct dentry > *dentry, int ia_valid) > > if (!evm_key_loaded()) > > return; > > > > + evm_reset_status(dentry->d_inode); > > + > > if (ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) > > evm_update_evmxattr(dentry, NULL, NULL, 0); > > }
