Hi Christian, On Wed, 2022-09-28 at 18:08 +0200, Christian Brauner wrote: > The current way of setting and getting posix acls through the generic > xattr interface is error prone and type unsafe. The vfs needs to > interpret and fixup posix acls before storing or reporting it to > userspace. Various hacks exist to make this work. The code is hard to > understand and difficult to maintain in it's current form. Instead of > making this work by hacking posix acls through xattr handlers we are > building a dedicated posix acl api around the get and set inode > operations. This removes a lot of hackiness and makes the codepaths > easier to maintain. A lot of background can be found in [1]. > > So far posix acls were passed as a void blob to the security and > integrity modules. Some of them like evm then proceed to interpret the > void pointer and convert it into the kernel internal struct posix acl > representation to perform their integrity checking magic. This is > obviously pretty problematic as that requires knowledge that only the > vfs is guaranteed to have and has lead to various bugs. Add a proper > security hook for setting posix acls and pass down the posix acls in > their appropriate vfs format instead of hacking it through a void > pointer stored in the uapi format. > > I spent considerate time in the security module and integrity > infrastructure and audited all codepaths. EVM is the only part that > really has restrictions based on the actual posix acl values passed > through it (e.g. i_mode). > Before this dedicated hook EVM used to translate from the > uapi posix acl format sent to it in the form of a void pointer into the > vfs format. This is not a good thing. Instead of hacking around in the > uapi struct give EVM the posix acls in the appropriate vfs format and > perform sane permissions checks that mirror what it used to to in the > generic xattr hook. > > IMA doesn't have any restrictions on posix acls. When posix acls are > changed it just wants to update its appraisal status. to trigger an EVM re-validation. > The removal of posix acls is equivalent to passing NULL to the posix set > acl hooks. This is the same as before through the generic xattr api. > > Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@xxxxxxxxxx [1] > Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx> > --- > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c > index 23d484e05e6f..7904786b610f 100644 > --- a/security/integrity/evm/evm_main.c > +++ b/security/integrity/evm/evm_main.c > @@ -8,7 +8,7 @@ > * > * File: evm_main.c > * implements evm_inode_setxattr, evm_inode_post_setxattr, > - * evm_inode_removexattr, and evm_verifyxattr > + * evm_inode_removexattr, evm_verifyxattr, and evm_inode_set_acl. > */ > > #define pr_fmt(fmt) "EVM: "fmt > @@ -670,6 +670,74 @@ int evm_inode_removexattr(struct user_namespace *mnt_userns, > return evm_protect_xattr(mnt_userns, dentry, xattr_name, NULL, 0); > } > > +static int evm_inode_set_acl_change(struct user_namespace *mnt_userns, > + struct dentry *dentry, const char *name, > + struct posix_acl *kacl) > +{ > +#ifdef CONFIG_FS_POSIX_ACL > + int rc; > + > + umode_t mode; > + struct inode *inode = d_backing_inode(dentry); > + > + if (!kacl) > + return 1; > + > + rc = posix_acl_update_mode(mnt_userns, inode, &mode, &kacl); > + if (rc || (inode->i_mode != mode)) acl_res in the existing evm_xattr_acl_change() code is based on the init_user_ns. Is that the same here? Is it guaranteed? > + return 1; > +#endif > + return 0; > +} > + > +/** > + * evm_inode_set_acl - protect the EVM extended attribute for posix acls ^from posix acls > + * @mnt_userns: user namespace of the idmapped mount > + * @dentry: pointer to the affected dentry > + * @acl_name: name of the posix acl > + * @kacl: pointer to the posix acls Prevent modifying posix acls causing the EVM HMAC to be re-calculated and 'security.evm' xattr updated, unless the existing 'security.evm' is valid. -- thanks, Mimi
