On Fri, Mar 24, 2023 at 6:18 AM Roberto Sassu <roberto.sassu@xxxxxxxxxxxxxxx> wrote: > On Thu, 2023-03-23 at 20:09 -0400, Paul Moore wrote: > > On Tue, Mar 14, 2023 at 4:19 AM Roberto Sassu > > <roberto.sassu@xxxxxxxxxxxxxxx> wrote: > > > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx> > > > > > > Currently, security_inode_init_security() supports only one LSM providing > > > an xattr and EVM calculating the HMAC on that xattr, plus other inode > > > metadata. > > > > > > Allow all LSMs to provide one or multiple xattrs, by extending the security > > > blob reservation mechanism. Introduce the new lbs_xattr field of the > > > lsm_blob_sizes structure, so that each LSM can specify how many xattrs it > > > needs, and the LSM infrastructure knows how many xattr slots it should > > > allocate. > > > > > > Dynamically allocate the xattrs array to be populated by LSMs with the > > > inode_init_security hook, and pass it to the latter instead of the > > > name/value/len triple. Update the documentation accordingly, and fix the > > > description of the xattr name, as it is not allocated anymore. > > > > > > Since the LSM infrastructure, at initialization time, updates the number of > > > the requested xattrs provided by each LSM with a corresponding offset in > > > the security blob (in this case the xattr array), it makes straightforward > > > for an LSM to access the right position in the xattr array. > > > > > > There is still the issue that an LSM might not fill the xattr, even if it > > > requests it (legitimate case, for example it might have been loaded but not > > > initialized with a policy). Since users of the xattr array (e.g. the > > > initxattrs() callbacks) detect the end of the xattr array by checking if > > > the xattr name is NULL, not filling an xattr would cause those users to > > > stop scanning xattrs prematurely. > > > > > > Solve that issue by introducing security_check_compact_filled_xattrs(), > > > which does a basic check of the xattr array (if the xattr name is filled, > > > the xattr value should be too, and viceversa), and compacts the xattr array > > > by removing the holes. > > > > > > An alternative solution would be to let users of the xattr array know the > > > number of elements of that array, so that they don't have to check the > > > termination. However, this seems more invasive, compared to a simple move > > > of few array elements. > > > > > > security_check_compact_filled_xattrs() also determines how many xattrs in > > > the xattr array have been filled. If there is none, skip > > > evm_inode_init_security() and initxattrs(). Skipping the former also avoids > > > EVM to crash the kernel, as it is expecting a filled xattr. > > > > > > Finally, adapt both SELinux and Smack to use the new definition of the > > > inode_init_security hook, and to correctly fill the designated slots in the > > > xattr array. For Smack, reserve space for the other defined xattrs although > > > they are not set yet in smack_inode_init_security(). > > > > > > Reported-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx> (EVM crash) > > > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/ > > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx> > > > Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx> > > > Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx> > > > --- > > > include/linux/lsm_hook_defs.h | 3 +- > > > include/linux/lsm_hooks.h | 1 + > > > security/security.c | 119 +++++++++++++++++++++++++++++----- > > > security/selinux/hooks.c | 19 ++++-- > > > security/smack/smack_lsm.c | 33 ++++++---- > > > 5 files changed, 137 insertions(+), 38 deletions(-) ... > > > @@ -1604,33 +1654,66 @@ int security_inode_init_security(struct inode *inode, struct inode *dir, > > > const struct qstr *qstr, > > > const initxattrs initxattrs, void *fs_data) > > > { > > > - struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1]; > > > - struct xattr *lsm_xattr, *evm_xattr, *xattr; > > > - int ret; > > > + struct security_hook_list *P; > > > + struct xattr *new_xattrs; > > > + struct xattr *xattr; > > > + int ret = -EOPNOTSUPP, num_filled_xattrs = 0; > > > > > > if (unlikely(IS_PRIVATE(inode))) > > > return 0; > > > > > > + if (!blob_sizes.lbs_xattr) > > > + return 0; > > > + > > > if (!initxattrs) > > > return call_int_hook(inode_init_security, -EOPNOTSUPP, inode, > > > - dir, qstr, NULL, NULL, NULL); > > > - memset(new_xattrs, 0, sizeof(new_xattrs)); > > > - lsm_xattr = new_xattrs; > > > - ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr, > > > - &lsm_xattr->name, > > > - &lsm_xattr->value, > > > - &lsm_xattr->value_len); > > > - if (ret) > > > + dir, qstr, NULL); > > > + /* Allocate +1 for EVM and +1 as terminator. */ > > > + new_xattrs = kcalloc(blob_sizes.lbs_xattr + 2, sizeof(*new_xattrs), > > > + GFP_NOFS); > > > + if (!new_xattrs) > > > + return -ENOMEM; > > > + > > > + hlist_for_each_entry(P, &security_hook_heads.inode_init_security, > > > + list) { > > > + ret = P->hook.inode_init_security(inode, dir, qstr, new_xattrs); > > > + if (ret && ret != -EOPNOTSUPP) > > > + goto out; > > > + /* > > > + * As documented in lsm_hooks.h, -EOPNOTSUPP in this context > > > + * means that the LSM is not willing to provide an xattr, not > > > + * that it wants to signal an error. Thus, continue to invoke > > > + * the remaining LSMs. > > > + */ > > > + if (ret == -EOPNOTSUPP) > > > + continue; > > > + /* > > > + * As the number of xattrs reserved by LSMs is not directly > > > + * available, directly use the total number blob_sizes.lbs_xattr > > > + * to keep the code simple, while being not the most efficient > > > + * way. > > > + */ > > > > Is there a good reason why the LSM can't return the number of xattrs > > it is adding to the xattr array? It seems like it should be fairly > > trivial for the individual LSMs to determine and it could save a lot > > of work. However, given we're at v8 on this patchset I'm sure I'm > > missing something obvious, can you help me understand why the idea > > above is crazy stupid? ;) > > Ok, I looked back at what I did for v3. > > Moving from v3 to v4, I decided to put less burden on LSMs, and to make > all the processing from the LSM infrastructure side. As a general rule I think it's a good goal to keep the LSM layer as small as possible; I believe it allows us to be more flexible with the LSMs and it keeps the LSM as simple as possible. I mean less code, less bugs, amirite? ... ;) > v3 had some safeguards to prevent some programming mistakes by LSMs, > which maybe made the code less understandable. > > However, if we say we keep things as simple as possible and assume that > LSMs implement this correctly, we can just pass num_filled_xattrs to > them and they simply increment it. > > The EVM bug should not arise (accessing xattr->name = NULL), even if > BPF LSM alone returns zero, due to the check of num_filled_xattrs > before calling evm_inode_init_security(). > > Patch 6 (at the end) will prevent the bug from arising when EVM is > moved to the LSM infrastructure (no num_filled_xattrs check anymore). > There is a loop that stops if xattr->name is NULL, so > evm_protected_xattr() will not be called. > > Or, like you suggested, we just return a positive value from LSMs and > we keep num_filled_xattrs in security_inode_init_security(). I like the idea of individual LSMs simply reporting the number of xattrs they've generated instead of incrementing the num_filled_xattrs variable. It seems like returning the xattr count as a positive return value should work just fine, leaving negative values for errors, but if you run into problems you can always pass the value back in a new parameter pointer if needed. > > > @@ -2868,11 +2870,11 @@ static int selinux_dentry_create_files_as(struct dentry *dentry, int mode, > > > > > > static int selinux_inode_init_security(struct inode *inode, struct inode *dir, > > > const struct qstr *qstr, > > > - const char **name, > > > - void **value, size_t *len) > > > + struct xattr *xattrs) > > > { > > > const struct task_security_struct *tsec = selinux_cred(current_cred()); > > > struct superblock_security_struct *sbsec; > > > + struct xattr *xattr = NULL; > > > u32 newsid, clen; > > > int rc; > > > char *context; > > > @@ -2899,16 +2901,18 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir, > > > !(sbsec->flags & SBLABEL_MNT)) > > > return -EOPNOTSUPP; > > > > > > - if (name) > > > - *name = XATTR_SELINUX_SUFFIX; > > > + if (xattrs) > > > + xattr = xattrs + selinux_blob_sizes.lbs_xattr; > > > > Please abstract that away to an inline function similar to > > selinux_cred(), selinux_file(), selinux_inode(), etc. > > Ok. > > > > + if (xattr) { > > > + xattr->name = XATTR_SELINUX_SUFFIX; > > > > I'm guessing the xattr->name assignment is always done, regardless of > > if security_sid_to_context_force() is successful, due to the -EINVAL > > check in security_check_compact_filled_xattrs()? If yes, it would be > > good to make note of that here in the code. If not, it would be nice > > to move this down the function to go with the other xattr->XXX > > assignments, unless there is another reason for its placement that I'm > > missing. > > Uhm, if an LSM returns an error, security_inode_init_security() stops > and does the cleanup. It should not matter if xattr->name was set. Okay, I thought I might be missing something during the review. Since there is no special reason for putting the xattr->name assignment up there, please move it down below with the other xattr->XXX assignments. Thanks. -- paul-moore.com
