From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DF1B0C77B6F for ; Mon, 27 Mar 2023 07:37:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232615AbjC0HhK (ORCPT ); Mon, 27 Mar 2023 03:37:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41452 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232456AbjC0HhI (ORCPT ); Mon, 27 Mar 2023 03:37:08 -0400 Received: from frasgout11.his.huawei.com (frasgout11.his.huawei.com [14.137.139.23]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A3A30559A; Mon, 27 Mar 2023 00:36:40 -0700 (PDT) Received: from mail02.huawei.com (unknown [172.18.147.228]) by frasgout11.his.huawei.com (SkyGuard) with ESMTP id 4PlPXS5Ndjz9v7Vl; Mon, 27 Mar 2023 15:27:20 +0800 (CST) Received: from roberto-ThinkStation-P620 (unknown [10.204.63.22]) by APP1 (Coremail) with SMTP id LxC2BwAHjQhWRyFku1LSAQ--.62049S2; Mon, 27 Mar 2023 08:36:01 +0100 (CET) Message-ID: <59def311993b839bb4fa623daa973a3bebb52359.camel@huaweicloud.com> Subject: Re: [PATCH v8 4/6] security: Allow all LSMs to provide xattrs for inode_init_security hook From: Roberto Sassu To: Paul Moore Cc: mark@fasheh.com, jlbec@evilplan.org, joseph.qi@linux.alibaba.com, zohar@linux.ibm.com, dmitry.kasatkin@gmail.com, jmorris@namei.org, serge@hallyn.com, stephen.smalley.work@gmail.com, eparis@parisplace.org, casey@schaufler-ca.com, ocfs2-devel@oss.oracle.com, reiserfs-devel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, linux-kernel@vger.kernel.org, keescook@chromium.org, nicolas.bouchinet@clip-os.org, Roberto Sassu Date: Mon, 27 Mar 2023 09:35:47 +0200 In-Reply-To: References: <20230314081720.4158676-1-roberto.sassu@huaweicloud.com> <20230314081720.4158676-5-roberto.sassu@huaweicloud.com> <939e6c88662ad90b963993c4cc1b702083e74a7a.camel@huaweicloud.com> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.36.5-0ubuntu1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-CM-TRANSID: LxC2BwAHjQhWRyFku1LSAQ--.62049S2 X-Coremail-Antispam: 1UD129KBjvJXoW3GFyrKF13Kw1kur1Utr4UXFb_yoWDJF4kpF WUt3Wj9r4kJFy7GrySqa18u3WS9rWfKrW7WrnxGry7AFyqyr1xtFyFyr15uFykZr4kGF1q qr42yrsxu3s8AaDanT9S1TB71UUUUUUqnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDU0xBIdaVrnRJUUUk0b4IE77IF4wAFF20E14v26ryj6rWUM7CY07I20VC2zVCF04k2 6cxKx2IYs7xG6rWj6s0DM7CIcVAFz4kK6r1j6r18M28lY4IEw2IIxxk0rwA2F7IY1VAKz4 vEj48ve4kI8wA2z4x0Y4vE2Ix0cI8IcVAFwI0_Jr0_JF4l84ACjcxK6xIIjxv20xvEc7Cj xVAFwI0_Gr0_Cr1l84ACjcxK6I8E87Iv67AKxVWUJVW8JwA2z4x0Y4vEx4A2jsIEc7CjxV AFwI0_Gr0_Gr1UM2AIxVAIcxkEcVAq07x20xvEncxIr21l5I8CrVACY4xI64kE6c02F40E x7xfMcIj6xIIjxv20xvE14v26r106r15McIj6I8E87Iv67AKxVWUJVW8JwAm72CE4IkC6x 0Yz7v_Jr0_Gr1lF7xvr2IY64vIr41lFIxGxcIEc7CjxVA2Y2ka0xkIwI1l42xK82IYc2Ij 64vIr41l4I8I3I0E4IkC6x0Yz7v_Jr0_Gr1lx2IqxVAqx4xG67AKxVWUJVWUGwC20s026x 8GjcxK67AKxVWUGVWUWwC2zVAF1VAY17CE14v26r4a6rW5MIIYrxkI7VAKI48JMIIF0xvE 2Ix0cI8IcVAFwI0_Jr0_JF4lIxAIcVC0I7IYx2IY6xkF7I0E14v26r4j6F4UMIIF0xvE42 xK8VAvwI8IcIk0rVWrJr0_WFyUJwCI42IY6I8E87Iv67AKxVWUJVW8JwCI42IY6I8E87Iv 6xkF7I0E14v26r4j6r4UJbIYCTnIWIevJa73UjIFyTuYvjxUo0eHDUUUU X-CM-SenderInfo: purev21wro2thvvxqx5xdzvxpfor3voofrz/1tbiAgAJBF1jj4cvOgABst X-CFilter-Loop: Reflected Precedence: bulk List-ID: On Fri, 2023-03-24 at 17:19 -0400, Paul Moore wrote: > On Fri, Mar 24, 2023 at 6:18 AM Roberto Sassu > wrote: > > On Thu, 2023-03-23 at 20:09 -0400, Paul Moore wrote: > > > On Tue, Mar 14, 2023 at 4:19 AM Roberto Sassu > > > wrote: > > > > From: Roberto Sassu > > > > > > > > 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 (EVM crash) > > > > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/ > > > > Signed-off-by: Roberto Sassu > > > > Reviewed-by: Casey Schaufler > > > > Reviewed-by: Mimi Zohar > > > > --- > > > > 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. Ok, will do. Thanks Roberto