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Message-ID: <ffc86b3907f7b87d3c568ae62bea3cdb3275be4e.camel@huaweicloud.com>
Date: Fri, 24 Mar 2023 14:25:21 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Paul Moore <paul@...l-moore.com>
Cc: mark@...heh.com, jlbec@...lplan.org, joseph.qi@...ux.alibaba.com,
zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, jmorris@...ei.org,
serge@...lyn.com, stephen.smalley.work@...il.com,
eparis@...isplace.org, casey@...aufler-ca.com,
ocfs2-devel@....oracle.com, reiserfs-devel@...r.kernel.org,
linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
linux-kernel@...r.kernel.org, keescook@...omium.org,
nicolas.bouchinet@...p-os.org,
Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v8 4/6] security: Allow all LSMs to provide xattrs for
inode_init_security hook
On Fri, 2023-03-24 at 11:18 +0100, 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
> > <roberto.sassu@...weicloud.com> wrote:
> > > From: Roberto Sassu <roberto.sassu@...wei.com>
> > >
> > > 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@...p-os.org> (EVM crash)
> > > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> > > Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> > > Reviewed-by: Casey Schaufler <casey@...aufler-ca.com>
> > > Reviewed-by: Mimi Zohar <zohar@...ux.ibm.com>
> > > ---
> > > 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(-)
> > >
> > > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> > > index 6bb55e61e8e..b814955ae70 100644
> > > --- a/include/linux/lsm_hook_defs.h
> > > +++ b/include/linux/lsm_hook_defs.h
> > > @@ -112,8 +112,7 @@ LSM_HOOK(int, 0, path_notify, const struct path *path, u64 mask,
> > > LSM_HOOK(int, 0, inode_alloc_security, struct inode *inode)
> > > LSM_HOOK(void, LSM_RET_VOID, inode_free_security, struct inode *inode)
> > > LSM_HOOK(int, 0, inode_init_security, struct inode *inode,
> > > - struct inode *dir, const struct qstr *qstr, const char **name,
> > > - void **value, size_t *len)
> > > + struct inode *dir, const struct qstr *qstr, struct xattr *xattrs)
> > > LSM_HOOK(int, 0, inode_init_security_anon, struct inode *inode,
> > > const struct qstr *name, const struct inode *context_inode)
> > > LSM_HOOK(int, 0, inode_create, struct inode *dir, struct dentry *dentry,
> > > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > > index c2be66c669a..75a2f85b49d 100644
> > > --- a/include/linux/lsm_hooks.h
> > > +++ b/include/linux/lsm_hooks.h
> > > @@ -63,6 +63,7 @@ struct lsm_blob_sizes {
> > > int lbs_ipc;
> > > int lbs_msg_msg;
> > > int lbs_task;
> > > + int lbs_xattr; /* number of xattr slots in new_xattrs array */
> >
> > No need for the comment, we don't do it for the other fields.
> >
> > > };
> > >
> > > /*
> > > diff --git a/security/security.c b/security/security.c
> > > index f4170efcddd..f1f5f62f7fa 100644
> > > --- a/security/security.c
> > > +++ b/security/security.c
> > > @@ -1579,6 +1579,52 @@ int security_dentry_create_files_as(struct dentry *dentry, int mode,
> > > }
> > > EXPORT_SYMBOL(security_dentry_create_files_as);
> > >
> > > +/**
> > > + * security_check_compact_filled_xattrs - check xattrs and make array contiguous
> > > + * @xattrs: xattr array filled by LSMs
> > > + * @num_xattrs: length of xattr array
> > > + * @num_filled_xattrs: number of already processed xattrs
> > > + *
> > > + * Ensure that each xattr slot is correctly filled and close the gaps in the
> > > + * xattr array if an LSM didn't provide an xattr for which it asked space
> > > + * (legitimate case, it might have been loaded but not initialized). An LSM
> > > + * might request space in the xattr array for one or multiple xattrs. The LSM
> > > + * infrastructure ensures that all requests by LSMs are satisfied.
> > > + *
> > > + * Track the number of filled xattrs in @num_filled_xattrs, so that it is easy
> > > + * to determine whether the currently processed xattr is fine in its position
> > > + * (if all previous xattrs were filled) or it should be moved after the last
> > > + * filled xattr.
> > > + *
> > > + * Return: zero if all xattrs are valid, -EINVAL otherwise.
> > > + */
> > > +static int security_check_compact_filled_xattrs(struct xattr *xattrs,
> > > + int num_xattrs,
> > > + int *num_filled_xattrs)
> >
> > That is one long name :)
> >
> > Since you're making some other changes to this patch, can you rename
> > this to security_xattr_compact() or something like that?
>
> Yes, definitely!
>
> > > +{
> > > + int i;
> > > +
> > > + for (i = *num_filled_xattrs; i < num_xattrs; i++) {
> > > + if ((!xattrs[i].name && xattrs[i].value) ||
> > > + (xattrs[i].name && !xattrs[i].value))
> > > + return -EINVAL;
> > > +
> > > + if (!xattrs[i].name)
> > > + continue;
> > > +
> > > + if (i == *num_filled_xattrs) {
> > > + (*num_filled_xattrs)++;
> > > + continue;
> > > + }
> > > +
> > > + memcpy(xattrs + (*num_filled_xattrs)++, xattrs + i,
> > > + sizeof(*xattrs));
> > > + memset(xattrs + i, 0, sizeof(*xattrs));
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +
> > > /**
> > > * security_inode_init_security() - Initialize an inode's LSM context
> > > * @inode: the inode
> > > @@ -1591,9 +1637,13 @@ EXPORT_SYMBOL(security_dentry_create_files_as);
> > > * created inode and set up the incore security field for the new inode. This
> > > * hook is called by the fs code as part of the inode creation transaction and
> > > * provides for atomic labeling of the inode, unlike the post_create/mkdir/...
> > > - * hooks called by the VFS. The hook function is expected to allocate the name
> > > - * and value via kmalloc, with the caller being responsible for calling kfree
> > > - * after using them. If the security module does not use security attributes
> > > + * hooks called by the VFS. The hook function is expected to populate the
> > > + * @xattrs array, depending on how many xattrs have been specified by the
> > > + * security module in the lbs_xattr field of the lsm_blob_sizes structure. For
> > > + * each array element, the hook function is expected to set ->name to the
> > > + * attribute name suffix (e.g. selinux), to allocate ->value (will be freed by
> > > + * the caller) and set it to the attribute value, to set ->value_len to the
> > > + * length of the value. If the security module does not use security attributes
> > > * or does not wish to put a security attribute on this particular inode, then
> > > * it should return -EOPNOTSUPP to skip this processing.
> > > *
> > > @@ -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? ;)
Much simple answer. Yes, LSMs could return the number of xattrs set,
but security_check_compact_filled_xattrs() also needs to know from
which offset (the lbs_xattr of each LSM) it should start compacting.
Example: suppose that you have three LSMs with:
LSM#1: lbs_xattr 1
LSM#2: lbs_xattr 2 (disabled)
LSM#3: lbs_xattr 1
The current compaction interval is: already compacted xattrs - end of
new_xattr array.
When the security_inode_init_security() loop calls LSM#3, the
compaction interval is: 1 - 2 (LSM#2 returns 0), which clearly isn't
right. The correct compaction interval should be: 3 - 4.
Going to the end of new_xattrs is an approximation, but it ensures
that security_check_compact_filled_xattrs() reaches the xattr set by
LSM#3.
The alternative I was mentioning of passing num_filled_xattrs to LSMs
goes again in the direction of doing on-the-fly compaction, while LSMs
are more familiar with using the lbs_* fields.
I suggest to keep this part as it is, if you agree.
Thanks
Roberto
> 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.
>
> 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().
>
> Ok, I'll go for your proposal.
>
> > > + ret = security_check_compact_filled_xattrs(new_xattrs,
> > > + blob_sizes.lbs_xattr,
> > > + &num_filled_xattrs);
> > > + if (ret < 0) {
> > > + ret = -ENOMEM;
> > > + goto out;
> > > + }
> > > + }
> > > +
> > > + if (!num_filled_xattrs)
> > > goto out;
> > >
> > > - evm_xattr = lsm_xattr + 1;
> > > - ret = evm_inode_init_security(inode, lsm_xattr, evm_xattr);
> > > + ret = evm_inode_init_security(inode, new_xattrs,
> > > + new_xattrs + num_filled_xattrs);
> > > if (ret)
> > > goto out;
> > > ret = initxattrs(inode, new_xattrs, fs_data);
> > > out:
> > > for (xattr = new_xattrs; xattr->value != NULL; xattr++)
> > > kfree(xattr->value);
> > > + kfree(new_xattrs);
> > > return (ret == -EOPNOTSUPP) ? 0 : ret;
> > > }
> > > EXPORT_SYMBOL(security_inode_init_security);
> > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > index 9a5bdfc2131..3e4308dd336 100644
> > > --- a/security/selinux/hooks.c
> > > +++ b/security/selinux/hooks.c
> > > @@ -104,6 +104,8 @@
> > > #include "audit.h"
> > > #include "avc_ss.h"
> > >
> > > +#define SELINUX_INODE_INIT_XATTRS 1
> > > +
> > > struct selinux_state selinux_state;
> > >
> > > /* SECMARK reference count */
> > > @@ -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.
>
> Thanks
>
> Roberto
>
> > > - if (value && len) {
> > > rc = security_sid_to_context_force(&selinux_state, newsid,
> > > &context, &clen);
> > > if (rc)
> > > return rc;
> > > - *value = context;
> > > - *len = clen;
> > > + xattr->value = context;
> > > + xattr->value_len = clen;
> > > }
> > >
> > > return 0;
> > > @@ -6918,6 +6922,7 @@ struct lsm_blob_sizes selinux_blob_sizes __lsm_ro_after_init = {
> > > .lbs_ipc = sizeof(struct ipc_security_struct),
> > > .lbs_msg_msg = sizeof(struct msg_security_struct),
> > > .lbs_superblock = sizeof(struct superblock_security_struct),
> > > + .lbs_xattr = SELINUX_INODE_INIT_XATTRS,
> > > };
> > >
> > > #ifdef CONFIG_PERF_EVENTS
> > > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> > > index cfcbb748da2..c8cf8df268b 100644
> > > --- a/security/smack/smack_lsm.c
> > > +++ b/security/smack/smack_lsm.c
> > > @@ -52,6 +52,15 @@
> > > #define SMK_RECEIVING 1
> > > #define SMK_SENDING 2
> > >
> > > +/*
> > > + * Smack uses multiple xattrs.
> > > + * SMACK64 - for access control, SMACK64EXEC - label for the program,
> > > + * SMACK64MMAP - controls library loading,
> > > + * SMACK64TRANSMUTE - label initialization,
> > > + * Not saved on files - SMACK64IPIN and SMACK64IPOUT
> > > + */
> > > +#define SMACK_INODE_INIT_XATTRS 4
> > > +
> > > #ifdef SMACK_IPV6_PORT_LABELING
> > > static DEFINE_MUTEX(smack_ipv6_lock);
> > > static LIST_HEAD(smk_ipv6_port_list);
> > > @@ -939,26 +948,27 @@ static int smack_inode_alloc_security(struct inode *inode)
> > > * @inode: the newly created inode
> > > * @dir: containing directory object
> > > * @qstr: unused
> > > - * @name: where to put the attribute name
> > > - * @value: where to put the attribute value
> > > - * @len: where to put the length of the attribute
> > > + * @xattrs: where to put the attributes
> > > *
> > > * Returns 0 if it all works out, -ENOMEM if there's no memory
> > > */
> > > static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> > > - const struct qstr *qstr, const char **name,
> > > - void **value, size_t *len)
> > > + const struct qstr *qstr,
> > > + struct xattr *xattrs)
> > > {
> > > struct inode_smack *issp = smack_inode(inode);
> > > struct smack_known *skp = smk_of_current();
> > > struct smack_known *isp = smk_of_inode(inode);
> > > struct smack_known *dsp = smk_of_inode(dir);
> > > + struct xattr *xattr = NULL;
> > > int may;
> > >
> > > - if (name)
> > > - *name = XATTR_SMACK_SUFFIX;
> > > + if (xattrs)
> > > + xattr = xattrs + smack_blob_sizes.lbs_xattr;
> > > +
> > > + if (xattr) {
> > > + xattr->name = XATTR_SMACK_SUFFIX;
> > >
> > > - if (value && len) {
> > > rcu_read_lock();
> > > may = smk_access_entry(skp->smk_known, dsp->smk_known,
> > > &skp->smk_rules);
> > > @@ -976,11 +986,11 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> > > issp->smk_flags |= SMK_INODE_CHANGED;
> > > }
> > >
> > > - *value = kstrdup(isp->smk_known, GFP_NOFS);
> > > - if (*value == NULL)
> > > + xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
> > > + if (xattr->value == NULL)
> > > return -ENOMEM;
> > >
> > > - *len = strlen(isp->smk_known);
> > > + xattr->value_len = strlen(isp->smk_known);
> > > }
> > >
> > > return 0;
> > > @@ -4854,6 +4864,7 @@ struct lsm_blob_sizes smack_blob_sizes __lsm_ro_after_init = {
> > > .lbs_ipc = sizeof(struct smack_known *),
> > > .lbs_msg_msg = sizeof(struct smack_known *),
> > > .lbs_superblock = sizeof(struct superblock_smack),
> > > + .lbs_xattr = SMACK_INODE_INIT_XATTRS,
> > > };
> > >
> > > static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
> > > --
> > > 2.25.1
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