[PATCH v35 28/29] LSM: Add /proc attr entry for full LSM context

Casey Schaufler
2022-04-18T15:45:38Z

Add an entry /proc/.../attr/context which displays the full
process security "context" in compound format:
lsm1\0value\0lsm2\0value\0...
This entry is not writable.

A security module may decide that its policy does not allow
this information to be displayed. In this case none of the
information will be displayed.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Cc: linux-api@vger.kernel.org
Cc: linux-doc@vger.kernel.org
---
Documentation/security/lsm.rst | 14 +++++
fs/proc/base.c | 1 +
include/linux/lsm_hooks.h | 6 +++
security/apparmor/include/procattr.h | 2 +-
security/apparmor/lsm.c | 8 ++-
security/apparmor/procattr.c | 22 ++++----
security/security.c | 79 ++++++++++++++++++++++++++++
security/selinux/hooks.c | 2 +-
security/smack/smack_lsm.c | 2 +-
9 files changed, 121 insertions(+), 15 deletions(-)

diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst
index b77b4a540391..070225ae6ceb 100644
--- a/Documentation/security/lsm.rst
+++ b/Documentation/security/lsm.rst
@@ -143,3 +143,17 @@ separated list of the active security modules.
The file ``/proc/pid/attr/interface_lsm`` contains the name of the security
module for which the ``/proc/pid/attr/current`` interface will
apply. This interface can be written to.
+
+The infrastructure does provide an interface for the special
+case where multiple security modules provide a process context.
+This is provided in compound context format.
+
+- `lsm\0value\0lsm\0value\0`
+
+The `lsm` and `value` fields are NUL-terminated bytestrings.
+Each field may contain whitespace or non-printable characters.
+The NUL bytes are included in the size of a compound context.
+The context ``Bell\0Secret\0Biba\0Loose\0`` has a size of 23.
+
+The file ``/proc/pid/attr/context`` provides the security
+context of the identified process.
diff --git a/fs/proc/base.c b/fs/proc/base.c
index f2d15348bdff..f8aed4404e7e 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2828,6 +2828,7 @@ static const struct pid_entry attr_dir_stuff[] = {
ATTR(NULL, "keycreate", 0666),
ATTR(NULL, "sockcreate", 0666),
ATTR(NULL, "interface_lsm", 0666),
+ ATTR(NULL, "context", 0444),
#ifdef CONFIG_SECURITY_SMACK
DIR("smack", 0555,
proc_smack_attr_dir_inode_ops, proc_smack_attr_dir_ops),
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index fd63ae215104..425538ebc606 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1401,6 +1401,12 @@
* @pages contains the number of pages.
* Return 0 if permission is granted.
*
+ * @getprocattr:
+ * Provide the named process attribute for display in special files in
+ * the /proc/.../attr directory. Attribute naming and the data displayed
+ * is at the discretion of the security modules. The exception is the
+ * "context" attribute, which will contain the security context of the
+ * task as a nul terminated text string without trailing whitespace.
* @ismaclabel:
* Check if the extended attribute specified by @name
* represents a MAC label. Returns 1 if name is a MAC
diff --git a/security/apparmor/include/procattr.h b/security/apparmor/include/procattr.h
index 31689437e0e1..03dbfdb2f2c0 100644
--- a/security/apparmor/include/procattr.h
+++ b/security/apparmor/include/procattr.h
@@ -11,7 +11,7 @@
#ifndef __AA_PROCATTR_H
#define __AA_PROCATTR_H

-int aa_getprocattr(struct aa_label *label, char **string);
+int aa_getprocattr(struct aa_label *label, char **string, bool newline);
int aa_setprocattr_changehat(char *args, size_t size, int flags);

#endif /* __AA_PROCATTR_H */
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 29181bc8c693..1ee58c1491ab 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -602,6 +602,7 @@ static int apparmor_getprocattr(struct task_struct *task, char *name,
const struct cred *cred = get_task_cred(task);
struct aa_task_ctx *ctx = task_ctx(current);
struct aa_label *label = NULL;
+ bool newline = true;

if (strcmp(name, "current") == 0)
label = aa_get_newest_label(cred_label(cred));
@@ -609,11 +610,14 @@ static int apparmor_getprocattr(struct task_struct *task, char *name,
label = aa_get_newest_label(ctx->previous);
else if (strcmp(name, "exec") == 0 && ctx->onexec)
label = aa_get_newest_label(ctx->onexec);
- else
+ else if (strcmp(name, "context") == 0) {
+ label = aa_get_newest_label(cred_label(cred));
+ newline = false;
+ } else
error = -EINVAL;

if (label)
- error = aa_getprocattr(label, value);
+ error = aa_getprocattr(label, value, newline);

aa_put_label(label);
put_cred(cred);
diff --git a/security/apparmor/procattr.c b/security/apparmor/procattr.c
index fde332e0ea7d..172550f67fc0 100644
--- a/security/apparmor/procattr.c
+++ b/security/apparmor/procattr.c
@@ -20,6 +20,7 @@
* aa_getprocattr - Return the profile information for @profile
* @profile: the profile to print profile info about (NOT NULL)
* @string: Returns - string containing the profile info (NOT NULL)
+ * @newline: Should a newline be added to @string.
*
* Requires: profile != NULL
*
@@ -28,20 +29,21 @@
*
* Returns: size of string placed in @string else error code on failure
*/
-int aa_getprocattr(struct aa_label *label, char **string)
+int aa_getprocattr(struct aa_label *label, char **string, bool newline)
{
struct aa_ns *ns = labels_ns(label);
struct aa_ns *current_ns = aa_get_current_ns();
+ int flags = FLAG_VIEW_SUBNS | FLAG_HIDDEN_UNCONFINED;
int len;

if (!aa_ns_visible(current_ns, ns, true)) {
aa_put_ns(current_ns);
return -EACCES;
}
+ if (newline)
+ flags |= FLAG_SHOW_MODE;

- len = aa_label_snxprint(NULL, 0, current_ns, label,
- FLAG_SHOW_MODE | FLAG_VIEW_SUBNS |
- FLAG_HIDDEN_UNCONFINED);
+ len = aa_label_snxprint(NULL, 0, current_ns, label, flags);
AA_BUG(len < 0);

*string = kmalloc(len + 2, GFP_KERNEL);
@@ -50,19 +52,19 @@ int aa_getprocattr(struct aa_label *label, char **string)
return -ENOMEM;
}

- len = aa_label_snxprint(*string, len + 2, current_ns, label,
- FLAG_SHOW_MODE | FLAG_VIEW_SUBNS |
- FLAG_HIDDEN_UNCONFINED);
+ len = aa_label_snxprint(*string, len + 2, current_ns, label, flags);
if (len < 0) {
aa_put_ns(current_ns);
return len;
}

- (*string)[len] = '\n';
- (*string)[len + 1] = 0;
+ if (newline) {
+ (*string)[len] = '\n';
+ (*string)[++len] = 0;
+ }

aa_put_ns(current_ns);
- return len + 1;
+ return len;
}

/**
diff --git a/security/security.c b/security/security.c
index be6682768760..4c6d4171ded6 100644
--- a/security/security.c
+++ b/security/security.c
@@ -802,6 +802,57 @@ static int lsm_superblock_alloc(struct super_block *sb)
return 0;
}

+/**
+ * append_ctx - append a lsm/context pair to a compound context
+ * @ctx: the existing compound context
+ * @ctxlen: size of the old context, including terminating nul byte
+ * @lsm: new lsm name, nul terminated
+ * @new: new context, possibly nul terminated
+ * @newlen: maximum size of @new
+ *
+ * replace @ctx with a new compound context, appending @newlsm and @new
+ * to @ctx. On exit the new data replaces the old, which is freed.
+ * @ctxlen is set to the new size, which includes a trailing nul byte.
+ *
+ * Returns 0 on success, -ENOMEM if no memory is available.
+ */
+static int append_ctx(char **ctx, int *ctxlen, const char *lsm, char *new,
+ int newlen)
+{
+ char *final;
+ size_t llen;
+ size_t nlen;
+ size_t flen;
+
+ llen = strlen(lsm) + 1;
+ /*
+ * A security module may or may not provide a trailing nul on
+ * when returning a security context. There is no definition
+ * of which it should be, and there are modules that do it
+ * each way.
+ */
+ nlen = strnlen(new, newlen);
+
+ flen = *ctxlen + llen + nlen + 1;
+ final = kzalloc(flen, GFP_KERNEL);
+
+ if (final == NULL)
+ return -ENOMEM;
+
+ if (*ctxlen)
+ memcpy(final, *ctx, *ctxlen);
+
+ memcpy(final + *ctxlen, lsm, llen);
+ memcpy(final + *ctxlen + llen, new, nlen);
+
+ kfree(*ctx);
+
+ *ctx = final;
+ *ctxlen = flen;
+
+ return 0;
+}
+
/*
* The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
* can be accessed with:
@@ -2223,6 +2274,10 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
char **value)
{
struct security_hook_list *hp;
+ char *final = NULL;
+ char *cp;
+ int rc = 0;
+ int finallen = 0;
int ilsm = lsm_task_ilsm(current);
int slot = 0;

@@ -2250,6 +2305,30 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
return -ENOMEM;
}

+ if (!strcmp(name, "context")) {
+ hlist_for_each_entry(hp, &security_hook_heads.getprocattr,
+ list) {
+ rc = hp->hook.getprocattr(p, "context", &cp);
+ if (rc == -EINVAL)
+ continue;
+ if (rc < 0) {
+ kfree(final);
+ return rc;
+ }
+ rc = append_ctx(&final, &finallen, hp->lsmid->lsm,
+ cp, rc);
+ kfree(cp);
+ if (rc < 0) {
+ kfree(final);
+ return rc;
+ }
+ }
+ if (final == NULL)
+ return -EINVAL;
+ *value = final;
+ return finallen;
+ }
+
hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
continue;
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 653dd2e236f1..cd977493f734 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6346,7 +6346,7 @@ static int selinux_getprocattr(struct task_struct *p,
goto bad;
}

- if (!strcmp(name, "current"))
+ if (!strcmp(name, "current") || !strcmp(name, "context"))
sid = __tsec->sid;
else if (!strcmp(name, "prev"))
sid = __tsec->osid;
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 2190c03ae3d0..9e442c4495bf 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -3485,7 +3485,7 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
char *cp;
int slen;

- if (strcmp(name, "current") != 0)
+ if (strcmp(name, "current") != 0 && strcmp(name, "context") != 0)
return -EINVAL;

cp = kstrdup(skp->smk_known, GFP_KERNEL);
--
2.35.1