[PATCH v22 22/23] LSM: Add /proc attr entry for full LSM context

From: Casey Schaufler
Date: Wed Nov 04 2020 - 20:15:06 EST

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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@xxxxxxxxxxxx>
Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
---
Documentation/security/lsm.rst | 28 ++++++++++
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, 135 insertions(+), 15 deletions(-)

diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst
index 6a2a2e973080..3a1403b7cf10 100644
--- a/Documentation/security/lsm.rst
+++ b/Documentation/security/lsm.rst
@@ -129,3 +129,31 @@ to identify it as the first security module to be registered.
The capabilities security module does not use the general security
blobs, unlike other modules. The reasons are historical and are
based on overhead, complexity and performance concerns.
+
+LSM External Interfaces
+=======================
+
+The LSM infrastructure does not generally provide external interfaces.
+The individual security modules provide what external interfaces they
+require.
+
+The file ``/sys/kernel/security/lsm`` provides a comma
+separated list of the active security modules.
+
+The file ``/proc/pid/attr/display`` 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 7432f24f0132..1894ed42a2a6 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2807,6 +2807,7 @@ static const struct pid_entry attr_dir_stuff[] = {
ATTR(NULL, "keycreate", 0666),
ATTR(NULL, "sockcreate", 0666),
ATTR(NULL, "display", 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 c2f5adef0ad7..5218aae856e7 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1372,6 +1372,12 @@
* @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 31a6f11890f1..7ce570b0f491 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -593,6 +593,7 @@ 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));
@@ -600,11 +601,14 @@ 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 c929bf4a3df1..be3b083d9b74 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.
*
* Returns: length of @string on success else error on failure
*
@@ -30,20 +31,21 @@
*
* 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);
@@ -52,19 +54,19 @@ 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 1a78f7bb808b..e59a75e776cc 100644
--- a/security/security.c
+++ b/security/security.c
@@ -757,6 +757,57 @@ static void __init lsm_early_task(struct task_struct *task)
panic("%s: Early task alloc failed.\n", __func__);
}

+/**
+ * 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:
@@ -2142,6 +2193,10 @@ 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 display = lsm_task_display(current);
int slot = 0;

@@ -2169,6 +2224,30 @@ 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 cb124671c15a..97568224e7d1 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6292,7 +6292,7 @@ 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 913d41b4b96a..b4bb0f3676cf 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -3478,7 +3478,7 @@ 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);
--
2.24.1

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