2:52 p.m.
On 5/14/2021 12:23 PM, Kees Cook wrote:
On Thu, May 13, 2021 at 01:07:56PM -0700, Casey Schaufler wrote:
> Create a new entry "interface_lsm" in the procfs attr directory for
> controlling which LSM security information is displayed for a
> process. A process can only read or write its own display value.
>
> The name of an active LSM that supplies hooks for
> human readable data may be written to "interface_lsm" to set the
> value. The name of the LSM currently in use can be read from
> "interface_lsm". At this point there can only be one LSM capable
> of display active. A helper function lsm_task_ilsm() is
> provided to get the interface lsm slot for a task_struct.
>
> Setting the "interface_lsm" requires that all security modules using
> setprocattr hooks allow the action. Each security module is
> responsible for defining its policy.
>
> AppArmor hook provided by John Johansen <john.johansen(a)canonical.com>
> SELinux hook provided by Stephen Smalley <stephen.smalley.work(a)gmail.com>
>
> Signed-off-by: Casey Schaufler <casey(a)schaufler-ca.com>
> Cc: Kees Cook <keescook(a)chromium.org>
> Cc: Stephen Smalley <stephen.smalley.work(a)gmail.com>
> Cc: Paul Moore <paul(a)paul-moore.com>
> Cc: John Johansen <john.johansen(a)canonical.com>
> Cc: Greg Kroah-Hartman <gregkh(a)linuxfoundation.org>
> Cc: linux-api(a)vger.kernel.org
> Cc: linux-doc(a)vger.kernel.org
> ---
> .../ABI/testing/procfs-attr-lsm_display | 22 +++
> Documentation/security/lsm.rst | 14 ++
> fs/proc/base.c | 1 +
> include/linux/lsm_hooks.h | 17 ++
> security/apparmor/include/apparmor.h | 3 +-
> security/apparmor/lsm.c | 32 ++++
> security/security.c | 166 ++++++++++++++++--
> security/selinux/hooks.c | 11 ++
> security/selinux/include/classmap.h | 2 +-
> security/smack/smack_lsm.c | 7 +
> 10 files changed, 256 insertions(+), 19 deletions(-)
> create mode 100644 Documentation/ABI/testing/procfs-attr-lsm_display
>
> diff --git a/Documentation/ABI/testing/procfs-attr-lsm_display
b/Documentation/ABI/testing/procfs-attr-lsm_display
> new file mode 100644
> index 000000000000..0f60005c235c
> --- /dev/null
> +++ b/Documentation/ABI/testing/procfs-attr-lsm_display
> @@ -0,0 +1,22 @@
> +What: /proc/*/attr/lsm_display
> +Contact: linux-security-module(a)vger.kernel.org,
> +Description: The name of the Linux security module (LSM) that will
> + provide information in the /proc/*/attr/current,
> + /proc/*/attr/prev and /proc/*/attr/exec interfaces.
> + The details of permissions required to read from
> + this interface are dependent on the LSMs active on the
> + system.
> + A process cannot write to this interface unless it
> + refers to itself.
> + The other details of permissions required to write to
> + this interface are dependent on the LSMs active on the
> + system.
> + The format of the data used by this interface is a
> + text string identifying the name of an LSM. The values
> + accepted are:
> + selinux - the SELinux LSM
> + smack - the Smack LSM
> + apparmor - The AppArmor LSM
> + By convention the LSM names are lower case and do not
> + contain special characters.
> +Users: LSM user-space
> diff --git a/Documentation/security/lsm.rst b/Documentation/security/lsm.rst
> index 6a2a2e973080..b77b4a540391 100644
> --- a/Documentation/security/lsm.rst
> +++ b/Documentation/security/lsm.rst
> @@ -129,3 +129,17 @@ 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/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.
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 3851bfcdba56..10de522f3112 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, "fscreate", 0666),
> ATTR(NULL, "keycreate", 0666),
> ATTR(NULL, "sockcreate", 0666),
> + ATTR(NULL, "interface_lsm", 0666),
Anyone can open this file... (continued below)
> #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 c61a16f0a5bc..d2c4bc94d47f 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -1686,4 +1686,21 @@ static inline void security_delete_hooks(struct
security_hook_list *hooks,
>
> extern int lsm_inode_alloc(struct inode *inode);
>
> +/**
> + * lsm_task_ilsm - the "interface_lsm" for this task
> + * @task: The task to report on
> + *
> + * Returns the task's interface LSM slot.
> + */
> +static inline int lsm_task_ilsm(struct task_struct *task)
> +{
> +#ifdef CONFIG_SECURITY
> + int *ilsm = task->security;
> +
> + if (ilsm)
> + return *ilsm;
> +#endif
> + return LSMBLOB_INVALID;
> +}
> +
> #endif /* ! __LINUX_LSM_HOOKS_H */
> diff --git a/security/apparmor/include/apparmor.h
b/security/apparmor/include/apparmor.h
> index 1fbabdb565a8..b1622fcb4394 100644
> --- a/security/apparmor/include/apparmor.h
> +++ b/security/apparmor/include/apparmor.h
> @@ -28,8 +28,9 @@
> #define AA_CLASS_SIGNAL 10
> #define AA_CLASS_NET 14
> #define AA_CLASS_LABEL 16
> +#define AA_CLASS_DISPLAY_LSM 17
>
> -#define AA_CLASS_LAST AA_CLASS_LABEL
> +#define AA_CLASS_LAST AA_CLASS_DISPLAY_LSM
>
> /* Control parameters settable through module/boot flags */
> extern enum audit_mode aa_g_audit;
> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
> index 392e25940d1f..4237536106aa 100644
> --- a/security/apparmor/lsm.c
> +++ b/security/apparmor/lsm.c
> @@ -621,6 +621,25 @@ static int apparmor_getprocattr(struct task_struct *task, char
*name,
> return error;
> }
>
> +
> +static int profile_interface_lsm(struct aa_profile *profile,
> + struct common_audit_data *sa)
> +{
> + struct aa_perms perms = { };
> + unsigned int state;
> +
> + state = PROFILE_MEDIATES(profile, AA_CLASS_DISPLAY_LSM);
> + if (state) {
> + aa_compute_perms(profile->policy.dfa, state, &perms);
> + aa_apply_modes_to_perms(profile, &perms);
> + aad(sa)->label = &profile->label;
> +
> + return aa_check_perms(profile, &perms, AA_MAY_WRITE, sa, NULL);
> + }
> +
> + return 0;
> +}
> +
> static int apparmor_setprocattr(const char *name, void *value,
> size_t size)
> {
> @@ -632,6 +651,19 @@ static int apparmor_setprocattr(const char *name, void *value,
> if (size == 0)
> return -EINVAL;
>
> + /* LSM infrastructure does actual setting of interface_lsm if allowed */
> + if (!strcmp(name, "interface_lsm")) {
> + struct aa_profile *profile;
> + struct aa_label *label;
> +
> + aad(&sa)->info = "set interface lsm";
> + label = begin_current_label_crit_section();
> + error = fn_for_each_confined(label, profile,
> + profile_interface_lsm(profile, &sa));
> + end_current_label_crit_section(label);
> + return error;
> + }
> +
> /* AppArmor requires that the buffer must be null terminated atm */
> if (args[size - 1] != '\0') {
> /* null terminate */
> diff --git a/security/security.c b/security/security.c
> index f5407a85641e..1ce125c01782 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -77,7 +77,16 @@ static struct kmem_cache *lsm_file_cache;
> static struct kmem_cache *lsm_inode_cache;
>
> char *lsm_names;
> -static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init;
> +
> +/*
> + * The task blob includes the "interface_lsm" slot used for
> + * chosing which module presents contexts.
> + * Using a long to avoid potential alignment issues with
> + * module assigned task blobs.
> + */
> +static struct lsm_blob_sizes blob_sizes __lsm_ro_after_init = {
> + .lbs_task = sizeof(long),
> +};
>
> /* Boot-time LSM user choice */
> static __initdata const char *chosen_lsm_order;
> @@ -669,6 +678,8 @@ int lsm_inode_alloc(struct inode *inode)
> */
> static int lsm_task_alloc(struct task_struct *task)
> {
> + int *ilsm;
> +
> if (blob_sizes.lbs_task == 0) {
> task->security = NULL;
> return 0;
> @@ -677,6 +688,15 @@ static int lsm_task_alloc(struct task_struct *task)
> task->security = kzalloc(blob_sizes.lbs_task, GFP_KERNEL);
> if (task->security == NULL)
> return -ENOMEM;
> +
> + /*
> + * The start of the task blob contains the "interface" LSM slot number.
> + * Start with it set to the invalid slot number, indicating that the
> + * default first registered LSM be displayed.
> + */
> + ilsm = task->security;
> + *ilsm = LSMBLOB_INVALID;
> +
> return 0;
> }
>
> @@ -1732,14 +1752,26 @@ int security_file_open(struct file *file)
>
> int security_task_alloc(struct task_struct *task, unsigned long clone_flags)
> {
> + int *oilsm = current->security;
> + int *nilsm;
> int rc = lsm_task_alloc(task);
>
> - if (rc)
> + if (unlikely(rc))
> return rc;
> +
> rc = call_int_hook(task_alloc, 0, task, clone_flags);
> - if (unlikely(rc))
> + if (unlikely(rc)) {
> security_task_free(task);
> - return rc;
> + return rc;
> + }
> +
> + if (oilsm) {
> + nilsm = task->security;
> + if (nilsm)
> + *nilsm = *oilsm;
> + }
> +
> + return 0;
> }
>
> void security_task_free(struct task_struct *task)
> @@ -2171,23 +2203,110 @@ int security_getprocattr(struct task_struct *p, const char
*lsm, char *name,
> char **value)
> {
> struct security_hook_list *hp;
> + int ilsm = lsm_task_ilsm(current);
> + int slot = 0;
> +
> + if (!strcmp(name, "interface_lsm")) {
> + /*
> + * lsm_slot will be 0 if there are no displaying modules.
> + */
> + if (lsm_slot == 0)
> + return -EINVAL;
> +
> + /*
> + * Only allow getting the current process' interface_lsm.
> + * There are too few reasons to get another process'
> + * interface_lsm and too many LSM policy issues.
> + */
> + if (current != p)
> + return -EINVAL;
... but context isn't established by just checking "current", as this
file handle may have been given to another process.
I suspect the security_get/setprocattr needs to gain a pointer to "file"
so that the f_cred struct can be examined[1] (i.e. compare opener
against reader/writer).
[1]
https://www.kernel.org/doc/html/latest/security/credentials.html#open-fil...
It's not credentials being checked here. The check is whether the task that
would be affected is "current". Process A can't open
/proc/B/attr/interface_lsm
with write access. The only process that can open it for write access is B.
If process B opens /proc/B/attr/interface_lsm for write access it could send
the file handle to process A, but process A can't write to the file because
(current != p) that is, (A != B).
> +
> + ilsm = lsm_task_ilsm(p);
> + if (ilsm != LSMBLOB_INVALID)
> + slot = ilsm;
> + *value = kstrdup(lsm_slotlist[slot]->lsm, GFP_KERNEL);
> + if (*value)
> + return strlen(*value);
> + return -ENOMEM;
> + }
>
> hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
> if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
> continue;
> + if (lsm == NULL && ilsm != LSMBLOB_INVALID &&
> + ilsm != hp->lsmid->slot)
> + continue;
> return hp->hook.getprocattr(p, name, value);
> }
> return LSM_RET_DEFAULT(getprocattr);
> }
>
> +/**
> + * security_setprocattr - Set process attributes via /proc
> + * @lsm: name of module involved, or NULL
> + * @name: name of the attribute
> + * @value: value to set the attribute to
> + * @size: size of the value
> + *
> + * Set the process attribute for the specified security module
> + * to the specified value. Note that this can only be used to set
> + * the process attributes for the current, or "self" process.
> + * The /proc code has already done this check.
> + *
> + * Returns 0 on success, an appropriate code otherwise.
> + */
> int security_setprocattr(const char *lsm, const char *name, void *value,
> size_t size)
> {
> struct security_hook_list *hp;
> + char *termed;
> + char *copy;
> + int *ilsm = current->security;
> + int rc = -EINVAL;
> + int slot = 0;
> +
> + if (!strcmp(name, "interface_lsm")) {
> + /*
> + * Change the "interface_lsm" value only if all the security
> + * modules that support setting a procattr allow it.
> + * It is assumed that all such security modules will be
> + * cooperative.
> + */
> + if (size == 0)
> + return -EINVAL;
> +
> + hlist_for_each_entry(hp, &security_hook_heads.setprocattr,
> + list) {
> + rc = hp->hook.setprocattr(name, value, size);
> + if (rc < 0)
> + return rc;
> + }
Similarly here -- how do the LSMs reason about who actually _opened_
this file?
-Kees
> +
> + rc = -EINVAL;
> +
> + copy = kmemdup_nul(value, size, GFP_KERNEL);
> + if (copy == NULL)
> + return -ENOMEM;
> +
> + termed = strsep(©, " \n");
> +
> + for (slot = 0; slot < lsm_slot; slot++)
> + if (!strcmp(termed, lsm_slotlist[slot]->lsm)) {
> + *ilsm = lsm_slotlist[slot]->slot;
> + rc = size;
> + break;
> + }
> +
> + kfree(termed);
> + return rc;
> + }
>
> hlist_for_each_entry(hp, &security_hook_heads.setprocattr, list) {
> if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
> continue;
> + if (lsm == NULL && *ilsm != LSMBLOB_INVALID &&
> + *ilsm != hp->lsmid->slot)
> + continue;
> return hp->hook.setprocattr(name, value, size);
> }
> return LSM_RET_DEFAULT(setprocattr);
> @@ -2207,15 +2326,15 @@ EXPORT_SYMBOL(security_ismaclabel);
> int security_secid_to_secctx(struct lsmblob *blob, char **secdata, u32 *seclen)
> {
> struct security_hook_list *hp;
> - int rc;
> + int ilsm = lsm_task_ilsm(current);
>
> hlist_for_each_entry(hp, &security_hook_heads.secid_to_secctx, list) {
> if (WARN_ON(hp->lsmid->slot < 0 || hp->lsmid->slot >=
lsm_slot))
> continue;
> - rc = hp->hook.secid_to_secctx(blob->secid[hp->lsmid->slot],
> - secdata, seclen);
> - if (rc != LSM_RET_DEFAULT(secid_to_secctx))
> - return rc;
> + if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot)
> + return hp->hook.secid_to_secctx(
> + blob->secid[hp->lsmid->slot],
> + secdata, seclen);
> }
>
> return LSM_RET_DEFAULT(secid_to_secctx);
> @@ -2226,16 +2345,15 @@ int security_secctx_to_secid(const char *secdata, u32
seclen,
> struct lsmblob *blob)
> {
> struct security_hook_list *hp;
> - int rc;
> + int ilsm = lsm_task_ilsm(current);
>
> lsmblob_init(blob, 0);
> hlist_for_each_entry(hp, &security_hook_heads.secctx_to_secid, list) {
> if (WARN_ON(hp->lsmid->slot < 0 || hp->lsmid->slot >=
lsm_slot))
> continue;
> - rc = hp->hook.secctx_to_secid(secdata, seclen,
> - &blob->secid[hp->lsmid->slot]);
> - if (rc != 0)
> - return rc;
> + if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot)
> + return hp->hook.secctx_to_secid(secdata, seclen,
> + &blob->secid[hp->lsmid->slot]);
> }
> return 0;
> }
> @@ -2243,7 +2361,14 @@ EXPORT_SYMBOL(security_secctx_to_secid);
>
> void security_release_secctx(char *secdata, u32 seclen)
> {
> - call_void_hook(release_secctx, secdata, seclen);
> + struct security_hook_list *hp;
> + int ilsm = lsm_task_ilsm(current);
> +
> + hlist_for_each_entry(hp, &security_hook_heads.release_secctx, list)
> + if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot) {
> + hp->hook.release_secctx(secdata, seclen);
> + return;
> + }
> }
> EXPORT_SYMBOL(security_release_secctx);
>
> @@ -2384,8 +2509,15 @@ EXPORT_SYMBOL(security_sock_rcv_skb);
> int security_socket_getpeersec_stream(struct socket *sock, char __user *optval,
> int __user *optlen, unsigned len)
> {
> - return call_int_hook(socket_getpeersec_stream, -ENOPROTOOPT, sock,
> - optval, optlen, len);
> + int ilsm = lsm_task_ilsm(current);
> + struct security_hook_list *hp;
> +
> + hlist_for_each_entry(hp, &security_hook_heads.socket_getpeersec_stream,
> + list)
> + if (ilsm == LSMBLOB_INVALID || ilsm == hp->lsmid->slot)
> + return hp->hook.socket_getpeersec_stream(sock, optval,
> + optlen, len);
> + return -ENOPROTOOPT;
> }
>
> int security_socket_getpeersec_dgram(struct socket *sock, struct sk_buff *skb,
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 0133b142e938..dba867721336 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6510,6 +6510,17 @@ static int selinux_setprocattr(const char *name, void *value,
size_t size)
> /*
> * Basic control over ability to set these attributes at all.
> */
> +
> + /*
> + * For setting interface_lsm, we only perform a permission check;
> + * the actual update to the interface_lsm value is handled by the
> + * LSM framework.
> + */
> + if (!strcmp(name, "interface_lsm"))
> + return avc_has_perm(&selinux_state,
> + mysid, mysid, SECCLASS_PROCESS2,
> + PROCESS2__SETDISPLAY, NULL);
> +
> if (!strcmp(name, "exec"))
> error = avc_has_perm(&selinux_state,
> mysid, mysid, SECCLASS_PROCESS,
> diff --git a/security/selinux/include/classmap.h
b/security/selinux/include/classmap.h
> index 62d19bccf3de..8f4b0dd6dd78 100644
> --- a/security/selinux/include/classmap.h
> +++ b/security/selinux/include/classmap.h
> @@ -53,7 +53,7 @@ struct security_class_mapping secclass_map[] = {
> "execmem", "execstack", "execheap",
"setkeycreate",
> "setsockcreate", "getrlimit", NULL } },
> { "process2",
> - { "nnp_transition", "nosuid_transition", NULL } },
> + { "nnp_transition", "nosuid_transition",
"setdisplay", NULL } },
> { "system",
> { "ipc_info", "syslog_read", "syslog_mod",
> "syslog_console", "module_request",
"module_load", NULL } },
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 5c10ad27be37..7aa7ea38f627 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -3508,6 +3508,13 @@ static int smack_setprocattr(const char *name, void *value,
size_t size)
> struct smack_known_list_elem *sklep;
> int rc;
>
> + /*
> + * Allow the /proc/.../attr/current and SO_PEERSEC "interface_lsm"
> + * to be reset at will.
> + */
> + if (strcmp(name, "interface_lsm") == 0)
> + return 0;
> +
> if (!smack_privileged(CAP_MAC_ADMIN) &&
list_empty(&tsp->smk_relabel))
> return -EPERM;
>
> --
> 2.29.2
>