> On 8 May 2024, at 03:45, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On Tue, May 7, 2024 at 8:01 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>
>> On Wed, May 08, 2024 at 12:10:45AM +0200, KP Singh wrote:
>>> [...]
>>> +/**
>>> + * security_toggle_hook - Toggle the state of the LSM hook.
>>> + * @hook_addr: The address of the hook to be toggled.
>>> + * @state: Whether to enable for disable the hook.
>>> + *
>>> + * Returns 0 on success, -EINVAL if the address is not found.
>>> + */
>>> +int security_toggle_hook(void *hook_addr, bool state)
>>> +{
>>> + struct lsm_static_call *scalls = ((void *)&static_calls_table);
>>> + unsigned long num_entries =
>>> + (sizeof(static_calls_table) / sizeof(struct lsm_static_call));
>>> + int i;
>>> +
>>> + for (i = 0; i < num_entries; i++) {
>>> + if (!scalls[i].hl)
>>> + continue;
>>> +
>>> + if (scalls[i].hl->hook.lsm_func_addr != hook_addr)
>>> + continue;
>>> +
>>> + if (state)
>>> + static_branch_enable(scalls[i].active);
>>> + else
>>> + static_branch_disable(scalls[i].active);
>>> + return 0;
>>> + }
>>> + return -EINVAL;
>>> +}
>>
>> First of all: patches 1-4 are great. They have a measurable performance
>> benefit; let's get those in.
>>
>> But here I come to patch 5 where I will suggest the exact opposite of
>> what Paul said in v9 for patch 5. :P
>
> For those looking up v9 of the patchset, you'll be looking for patch
> *4*, not patch 5, as there were only four patches in the v9 series.
> Patch 4/5 in the v10 series is a new addition to the stack.
>
> Beyond that, I'm guessing you are referring to my comment regarding
> bpf_lsm_toggle_hook() Kees? The one that starts with "More ugh. If
> we are going to solve things this way ..."?
>
>> I don't want to have a global function that can be used to disable LSMs.
>> We got an entire distro (RedHat) to change their SELinux configurations
>> to get rid of CONFIG_SECURITY_SELINUX_DISABLE (and therefore
>> CONFIG_SECURITY_WRITABLE_HOOKS), via commit f22f9aaf6c3d ("selinux:
>> remove the runtime disable functionality"). We cannot reintroduce that,
>> and I'm hoping Paul will agree, given this reminder of LSM history. :)
>>
>> Run-time hook changing should be BPF_LSM specific, if it exists at all.
One idea here is that only LSM hooks with default_state = false can be toggled.
This would also any ROPs that try to abuse this function. Maybe we can call "default_disabled" .toggleable (or dynamic)
and change the corresponding LSM_INIT_TOGGLEABLE. Kees, Paul, this may be a fair middle ground?
Something like:
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 4bd1d47bb9dc..5c0918ed6b80 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -117,7 +117,7 @@ struct security_hook_list {
struct lsm_static_call *scalls;
union security_list_options hook;
const struct lsm_id *lsmid;
- bool default_enabled;
+ bool toggleable;
} __randomize_layout;
/*
@@ -168,14 +168,18 @@ static inline struct xattr *lsm_get_xattr_slot(struct xat>
{ \
.scalls = static_calls_table.NAME, \
.hook = { .NAME = HOOK }, \
- .default_enabled = true \
+ .toggleable = false \
}
-#define LSM_HOOK_INIT_DISABLED(NAME, HOOK) \
+/*
+ * Toggleable LSM hooks are enabled at runtime with
+ * security_toggle_hook and are initialized as inactive.
+ */
+#define LSM_HOOK_INIT_TOGGLEABLE(NAME, HOOK) \
{ \
.scalls = static_calls_table.NAME, \
.hook = { .NAME = HOOK }, \
- .default_enabled = false \
+ .toggleable = true \
}
extern char *lsm_names;
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index ed864f7430a3..ba1c3a19fb12 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -9,7 +9,7 @@
static struct security_hook_list bpf_lsm_hooks[] __ro_after_init = {
#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
- LSM_HOOK_INIT_DISABLED(NAME, bpf_lsm_##NAME),
+ LSM_HOOK_INIT_TOGGLEABLE(NAME, bpf_lsm_##NAME),
#include <linux/lsm_hook_defs.h>
#undef LSM_HOOK
LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
+ * security_toggle_hook and are initialized as inactive.
+ */
+#define LSM_HOOK_INIT_TOGGLEABLE(NAME, HOOK) \
{ \
.scalls = static_calls_table.NAME, \
.hook = { .NAME = HOOK }, \
- .default_enabled = false \
+ .toggleable = true \
}
extern char *lsm_names;
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index ed864f7430a3..ba1c3a19fb12 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -9,7 +9,7 @@
static struct security_hook_list bpf_lsm_hooks[] __ro_after_init = {
#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
- LSM_HOOK_INIT_DISABLED(NAME, bpf_lsm_##NAME),
+ LSM_HOOK_INIT_TOGGLEABLE(NAME, bpf_lsm_##NAME),
#include <linux/lsm_hook_defs.h>
#undef LSM_HOOK
LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
kpsingh@kpsingh:~/projects/linux$ git diff
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 4bd1d47bb9dc..5c0918ed6b80 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -117,7 +117,7 @@ struct security_hook_list {
struct lsm_static_call *scalls;
union security_list_options hook;
const struct lsm_id *lsmid;
- bool default_enabled;
+ bool toggleable;
} __randomize_layout;
/*
@@ -168,14 +168,18 @@ static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
{ \
.scalls = static_calls_table.NAME, \
.hook = { .NAME = HOOK }, \
- .default_enabled = true \
+ .toggleable = false \
}
-#define LSM_HOOK_INIT_DISABLED(NAME, HOOK) \
+/*
+ * Toggleable LSM hooks are enabled at runtime with
+ * security_toggle_hook and are initialized as inactive.
+ */
+#define LSM_HOOK_INIT_TOGGLEABLE(NAME, HOOK) \
{ \
.scalls = static_calls_table.NAME, \
.hook = { .NAME = HOOK }, \
- .default_enabled = false \
+ .toggleable = true \
}
extern char *lsm_names;
diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index ed864f7430a3..ba1c3a19fb12 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -9,7 +9,7 @@
static struct security_hook_list bpf_lsm_hooks[] __ro_after_init = {
#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
- LSM_HOOK_INIT_DISABLED(NAME, bpf_lsm_##NAME),
+ LSM_HOOK_INIT_TOGGLEABLE(NAME, bpf_lsm_##NAME),
#include <linux/lsm_hook_defs.h>
#undef LSM_HOOK
LSM_HOOK_INIT(inode_free_security, bpf_inode_storage_free),
diff --git a/security/security.c b/security/security.c
index b3a92a67f325..a89eb8fe302b 100644
--- a/security/security.c
+++ b/security/security.c
@@ -407,7 +407,8 @@ static void __init lsm_static_call_init(struct security_hook_list *hl)
__static_call_update(scall->key, scall->trampoline,
hl->hook.lsm_func_addr);
scall->hl = hl;
- if (hl->default_enabled)
+ /* Toggleable hooks are inactive by default */
+ if (!hl->toggleable)
static_branch_enable(scall->active);
return;
}
@@ -901,6 +902,9 @@ int security_toggle_hook(void *hook_addr, bool state)
int i;
for (i = 0; i < num_entries; i++) {
+ if (!scalls[i].hl->toggleable)
+ continue;
+
if (!scalls[i].hl)
continue;
- KP
>
> I don't want individual LSMs manipulating the LSM hook state directly;
> they go through the LSM layer to register their hooks, they should go
> through the LSM layer to unregister or enable/disable their hooks.
> I'm going to be pretty inflexible on this point.
>
> Honestly, I see this more as a problem in the BPF LSM design (although
> one might argue it's an implementation issue?), just as I saw the
> SELinux runtime disable as a problem. If you're upset with the
> runtime hook disable, and you should be, fix the BPF LSM, don't force
> more bad architecture on the LSM layer.
>
> --
> paul-moore.com
