* [PATCH] kexec: Allow kexec_file() with appropriate IMA policy when locked down
@ 2019-03-18 23:03 Matthew Garrett
2019-03-19 2:47 ` Mimi Zohar
0 siblings, 1 reply; 2+ messages in thread
From: Matthew Garrett @ 2019-03-18 23:03 UTC (permalink / raw)
To: linux-integrity
Cc: dhowells, zohar, dmitry.kasatkin, kexec, dyoung, ebiederm,
Matthew Garrett, Matthew Garrett
Systems in lockdown mode should block the kexec of untrusted kernels.
For x86 and ARM we can ensure that a kernel is trustworthy by validating
a PE signature, but this isn't possible on other architectures. On those
platforms we can use IMA digital signatures instead. Add a function to
determine whether IMA has or will verify signatures for a given event type,
and if so permit kexec_file() even if the kernel is otherwise locked down.
This is restricted to cases where CONFIG_INTEGRITY_TRUSTED_KEYRING is set
in order to prevent an attacker from loading additional keys at runtime.
Signed-off-by: Matthew Garrett <mjg59@google.com>
---
Mimi, this relies on the lockdown patchset but I /think/ is independent
of the arch policy code - as a result I think it's possible to include
this in lockdown PR if that works for you, otherwise it can go through
your tree once the lockdown code is merged?
include/linux/ima.h | 9 ++++++
kernel/kexec_file.c | 7 +++-
security/integrity/ima/ima.h | 2 ++
security/integrity/ima/ima_main.c | 2 +-
security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++
5 files changed, 68 insertions(+), 2 deletions(-)
diff --git a/include/linux/ima.h b/include/linux/ima.h
index dc12fbcf484c..a78c04580a3c 100644
--- a/include/linux/ima.h
+++ b/include/linux/ima.h
@@ -132,4 +132,13 @@ static inline int ima_inode_removexattr(struct dentry *dentry,
return 0;
}
#endif /* CONFIG_IMA_APPRAISE */
+
+#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
+extern bool ima_appraise_signature(enum kernel_read_file_id func);
+#else
+static inline bool ima_appraise_kexec_signature(enum kernel_read_file_id func)
+{
+ return false;
+}
+#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
#endif /* _LINUX_IMA_H */
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 0cfe4f6f7f85..8ffa4b75c620 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -240,7 +240,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
ret = 0;
- if (kernel_is_locked_down(reason)) {
+ /* If IMA is guaranteed to appraise a signature on the kexec
+ * image, permit it even if the kernel is otherwise locked
+ * down.
+ */
+ if (!ima_appraise_signature(READING_KEXEC_IMAGE) &&
+ kernel_is_locked_down(reason)) {
ret = -EPERM;
goto out;
}
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index cc12f3449a72..fe03cc6f1ca4 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -115,6 +115,8 @@ struct ima_kexec_hdr {
u64 count;
};
+extern const int read_idmap[];
+
#ifdef CONFIG_HAVE_IMA_KEXEC
void ima_load_kexec_buffer(void);
#else
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 357edd140c09..927fe889201a 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -473,7 +473,7 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
return 0;
}
-static const int read_idmap[READING_MAX_ID] = {
+const int read_idmap[READING_MAX_ID] = {
[READING_FIRMWARE] = FIRMWARE_CHECK,
[READING_FIRMWARE_PREALLOC_BUFFER] = FIRMWARE_CHECK,
[READING_MODULE] = MODULE_CHECK,
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 8bc8a1c8cb3f..33e0b64305af 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -1337,3 +1337,53 @@ int ima_policy_show(struct seq_file *m, void *v)
return 0;
}
#endif /* CONFIG_IMA_READ_POLICY */
+
+#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
+/*
+ * ima_appraise_signature: whether IMA will appraise a given function using
+ * an IMA digital signature. This is restricted to cases where the kernel
+ * has a set of built-in trusted keys in order to avoid an attacker simply
+ * loading additional keys.
+ */
+bool ima_appraise_signature(enum kernel_read_file_id id)
+{
+ struct ima_rule_entry *entry;
+ bool found = false;
+ enum ima_hooks func;
+
+ if (id >= READING_MAX_ID)
+ return false;
+
+ func = read_idmap[id] ?: FILE_CHECK;
+
+ rcu_read_lock();
+ list_for_each_entry_rcu(entry, ima_rules, list) {
+ if (entry->action != APPRAISE)
+ continue;
+
+ /*
+ * A generic entry will match, but otherwise require that it
+ * match the func we're looking for
+ */
+ if (entry->func && entry->func != func)
+ continue;
+
+ /*
+ * We require this to be a digital signature, not a raw IMA
+ * hash.
+ */
+ if (entry->flags & IMA_DIGSIG_REQUIRED)
+ found = true;
+
+ /*
+ * We've found a rule that matches, so break now even if it
+ * didn't require a digital signature - a later rule that does
+ * won't override it, so would be a false positive.
+ */
+ break;
+ }
+
+ rcu_read_unlock();
+ return found;
+}
+#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
--
2.21.0.225.g810b269d1ac-goog
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] kexec: Allow kexec_file() with appropriate IMA policy when locked down
2019-03-18 23:03 [PATCH] kexec: Allow kexec_file() with appropriate IMA policy when locked down Matthew Garrett
@ 2019-03-19 2:47 ` Mimi Zohar
0 siblings, 0 replies; 2+ messages in thread
From: Mimi Zohar @ 2019-03-19 2:47 UTC (permalink / raw)
To: Matthew Garrett, linux-integrity
Cc: dhowells, dmitry.kasatkin, kexec, dyoung, ebiederm, Matthew Garrett
On Mon, 2019-03-18 at 16:03 -0700, Matthew Garrett wrote:
> Systems in lockdown mode should block the kexec of untrusted kernels.
> For x86 and ARM we can ensure that a kernel is trustworthy by validating
> a PE signature, but this isn't possible on other architectures. On those
> platforms we can use IMA digital signatures instead. Add a function to
> determine whether IMA has or will verify signatures for a given event type,
> and if so permit kexec_file() even if the kernel is otherwise locked down.
> This is restricted to cases where CONFIG_INTEGRITY_TRUSTED_KEYRING is set
> in order to prevent an attacker from loading additional keys at runtime.
>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
> ---
>
> Mimi, this relies on the lockdown patchset but I /think/ is independent
> of the arch policy code - as a result I think it's possible to include
> this in lockdown PR if that works for you, otherwise it can go through
> your tree once the lockdown code is merged?
Yes, please include this patch with the rest of the lockdown patchset.
thanks,
Mimi
>
> include/linux/ima.h | 9 ++++++
> kernel/kexec_file.c | 7 +++-
> security/integrity/ima/ima.h | 2 ++
> security/integrity/ima/ima_main.c | 2 +-
> security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++
> 5 files changed, 68 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index dc12fbcf484c..a78c04580a3c 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -132,4 +132,13 @@ static inline int ima_inode_removexattr(struct dentry *dentry,
> return 0;
> }
> #endif /* CONFIG_IMA_APPRAISE */
> +
> +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> +extern bool ima_appraise_signature(enum kernel_read_file_id func);
> +#else
> +static inline bool ima_appraise_kexec_signature(enum kernel_read_file_id func)
> +{
> + return false;
> +}
> +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
> #endif /* _LINUX_IMA_H */
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 0cfe4f6f7f85..8ffa4b75c620 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -240,7 +240,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
>
> ret = 0;
>
> - if (kernel_is_locked_down(reason)) {
> + /* If IMA is guaranteed to appraise a signature on the kexec
> + * image, permit it even if the kernel is otherwise locked
> + * down.
> + */
> + if (!ima_appraise_signature(READING_KEXEC_IMAGE) &&
> + kernel_is_locked_down(reason)) {
> ret = -EPERM;
> goto out;
> }
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index cc12f3449a72..fe03cc6f1ca4 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -115,6 +115,8 @@ struct ima_kexec_hdr {
> u64 count;
> };
>
> +extern const int read_idmap[];
> +
> #ifdef CONFIG_HAVE_IMA_KEXEC
> void ima_load_kexec_buffer(void);
> #else
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 357edd140c09..927fe889201a 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -473,7 +473,7 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
> return 0;
> }
>
> -static const int read_idmap[READING_MAX_ID] = {
> +const int read_idmap[READING_MAX_ID] = {
> [READING_FIRMWARE] = FIRMWARE_CHECK,
> [READING_FIRMWARE_PREALLOC_BUFFER] = FIRMWARE_CHECK,
> [READING_MODULE] = MODULE_CHECK,
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 8bc8a1c8cb3f..33e0b64305af 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -1337,3 +1337,53 @@ int ima_policy_show(struct seq_file *m, void *v)
> return 0;
> }
> #endif /* CONFIG_IMA_READ_POLICY */
> +
> +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> +/*
> + * ima_appraise_signature: whether IMA will appraise a given function using
> + * an IMA digital signature. This is restricted to cases where the kernel
> + * has a set of built-in trusted keys in order to avoid an attacker simply
> + * loading additional keys.
> + */
> +bool ima_appraise_signature(enum kernel_read_file_id id)
> +{
> + struct ima_rule_entry *entry;
> + bool found = false;
> + enum ima_hooks func;
> +
> + if (id >= READING_MAX_ID)
> + return false;
> +
> + func = read_idmap[id] ?: FILE_CHECK;
> +
> + rcu_read_lock();
> + list_for_each_entry_rcu(entry, ima_rules, list) {
> + if (entry->action != APPRAISE)
> + continue;
> +
> + /*
> + * A generic entry will match, but otherwise require that it
> + * match the func we're looking for
> + */
> + if (entry->func && entry->func != func)
> + continue;
> +
> + /*
> + * We require this to be a digital signature, not a raw IMA
> + * hash.
> + */
> + if (entry->flags & IMA_DIGSIG_REQUIRED)
> + found = true;
> +
> + /*
> + * We've found a rule that matches, so break now even if it
> + * didn't require a digital signature - a later rule that does
> + * won't override it, so would be a false positive.
> + */
> + break;
> + }
> +
> + rcu_read_unlock();
> + return found;
> +}
> +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
^ permalink raw reply [flat|nested] 2+ messages in thread
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