[PATCH 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS)

[Jann Horn]

If the system is running with PROC_MEM_FORCE_ALWAYS, LSMs currently have no
good opportunity to block a process from overwriting read-only code in its
own address space through FOLL_FORCE writes via /proc/self/mem.
The security_ptrace_access_check() LSM hook is bypassed when a process
opens /proc/self/mem because this is considered "introspection".

This causes a hole in SELinux EXECMEM enforcement, which tries to ensure
that a process cannot create executable anonymous pages.

PROC_MEM_FORCE_PTRACE prevents that and ensures that such FOLL_FORCE
accesses are only possible when the LSM allows ptrace() attachment; but it
is unclear how quickly PROC_MEM_FORCE_PTRACE can be deployed in
environments running lots of third-party code, such as Android.

So, introduce a new LSM hook that can forbid FOLL_FORCE specifically for
such "introspective" accesses.

Signed-off-by: Jann Horn <jannh at google.com>
---
 fs/proc/base.c                |  6 ++++++
 include/linux/lsm_hook_defs.h |  1 +
 include/linux/security.h      |  6 ++++++
 security/security.c           | 15 +++++++++++++++
 4 files changed, 28 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index bec6197329dc..3dfaef49bb70 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -851,6 +851,8 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 /* private_data for proc_mem_operations */
 struct mem_private {
 	struct mm_struct *mm;
+	/* Was the ptrace access check bypassed due to introspection? */
+	bool introspection;
 };
 
 static int mem_open(struct inode *inode, struct file *file)
@@ -864,12 +866,14 @@ static int mem_open(struct inode *inode, struct file *file)
 	priv->mm = proc_mem_open(inode, PTRACE_MODE_ATTACH);
 	if (IS_ERR_OR_NULL(priv->mm))
 		return priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
+	priv->introspection = priv->mm == current->mm;
 	file->private_data = no_free_ptr(priv);
 	return 0;
 }
 
 static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
 {
+	struct mem_private *priv = file->private_data;
 	struct task_struct *task;
 	bool ptrace_active = false;
 
@@ -886,6 +890,8 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm)
 		}
 		return ptrace_active;
 	default:
+		if (priv->introspection)
+			return security_introspect_mem_foll_force(file->f_cred) == 0;
 		return true;
 	}
 }
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 65c9609ec207..67452f71bedf 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -36,6 +36,7 @@ LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from,
 LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child,
 	 unsigned int mode)
 LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent)
+LSM_HOOK(int, 0, introspect_mem_foll_force, const struct cred *subject)
 LSM_HOOK(int, 0, capget, const struct task_struct *target, kernel_cap_t *effective,
 	 kernel_cap_t *inheritable, kernel_cap_t *permitted)
 LSM_HOOK(int, 0, capset, struct cred *new, const struct cred *old,
diff --git a/include/linux/security.h b/include/linux/security.h
index 153e9043058f..f8483be58bc8 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -338,6 +338,7 @@ int security_binder_transfer_file(const struct cred *from,
 				  const struct cred *to, const struct file *file);
 int security_ptrace_access_check(struct task_struct *child, unsigned int mode);
 int security_ptrace_traceme(struct task_struct *parent);
+int security_introspect_mem_foll_force(const struct cred *subject);
 int security_capget(const struct task_struct *target,
 		    kernel_cap_t *effective,
 		    kernel_cap_t *inheritable,
@@ -676,6 +677,11 @@ static inline int security_ptrace_traceme(struct task_struct *parent)
 	return cap_ptrace_traceme(parent);
 }
 
+static inline int security_introspect_mem_foll_force(const struct cred *subject)
+{
+	return 0;
+}
+
 static inline int security_capget(const struct task_struct *target,
 				   kernel_cap_t *effective,
 				   kernel_cap_t *inheritable,
diff --git a/security/security.c b/security/security.c
index 71aea8fdf014..d0f790a534eb 100644
--- a/security/security.c
+++ b/security/security.c
@@ -595,6 +595,21 @@ int security_ptrace_traceme(struct task_struct *parent)
 	return call_int_hook(ptrace_traceme, parent);
 }
 
+/**
+ * security_introspect_mem_foll_force() - Check if introspective FOLL_FORCE is allowed
+ * @subject: credentials of the process accessing its own memory
+ *
+ * Check if FOLL_FORCE is allowed for a process accessing its own memory, which
+ * bypasses the security_ptrace_access_check() hook.
+ * This is only used when the system is configured with PROC_MEM_FORCE_ALWAYS.
+ *
+ * Return: Returns 0 if permission is granted.
+ */
+int security_introspect_mem_foll_force(const struct cred *subject)
+{
+	return call_int_hook(introspect_mem_foll_force, subject);
+}
+
 /**
  * security_capget() - Get the capability sets for a process
  * @target: target process

-- 
2.55.0.737.g08866a6d13-goog