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From:   Adrian Reber <areber@redhat.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>,
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        Oleg Nesterov <oleg@redhat.com>,
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        Nicolas Viennot <Nicolas.Viennot@twosigma.com>,
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Cc:     Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>,
        Adrian Reber <areber@redhat.com>,
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        Eric Paris <eparis@parisplace.org>,
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Subject: [PATCH v4 3/3] prctl: Allow ptrace capable processes to change /proc/self/exe
Date:   Wed,  1 Jul 2020 08:49:06 +0200
Message-Id: <20200701064906.323185-4-areber@redhat.com>
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From: Nicolas Viennot <Nicolas.Viennot@twosigma.com>

Previously, the current process could only change the /proc/self/exe
link with local CAP_SYS_ADMIN.
This commit relaxes this restriction by permitting such change with
CAP_CHECKPOINT_RESTORE, and the ability to use ptrace.

With access to ptrace facilities, a process can do the following: fork a
child, execve() the target executable, and have the child use ptrace()
to replace the memory content of the current process. This technique
makes it possible to masquerade an arbitrary program as any executable,
even setuid ones.

Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
Signed-off-by: Adrian Reber <areber@redhat.com>
---
 include/linux/lsm_hook_defs.h |  1 +
 include/linux/security.h      |  6 ++++++
 kernel/sys.c                  | 12 ++++--------
 security/commoncap.c          | 26 ++++++++++++++++++++++++++
 security/security.c           |  5 +++++
 security/selinux/hooks.c      | 14 ++++++++++++++
 6 files changed, 56 insertions(+), 8 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 0098852bb56a..90e51d5e093b 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -211,6 +211,7 @@ LSM_HOOK(int, 0, task_kill, struct task_struct *p, struct kernel_siginfo *info,
 	 int sig, const struct cred *cred)
 LSM_HOOK(int, -ENOSYS, task_prctl, int option, unsigned long arg2,
 	 unsigned long arg3, unsigned long arg4, unsigned long arg5)
+LSM_HOOK(int, 0, prctl_set_mm_exe_file, struct file *exe_file)
 LSM_HOOK(void, LSM_RET_VOID, task_to_inode, struct task_struct *p,
 	 struct inode *inode)
 LSM_HOOK(int, 0, ipc_permission, struct kern_ipc_perm *ipcp, short flag)
diff --git a/include/linux/security.h b/include/linux/security.h
index 2797e7f6418e..0f594eb7e766 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -412,6 +412,7 @@ int security_task_kill(struct task_struct *p, struct kernel_siginfo *info,
 			int sig, const struct cred *cred);
 int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			unsigned long arg4, unsigned long arg5);
+int security_prctl_set_mm_exe_file(struct file *exe_file);
 void security_task_to_inode(struct task_struct *p, struct inode *inode);
 int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
 void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid);
@@ -1124,6 +1125,11 @@ static inline int security_task_prctl(int option, unsigned long arg2,
 	return cap_task_prctl(option, arg2, arg3, arg4, arg5);
 }
 
+static inline int security_prctl_set_mm_exe_file(struct file *exe_file)
+{
+	return cap_prctl_set_mm_exe_file(exe_file);
+}
+
 static inline void security_task_to_inode(struct task_struct *p, struct inode *inode)
 { }
 
diff --git a/kernel/sys.c b/kernel/sys.c
index 00a96746e28a..bb53e8408c63 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1851,6 +1851,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	if (err)
 		goto exit;
 
+	err = security_prctl_set_mm_exe_file(exe.file);
+	if (err)
+		goto exit;
+
 	/*
 	 * Forbid mm->exe_file change if old file still mapped.
 	 */
@@ -2006,14 +2010,6 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
 	}
 
 	if (prctl_map.exe_fd != (u32)-1) {
-		/*
-		 * Make sure the caller has the rights to
-		 * change /proc/pid/exe link: only local sys admin should
-		 * be allowed to.
-		 */
-		if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN))
-			return -EINVAL;
-
 		error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd);
 		if (error)
 			return error;
diff --git a/security/commoncap.c b/security/commoncap.c
index 59bf3c1674c8..663d00fe2ecc 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -1291,6 +1291,31 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 	}
 }
 
+/**
+ * cap_prctl_set_mm_exe_file - Determine whether /proc/self/exe can be changed
+ * by the current process.
+ * @exe_file: The new exe file
+ * Returns 0 if permission is granted, -ve if denied.
+ *
+ * The current process is permitted to change its /proc/self/exe link via two policies:
+ * 1) The current user can do checkpoint/restore. At the time of this writing,
+ *    this means CAP_SYS_ADMIN or CAP_CHECKPOINT_RESTORE capable.
+ * 2) The current user can use ptrace.
+ *
+ * With access to ptrace facilities, a process can do the following:
+ * fork a child, execve() the target executable, and have the child use
+ * ptrace() to replace the memory content of the current process.
+ * This technique makes it possible to masquerade an arbitrary program as the
+ * target executable, even if it is setuid.
+ */
+int cap_prctl_set_mm_exe_file(struct file *exe_file)
+{
+	if (checkpoint_restore_ns_capable(current_user_ns()))
+		return 0;
+
+	return security_ptrace_access_check(current, PTRACE_MODE_ATTACH_REALCREDS);
+}
+
 /**
  * cap_vm_enough_memory - Determine whether a new virtual mapping is permitted
  * @mm: The VM space in which the new mapping is to be made
@@ -1356,6 +1381,7 @@ static struct security_hook_list capability_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(mmap_file, cap_mmap_file),
 	LSM_HOOK_INIT(task_fix_setuid, cap_task_fix_setuid),
 	LSM_HOOK_INIT(task_prctl, cap_task_prctl),
+	LSM_HOOK_INIT(prctl_set_mm_exe_file, cap_prctl_set_mm_exe_file),
 	LSM_HOOK_INIT(task_setscheduler, cap_task_setscheduler),
 	LSM_HOOK_INIT(task_setioprio, cap_task_setioprio),
 	LSM_HOOK_INIT(task_setnice, cap_task_setnice),
diff --git a/security/security.c b/security/security.c
index 2bb912496232..13a1ed32f9e3 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1790,6 +1790,11 @@ int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 	return rc;
 }
 
+int security_prctl_set_mm_exe_file(struct file *exe_file)
+{
+	return call_int_hook(prctl_set_mm_exe_file, 0, exe_file);
+}
+
 void security_task_to_inode(struct task_struct *p, struct inode *inode)
 {
 	call_void_hook(task_to_inode, p, inode);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index ca901025802a..fca5581392b8 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4156,6 +4156,19 @@ static int selinux_task_kill(struct task_struct *p, struct kernel_siginfo *info,
 			    secid, task_sid(p), SECCLASS_PROCESS, perm, NULL);
 }
 
+static int selinux_prctl_set_mm_exe_file(struct file *exe_file)
+{
+	u32 sid = current_sid();
+
+	struct common_audit_data ad = {
+		.type = LSM_AUDIT_DATA_FILE,
+		.u.file = exe_file,
+	};
+
+	return avc_has_perm(&selinux_state, sid, sid,
+			    SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
+}
+
 static void selinux_task_to_inode(struct task_struct *p,
 				  struct inode *inode)
 {
@@ -7057,6 +7070,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(task_getscheduler, selinux_task_getscheduler),
 	LSM_HOOK_INIT(task_movememory, selinux_task_movememory),
 	LSM_HOOK_INIT(task_kill, selinux_task_kill),
+	LSM_HOOK_INIT(prctl_set_mm_exe_file, selinux_prctl_set_mm_exe_file),
 	LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode),
 
 	LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission),
-- 
2.26.2