CVE-2017-14140: Exposure of Sensitive Information to an Unauthorized Actor (CWE-200)
value: The move_pages system call in mm/migrate.c in the Linux kernel before 4.12.9 doesn't check the effective uid of the target process, enabling a local attacker to learn the memory layout of a setuid executable despite ASLR.
CWE description: The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information.
CVSS v3 base score: 5.5
Published: 2017-09-05T06:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: mm/migrate.c
Commit date: 2017-08-20 13:26:27 -0700
Commit message: Sanitize 'move_pages()' permission checks

The 'move_paghes()' system call was introduced long long ago with the
same permission checks as for sending a signal (except using
CAP_SYS_NICE instead of CAP_SYS_KILL for the overriding capability).

That turns out to not be a great choice - while the system call really
only moves physical page allocations around (and you need other
capabilities to do a lot of it), you can check the return value to map
out some the virtual address choices and defeat ASLR of a binary that
still shares your uid.

So change the access checks to the more common 'ptrace_may_access()'
model instead.

This tightens the access checks for the uid, and also effectively
changes the CAP_SYS_NICE check to CAP_SYS_PTRACE, but it's unlikely that
anybody really _uses_ this legacy system call any more (we hav ebetter
NUMA placement models these days), so I expect nobody to notice.

Famous last words.

Reported-by: Otto Ebeling <otto.ebeling@iki.fi>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Diff: diff --git a/mm/migrate.c b/mm/migrate.c
index d68a41da6abb07..e84eeb4e43566c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -39,10 +39,11 @@
 #include <linux/balloon_compaction.h>
 #include <linux/mmu_notifier.h>
 #include <linux/page_idle.h>
 #include <linux/page_owner.h>
 #include <linux/sched/mm.h>
+#include <linux/ptrace.h>
 
 #include <asm/tlbflush.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/migrate.h>
@@ -1650,11 +1651,10 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
 SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
 		const void __user * __user *, pages,
 		const int __user *, nodes,
 		int __user *, status, int, flags)
 {
-	const struct cred *cred = current_cred(), *tcred;
 	struct task_struct *task;
 	struct mm_struct *mm;
 	int err;
 	nodemask_t task_nodes;
 
@@ -1674,18 +1674,13 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
 	}
 	get_task_struct(task);
 
 	/*
 	 * Check if this process has the right to modify the specified
-	 * process. The right exists if the process has administrative
-	 * capabilities, superuser privileges or the same
-	 * userid as the target process.
+	 * process. Use the regular "ptrace_may_access()" checks.
 	 */
-	tcred = __task_cred(task);
-	if (!uid_eq(cred->euid, tcred->suid) && !uid_eq(cred->euid, tcred->uid) &&
-	    !uid_eq(cred->uid,  tcred->suid) && !uid_eq(cred->uid,  tcred->uid) &&
-	    !capable(CAP_SYS_NICE)) {
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
 		rcu_read_unlock();
 		err = -EPERM;
 		goto out;
 	}
 	rcu_read_unlock();

Vulnerable code: 	const struct cred *cred = current_cred(), *tcred;
	 * process. The right exists if the process has administrative
	 * capabilities, superuser privileges or the same
	 * userid as the target process.
	tcred = __task_cred(task);
	if (!uid_eq(cred->euid, tcred->suid) && !uid_eq(cred->euid, tcred->uid) &&
	    !uid_eq(cred->uid,  tcred->suid) && !uid_eq(cred->uid,  tcred->uid) &&
	    !capable(CAP_SYS_NICE)) {
Fixed code: #include <linux/ptrace.h>
	 * process. Use the regular "ptrace_may_access()" checks.
	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {