Commit 197e7e52 authored Aug 20, 2017 by Linus Torvalds

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>

parent 7f680d7e

mm/migrate.c

+3 −8

Original line number	Diff line number	Diff line
		@@ -41,6 +41,7 @@
		#include <linux/page_idle.h>
		#include <linux/page_owner.h>
		#include <linux/sched/mm.h>
		#include <linux/ptrace.h>

		#include <asm/tlbflush.h>

		@@ -1652,7 +1653,6 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_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;
		@@ -1676,14 +1676,9 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,

		/*
		* 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.
		*/
		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)) {
		* process. Use the regular "ptrace_may_access()" checks.
		*/
		if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
		rcu_read_unlock();
		err = -EPERM;
		goto out;