Patch "Sanitize 'move_pages()' permission checks" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

Sanitize 'move_pages()' permission checks

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sanitize-move_pages-permission-checks.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Linus Torvalds <[email protected]>
Date: Sun, 20 Aug 2017 13:26:27 -0700
Subject: Sanitize 'move_pages()' permission checks

From: Linus Torvalds <[email protected]>

commit 197e7e521384a23b9e585178f3f11c9fa08274b9 upstream.

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.

---
 mm/migrate.c |   13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

@@ -38,6 +38,7 @@
 #include <linux/balloon_compaction.h>
 #include <linux/mmu_notifier.h>
 #include <linux/page_idle.h>
+#include <linux/ptrace.h>
 
 #include <asm/tlbflush.h>
 

@@ -1483,7 +1484,6 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid,
 		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;

@@ -1507,14 +1507,9 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid,
 
 	/*
 	 * 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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/x86-asm-64-clear-ac-on-nmi-entries.patch queue-4.4/sanitize-move_pages-permission-checks.patch queue-4.4/mm-mempolicy-fix-use-after-free-when-calling-get_mempolicy.patch queue-4.4/mm-revert-x86_64-and-arm64-elf_et_dyn_base-base-changes.patch