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>

1 files changed, 3 insertions, 8 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index d68a41da6abb..e84eeb4e4356 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -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.
+         * 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;