[PATCH 3.2 105/106] Sanitize 'move_pages()' permission checks

[Ben Hutchings]

3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

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.

Reported-by: Otto Ebeling <otto.ebeling@xxxxxx>
Acked-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Willy Tarreau <w@xxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/migrate.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -34,6 +34,7 @@
 #include <linux/syscalls.h>
 #include <linux/hugetlb.h>
 #include <linux/gfp.h>
+#include <linux/ptrace.h>
 
 #include <asm/tlbflush.h>
 
@@ -1366,7 +1367,6 @@ 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;
@@ -1390,14 +1390,9 @@ 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.
+	 * process. Use the regular "ptrace_may_access()" checks.
 	 */
-	tcred = __task_cred(task);
-	if (cred->euid != tcred->suid && cred->euid != tcred->uid &&
-	    cred->uid  != tcred->suid && cred->uid  != tcred->uid &&
-	    !capable(CAP_SYS_NICE)) {
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
 		rcu_read_unlock();
 		err = -EPERM;
 		goto out;