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Subject[PATCH 3.16 233/233] Sanitize 'move_pages()' permission checks
3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Linus Torvalds <torvalds@linux-foundation.org>

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@iki.fi>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
mm/migrate.c | 11 +++--------
1 file changed, 3 insertions(+), 8 deletions(-)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -38,6 +38,7 @@
#include <linux/gfp.h>
#include <linux/balloon_compaction.h>
#include <linux/mmu_notifier.h>
+#include <linux/ptrace.h>

#include <asm/tlbflush.h>

@@ -1484,7 +1485,6 @@ 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;
@@ -1508,14 +1508,9 @@ 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;
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