[SRU][Trusty][PATCH 1/1] Sanitize 'move_pages()' permission checks

Kleber Sacilotto de Souza kleber.souza at canonical.com
Thu Dec 7 11:56:46 UTC 2017

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

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 at iki.fi>
Acked-by: Eric W. Biederman <ebiederm at xmission.com>
Cc: Willy Tarreau <w at 1wt.eu>
Cc: stable at kernel.org
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

(backported from commit 197e7e521384a23b9e585178f3f11c9fa08274b9)
[klebers: Trusty kernel doesn't have caaee6234d05a, which introduced
 the check for PTRACE_MODE_READ_REALCREDS. However, checking for
 PTRACE_MODE_READ should be enough, as discussed at
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
 mm/migrate.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 70eb6bb9a8e6..21f477b133c4 100644
--- 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>
@@ -1441,7 +1442,6 @@ 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;
@@ -1465,14 +1465,9 @@ 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)) {
 		err = -EPERM;
 		goto out;

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