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Subject[patch 24/28] mm_for_maps: simplify, use ptrace_may_access()
2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Oleg Nesterov <oleg@redhat.com>

commit 13f0feafa6b8aead57a2a328e2fca6a5828bf286 upstream.

It would be nice to kill __ptrace_may_access(). It requires task_lock(),
but this lock is only needed to read mm->flags in the middle.

Convert mm_for_maps() to use ptrace_may_access(), this also simplifies
the code a little bit.

Also, we do not need to take ->mmap_sem in advance. In fact I think
mm_for_maps() should not play with ->mmap_sem at all, the caller should
take this lock.

With or without this patch, without ->cred_guard_mutex held we can race
with exec() and get the new ->mm but check old creds.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/proc/base.c | 23 +++++++++++------------
1 file changed, 11 insertions(+), 12 deletions(-)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -242,20 +242,19 @@ struct mm_struct *mm_for_maps(struct tas
struct mm_struct *mm = get_task_mm(task);
if (!mm)
return NULL;
+ if (mm != current->mm) {
+ /*
+ * task->mm can be changed before security check,
+ * in that case we must notice the change after.
+ */
+ if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
+ mm != task->mm) {
+ mmput(mm);
+ return NULL;
+ }
+ }
down_read(&mm->mmap_sem);
- task_lock(task);
- if (task->mm != mm)
- goto out;
- if (task->mm != current->mm &&
- __ptrace_may_access(task, PTRACE_MODE_READ) < 0)
- goto out;
- task_unlock(task);
return mm;
-out:
- task_unlock(task);
- up_read(&mm->mmap_sem);
- mmput(mm);
- return NULL;
}

static int proc_pid_cmdline(struct task_struct *task, char * buffer)



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