mm, mempolicy: fix printing stack contents in numa_maps

When reading /proc/pid/numa_maps, it's possible to return the contents of
the stack where the mempolicy string should be printed if the policy gets
freed from beneath us.

This happens because mpol_to_str() may return an error the
stack-allocated buffer is then printed without ever being stored.

There are two possible error conditions in mpol_to_str():

 - if the buffer allocated is insufficient for the string to be stored,
   and

 - if the mempolicy has an invalid mode.

The first error condition is not triggered in any of the callers to
mpol_to_str(): at least 50 bytes is always allocated on the stack and this
is sufficient for the string to be written.  A future patch should convert
this into BUILD_BUG_ON() since we know the maximum strlen possible, but
that's not -rc material.

The second error condition is possible if a race occurs in dropping a
reference to a task's mempolicy causing it to be freed during the read().
The slab poison value is then used for the mode and mpol_to_str() returns
-EINVAL.

This race is only possible because get_vma_policy() believes that
mm->mmap_sem protects task->mempolicy, which isn't true.  The exit path
does not hold mm->mmap_sem when dropping the reference or setting
task->mempolicy to NULL: it uses task_lock(task) instead.

Thus, it's required for the caller of a task mempolicy to hold
task_lock(task) while grabbing the mempolicy and reading it.  Callers with
a vma policy store their mempolicy earlier and can simply increment the
reference count so it's guaranteed not to be freed.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 79827ce..14df880 100644 (file)
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1158,6 +1158,7 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
        struct vm_area_struct *vma = v;
        struct numa_maps *md = &numa_priv->md;
        struct file *file = vma->vm_file;
+       struct task_struct *task = proc_priv->task;
        struct mm_struct *mm = vma->vm_mm;
        struct mm_walk walk = {};
        struct mempolicy *pol;
@@ -1177,9 +1178,11 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
        walk.private = md;
        walk.mm = mm;
 
-       pol = get_vma_policy(proc_priv->task, vma, vma->vm_start);
+       task_lock(task);
+       pol = get_vma_policy(task, vma, vma->vm_start);
        mpol_to_str(buffer, sizeof(buffer), pol, 0);
        mpol_cond_put(pol);
+       task_unlock(task);
 
        seq_printf(m, "%08lx %s", vma->vm_start, buffer);
 
@@ -1189,7 +1192,7 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
        } else if (vma->vm_start <= mm->brk && vma->vm_end >= mm->start_brk) {
                seq_printf(m, " heap");
        } else {
-               pid_t tid = vm_is_stack(proc_priv->task, vma, is_pid);
+               pid_t tid = vm_is_stack(task, vma, is_pid);
                if (tid != 0) {
                        /*
                         * Thread stack in /proc/PID/task/TID/maps or

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 0b78fb9..d04a8a5 100644 (file)
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1536,9 +1536,8 @@ asmlinkage long compat_sys_mbind(compat_ulong_t start, compat_ulong_t len,
  *
  * Returns effective policy for a VMA at specified address.
  * Falls back to @task or system default policy, as necessary.
- * Current or other task's task mempolicy and non-shared vma policies
- * are protected by the task's mmap_sem, which must be held for read by
- * the caller.
+ * Current or other task's task mempolicy and non-shared vma policies must be
+ * protected by task_lock(task) by the caller.
  * Shared policies [those marked as MPOL_F_SHARED] require an extra reference
  * count--added by the get_policy() vm_op, as appropriate--to protect against
  * freeing by another task.  It is the caller's responsibility to free the