[patch for-3.7] mm, mempolicy: fix printing stack contents innuma_maps

From: David Rientjes
Date: Tue Oct 16 2012 - 20:31:19 EST

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,

- 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

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@xxxxxxxxxx>
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
fs/proc/task_mmu.c | 7 +++++--
mm/mempolicy.c | 5 ++---
2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
--- 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);
+ 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
--- 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
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