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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>
This commit is contained in:
David Rientjes 2012-10-16 17:31:23 -07:00 committed by Linus Torvalds
parent dd8e8c4a2c
commit 32f8516a8c
2 changed files with 7 additions and 5 deletions
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7
fs/proc/task_mmu.c
View File

@ -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

5
mm/mempolicy.c
View File

@ -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