• David Rientjes's avatar
    mm, mempolicy: fix printing stack contents in numa_maps · 32f8516a
    David Rientjes authored
    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: default avatarDave Jones <davej@redhat.com>
    Signed-off-by: default avatarDavid Rientjes <rientjes@google.com>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    32f8516a
task_mmu.c 31.6 KB