From 32f8516a8c733d281faa9f6666b509035246505c Mon Sep 17 00:00:00 2001 From: David Rientjes Date: Tue, 16 Oct 2012 17:31:23 -0700 Subject: 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 Signed-off-by: David Rientjes Signed-off-by: Linus Torvalds --- mm/mempolicy.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) (limited to 'mm/mempolicy.c') diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 0b78fb9ea65b..d04a8a54c294 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 -- cgit v1.2.3-59-g8ed1b