Re: [PATCH] avoid null pointer access in vm_struct
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2011-08-17 19:40:56
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On Wed, 17 Aug 2011 22:28:48 +0900 Mitsuo Hayasaka [off-list ref] wrote:
The /proc/vmallocinfo shows information about vmalloc allocations in vmlist that is a linklist of vm_struct. It, however, may access pages field of vm_struct where a page was not allocated, which results in a null pointer access and leads to a kernel panic. Why this happen: For example, in __vmalloc_area_node, the nr_pages field of vm_struct are set to the expected number of pages to be allocated, before the actual pages allocations. At the same time, when the /proc/vmallocinfo is read, it accesses the pages field of vm_struct according to the nr_pages field at show_numa_info(). Thus, a null pointer access happens. Patch: This patch avoids accessing the pages field with unallocated page when show_numa_info() is called. So, it can solve this problem.
Do we have a similar race when running __vunmap() in parallel with show_numa_info()?
quoted hunk ↗ jump to hunk
index 7ef0903..e2ec5b0 100644--- a/mm/vmalloc.c +++ b/mm/vmalloc.c@@ -2472,13 +2472,16 @@ static void show_numa_info(struct seq_file *m, struct vm_struct *v) if (NUMA_BUILD) { unsigned int nr, *counters = m->private; - if (!counters) + if (!counters || !v->nr_pages || !v->pages) return; memset(counters, 0, nr_node_ids * sizeof(unsigned int)); - for (nr = 0; nr < v->nr_pages; nr++) + for (nr = 0; nr < v->nr_pages; nr++) { + if (!v->pages[nr]) + break; counters[page_to_nid(v->pages[nr])]++; + } for_each_node_state(nr, N_HIGH_MEMORY) if (counters[nr])
I think this has memory ordering issues: it requires that this CPU see the modification to ->nr_pages and ->pages in the same order as the CPU which is writing ->nr_pages, ->pages and ->pages[x]. Perhaps fixable by taking vmlist_lock appropriately. I suspect that the real bug is that __vmalloc_area_node() and its caller made the new vmap_area globally visible before it was fully initialised. If we were to fix that, the /proc/vmallocinfo read would not encounter this vm_struct at all. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>