Re: [PATCH V2] slub: avoid potential NULL dereference orcorruption

From: Pekka Enberg
Date: Wed Nov 23 2011 - 03:32:26 EST


On Tue, 22 Nov 2011, Eric Dumazet wrote:
show_slab_objects() can trigger NULL dereferences or memory corruption.

Another cpu can change its c->page to NULL or c->node to NUMA_NO_NODE
while we use them.

Use ACCESS_ONCE(c->page) and ACCESS_ONCE(c->node) to make sure this
cannot happen.

Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>

Did someone test this patch? Does it fix any of the reported issues?

---
V2: remove a not needed "if (!c)" test

mm/slub.c | 21 +++++++++++----------
1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 7d2a996..9deef7d 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4435,30 +4435,31 @@ static ssize_t show_slab_objects(struct kmem_cache *s,

for_each_possible_cpu(cpu) {
struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab, cpu);
+ int node = ACCESS_ONCE(c->node);
struct page *page;

- if (!c || c->node < 0)
+ if (node < 0)
continue;
-
- if (c->page) {
- if (flags & SO_TOTAL)
- x = c->page->objects;
+ page = ACCESS_ONCE(c->page);
+ if (page) {
+ if (flags & SO_TOTAL)
+ x = page->objects;
else if (flags & SO_OBJECTS)
- x = c->page->inuse;
+ x = page->inuse;
else
x = 1;

total += x;
- nodes[c->node] += x;
+ nodes[node] += x;
}
page = c->partial;

if (page) {
x = page->pobjects;
- total += x;
- nodes[c->node] += x;
+ total += x;
+ nodes[node] += x;
}
- per_cpu[c->node]++;
+ per_cpu[node]++;
}
}




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