Messages in this thread |  | | From | Cong Wang <> | Date | Tue, 16 Apr 2019 16:47:55 -0700 | Subject | Re: [PATCH 1/2] ras: fix an off-by-one error in __find_elem() |
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On Tue, Apr 16, 2019 at 4:28 PM Luck, Tony <tony.luck@intel.com> wrote: > > On Tue, Apr 16, 2019 at 04:18:57PM -0700, Cong Wang wrote: > > > The problem case occurs when we've seen enough distinct > > > errors that we have filled every entry, then we try to > > > look up a pfn that is larger that any seen before. > > > > > > The loop: > > > > > > while (min < max) { > > > ... > > > } > > > > > > will terminate with "min" set to MAX_ELEMS. Then we > > > execute: > > > > > > this_pfn = PFN(ca->array[min]); > > > > > > which references beyond the end of the space allocated > > > for ca->array. > > > > Exactly. > > Hmmm. But can we ever really have this happen? The call > sequence to get here looks like: > > > mutex_lock(&ce_mutex); > > if (ca->n == MAX_ELEMS) > WARN_ON(!del_lru_elem_unlocked(ca)); > > ret = find_elem(ca, pfn, &to); > > I.e. if the array was all the way full, we delete one element > before calling find_elem(). So when we get here: > > static int __find_elem(struct ce_array *ca, u64 pfn, unsigned int *to) > { > u64 this_pfn; > int min = 0, max = ca->n; > > The biggest value "max" can have is MAX_ELEMS-1
This is exactly the explanation for why the crash is inside memmove() rather than inside find_elem(). del_elem() actually accesses off-by-two once we pass its 'if' check in line 232:
229 static void del_elem(struct ce_array *ca, int idx) 230 { 231 /* Save us a function call when deleting the last element. */ 232 if (ca->n - (idx + 1)) 233 memmove((void *)&ca->array[idx], 234 (void *)&ca->array[idx + 1], 235 (ca->n - (idx + 1)) * sizeof(u64)); 236 237 ca->n--; 238 }
idx is ca->n and ca->n is MAX_ELEMS-1, then the above if statement becomes true, therefore idx+1 is MAX_ELEMS which is just beyond the valid range.
Thanks.
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