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SubjectRe: [PATCH] zram: fix null dereference of handle
On (09/19/17 11:34), Minchan Kim wrote:
[..]
> static void zram_meta_free(struct zram *zram, u64 disksize)
> {
> size_t num_pages = disksize >> PAGE_SHIFT;
> @@ -876,11 +855,18 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
> zram_slot_unlock(zram, index);
> }
>
> - if (zram_same_page_read(zram, index, page, 0, PAGE_SIZE))
> - return 0;
> -
> zram_slot_lock(zram, index);
> handle = zram_get_handle(zram, index);
> + if (unlikely(!handle || zram_test_flag(zram, index, ZRAM_SAME))) {
> + void *mem;


is this branch really unlikely()? ZRAM_SAME ratio really depends,
on some setups it can be quite likely, I suspect.


another question, "!handle == value & ZRAM_SAME"? if so, then why not
just check for `flags & ZRAM_SAME'? if not then:

- for `value & ZRAM_SAME' you fill the page with zram_get_element(zram, index)
and return 0. ok.

- for !handle.... you also fill the page with zram_get_element(zram, index)
and return 0. is this ok? shouldn't !handle return error in this case?


I really suspect that there are some paths that can lead to !handle
entry, that will not be ZRAM_SAME. e.g. error return from compression
path.

-ss

> + mem = kmap_atomic(page);
> + zram_fill_page(mem, PAGE_SIZE, zram_get_element(zram, index));
> + kunmap_atomic(mem);
> + zram_slot_unlock(zram, index);
> + return 0;
> + }
> +
> size = zram_get_obj_size(zram, index);
>
> src = zs_map_object(zram->mem_pool, handle, ZS_MM_RO);
> --
> 2.7.4
>

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