On Wed, Jul 19, 2023 at 03:41:37PM -0400, Rik van Riel wrote:
> The memblock_discard function frees the memblock.reserved.regions
> array, which is good.
>
> However, if a subsequent memblock_free (or memblock_phys_free) comes
> in later, from for example ima_free_kexec_buffer, that will result in
> a use after free bug in memblock_isolate_range.
>
> When running a kernel with CONFIG_KASAN enabled, this will cause a
> kernel panic very early in boot. Without CONFIG_KASAN, there is
> a chance that memblock_isolate_range might scribble on memory
> that is now in use by somebody else.
>
> Avoid those issues by making sure that memblock_discard points
> memblock.reserved.regions back at the static buffer.
>
> If memblock_discard is called while there is still memory
> in the memblock.reserved type, that will print a warning
> in memblock_remove_region.
I'm going to apply this with the last paragraph rewritten as
If memblock_free is called after memblock memory is discarded, that will
print a warning in memblock_remove_region.
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>
> ---
> mm/memblock.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 3feafea06ab2..068289a46903 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -374,6 +374,10 @@ void __init memblock_discard(void)
> kfree(memblock.reserved.regions);
> else
> memblock_free_late(addr, size);
> + /* Reset to prevent UAF from stray frees. */
> + memblock.reserved.regions = memblock_reserved_init_regions;
> + memblock.reserved.cnt = 1;
> + memblock_remove_region(&memblock.reserved, 0);
> }
>
> if (memblock.memory.regions != memblock_memory_init_regions) {
> --
> 2.34.1
>
>
--
Sincerely yours,
Mike.
