zram: fix slot write race condition

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: zram: fix slot write race condition Parallel concurrent writes to the same zram index result in leaked zsmalloc handles. Schematically we can have something like this: CPU0 CPU1 zram_slot_lock() zs_free(handle) zram_slot_lock() zram_slot_lock() zs_free(handle) zram_slot_lock() compress compress handle = zs_malloc() handle = zs_malloc() zram_slot_lock zram_set_handle(handle) zram_slot_lock zram_slot_lock zram_set_handle(handle) zram_slot_lock Either CPU0 or CPU1 zsmalloc handle will leak because zs_free() is done too early. In fact, we need to reset zram entry right before we set its new handle, all under the same slot lock scope.

References

• https://git.kernel.org/stable/c/ff750e9f2c4d63854c33967d1646b5e89a9a19a2
• https://git.kernel.org/stable/c/ce4be9e4307c5a60701ff6e0cafa74caffdc54ce