mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()

In the Linux kernel, the following vulnerability has been resolved:

mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()

Currently, zswap_cpu_comp_dead() calls crypto_free_acomp() while holding

the per-CPU acomp_ctx mutex. crypto_free_acomp() then holds scomp_lock

(through crypto_exit_scomp_ops_async()).

On the other hand, crypto_alloc_acomp_node() holds the scomp_lock (through

crypto_scomp_init_tfm()), and then allocates memory. If the allocation

results in reclaim, we may attempt to hold the per-CPU acomp_ctx mutex.

The above dependencies can cause an ABBA deadlock. For example in the

following scenario:

(1) Task A running on CPU #1:

crypto_alloc_acomp_node()

Holds scomp_lock

Enters reclaim

Reads per_cpu_ptr(pool->acomp_ctx, 1)

(2) Task A is descheduled

(3) CPU #1 goes offline

zswap_cpu_comp_dead(CPU #1)

Holds per_cpu_ptr(pool->acomp_ctx, 1))

Calls crypto_free_acomp()

Waits for scomp_lock

(4) Task A running on CPU #2:

Waits for per_cpu_ptr(pool->acomp_ctx, 1) // Read on CPU #1

DEADLOCK

Since there is no requirement to call crypto_free_acomp() with the per-CPU

acomp_ctx mutex held in zswap_cpu_comp_dead(), move it after the mutex is

unlocked. Also move the acomp_request_free() and kfree() calls for

consistency and to avoid any potential sublte locking dependencies in the

future.

With this, only setting acomp_ctx fields to NULL occurs with the mutex

held. This is similar to how zswap_cpu_comp_prepare() only initializes

acomp_ctx fields with the mutex held, after performing all allocations

before holding the mutex.

Opportunistically, move the NULL check on acomp_ctx so that it takes place

before the mutex dereference.