On 2024/12/20 05:24, Yosry Ahmed wrote:
In zswap_compress() and zswap_decompress(), the per-CPU acomp_ctx of the
current CPU at the beginning of the operation is retrieved and used
throughout. However, since neither preemption nor migration are
disabled, it is possible that the operation continues on a different
CPU.
If the original CPU is hotunplugged while the acomp_ctx is still in use,
we run into a UAF bug as the resources attached to the acomp_ctx are
freed during hotunplug in zswap_cpu_comp_dead().
The problem was introduced in commit 1ec3b5fe6eec ("mm/zswap: move to
use crypto_acomp API for hardware acceleration") when the switch to the
crypto_acomp API was made. Prior to that, the per-CPU crypto_comp was
retrieved using get_cpu_ptr() which disables preemption and makes sure
the CPU cannot go away from under us. Preemption cannot be disabled with
the crypto_acomp API as a sleepable context is needed.
Commit 8ba2f844f050 ("mm/zswap: change per-cpu mutex and buffer to
per-acomp_ctx") increased the UAF surface area by making the per-CPU
buffers dynamic, adding yet another resource that can be freed from
under zswap compression/decompression by CPU hotunplug.
There are a few ways to fix this:
(a) Add a refcount for acomp_ctx.
(b) Disable migration while using the per-CPU acomp_ctx.
(c) Disable CPU hotunplug while using the per-CPU acomp_ctx by holding
the CPUs read lock.
Implement (c) since it's simpler than (a), and (b) involves using
migrate_disable() which is apparently undesired (see huge comment in
include/linux/preempt.h).
Fixes: 1ec3b5fe6eec ("mm/zswap: move to use crypto_acomp API for hardware acceleration")
Reported-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/20241113213007.GB1564047@xxxxxxxxxxx/
Reported-by: Sam Sun <samsun1006219@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/CAEkJfYMtSdM5HceNsXUDf5haghD5+o2e7Qv4OcuruL4tPg6OaQ@xxxxxxxxxxxxxx/
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
---
Good analysis and solution!
Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
Thanks.
