* Re: Patch "mm: zswap: fix race between [de]compression and CPU hotunplug" has been added to the 6.12-stable tree
[not found] <20250113140408.1739713-1-sashal@kernel.org>
@ 2025-01-13 15:59 ` Yosry Ahmed
2025-01-14 1:02 ` Sasha Levin
0 siblings, 1 reply; 2+ messages in thread
From: Yosry Ahmed @ 2025-01-13 15:59 UTC (permalink / raw)
To: stable
Cc: stable-commits, Johannes Weiner, Nhat Pham, Chengming Zhou,
Andrew Morton
On Mon, Jan 13, 2025 at 6:04 AM Sasha Levin <sashal@kernel.org> wrote:
>
> This is a note to let you know that I've just added the patch titled
>
> mm: zswap: fix race between [de]compression and CPU hotunplug
>
> to the 6.12-stable tree which can be found at:
> http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
>
> The filename of the patch is:
> mm-zswap-fix-race-between-de-compression-and-cpu-hot.patch
> and it can be found in the queue-6.12 subdirectory.
>
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.
Please drop this patch from v6.12 (and all stable trees) as it has a
bug, as I pointed out in [1] (was that the correct place to surface
this?).
Andrew's latest PR contains a revert of this patch (and alternative fix) [2].
Thanks!
[1]https://lore.kernel.org/stable/CAJD7tkaiUA6J1nb0=1ELpUD0OgjoD+tft-iPqbPdyXCpS=2AGQ@mail.gmail.com/
[2]https://lore.kernel.org/lkml/20250113000543.862792e948c2646032a477e0@linux-foundation.org/
>
>
>
> commit c56e79d453ef5b5fc6ded252bc6ba461f12946ba
> Author: Yosry Ahmed <yosryahmed@google.com>
> Date: Thu Dec 19 21:24:37 2024 +0000
>
> mm: zswap: fix race between [de]compression and CPU hotunplug
>
> [ Upstream commit eaebeb93922ca6ab0dd92027b73d0112701706ef ]
>
> 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).
>
> Link: https://lkml.kernel.org/r/20241219212437.2714151-1-yosryahmed@google.com
> Fixes: 1ec3b5fe6eec ("mm/zswap: move to use crypto_acomp API for hardware acceleration")
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Reported-by: Johannes Weiner <hannes@cmpxchg.org>
> Closes: https://lore.kernel.org/lkml/20241113213007.GB1564047@cmpxchg.org/
> Reported-by: Sam Sun <samsun1006219@gmail.com>
> Closes: https://lore.kernel.org/lkml/CAEkJfYMtSdM5HceNsXUDf5haghD5+o2e7Qv4OcuruL4tPg6OaQ@mail.gmail.com/
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
> Acked-by: Barry Song <baohua@kernel.org>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> Cc: Vitaly Wool <vitalywool@gmail.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 0030ce8fecfc..c86d4bcbb447 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -875,6 +875,18 @@ static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)
> return 0;
> }
>
> +/* Prevent CPU hotplug from freeing up the per-CPU acomp_ctx resources */
> +static struct crypto_acomp_ctx *acomp_ctx_get_cpu(struct crypto_acomp_ctx __percpu *acomp_ctx)
> +{
> + cpus_read_lock();
> + return raw_cpu_ptr(acomp_ctx);
> +}
> +
> +static void acomp_ctx_put_cpu(void)
> +{
> + cpus_read_unlock();
> +}
> +
> static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
> {
> struct crypto_acomp_ctx *acomp_ctx;
> @@ -887,8 +899,7 @@ static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
> gfp_t gfp;
> u8 *dst;
>
> - acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> -
> + acomp_ctx = acomp_ctx_get_cpu(entry->pool->acomp_ctx);
> mutex_lock(&acomp_ctx->mutex);
>
> dst = acomp_ctx->buffer;
> @@ -944,6 +955,7 @@ static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
> zswap_reject_alloc_fail++;
>
> mutex_unlock(&acomp_ctx->mutex);
> + acomp_ctx_put_cpu();
> return comp_ret == 0 && alloc_ret == 0;
> }
>
> @@ -954,7 +966,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio)
> struct crypto_acomp_ctx *acomp_ctx;
> u8 *src;
>
> - acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> + acomp_ctx = acomp_ctx_get_cpu(entry->pool->acomp_ctx);
> mutex_lock(&acomp_ctx->mutex);
>
> src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
> @@ -984,6 +996,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio)
>
> if (src != acomp_ctx->buffer)
> zpool_unmap_handle(zpool, entry->handle);
> + acomp_ctx_put_cpu();
> }
>
> /*********************************
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