The patch titled Subject: mm: zswap: disable migration while using per-CPU acomp_ctx has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-zswap-disable-migration-while-using-per-cpu-acomp_ctx.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Yosry Ahmed yosryahmed@google.com Subject: mm: zswap: disable migration while using per-CPU acomp_ctx Date: Tue, 7 Jan 2025 22:22:35 +0000
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.
This cannot be fixed by holding cpus_read_lock(), as it is possible for code already holding the lock to fall into reclaim and enter zswap (causing a deadlock). It also cannot be fixed by wrapping the usage of acomp_ctx in an SRCU critical section and using synchronize_srcu() in zswap_cpu_comp_dead(), because synchronize_srcu() is not allowed in CPU-hotplug notifiers (see Documentation/RCU/Design/Requirements/Requirements.rst).
This can be fixed by refcounting the acomp_ctx, but it involves complexity in handling the race between the refcount dropping to zero in zswap_[de]compress() and the refcount being re-initialized when the CPU is onlined.
Keep things simple for now and just disable migration while using the per-CPU acomp_ctx to block CPU hotunplug until the usage is over.
Link: https://lkml.kernel.org/r/20250107222236.2715883-2-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+o2e7Qv4OcuruL4tPg... Cc: Barry Song baohua@kernel.org Cc: Chengming Zhou chengming.zhou@linux.dev Cc: Kanchana P Sridhar kanchana.p.sridhar@intel.com Cc: Nhat Pham nphamcs@gmail.com Cc: syzbot syzkaller@googlegroups.com Cc: Vitaly Wool vitalywool@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/zswap.c | 19 ++++++++++++++++--- 1 file changed, 16 insertions(+), 3 deletions(-)
--- a/mm/zswap.c~mm-zswap-disable-migration-while-using-per-cpu-acomp_ctx +++ a/mm/zswap.c @@ -880,6 +880,18 @@ static int zswap_cpu_comp_dead(unsigned return 0; }
+/* Remain on the CPU while using its acomp_ctx to stop it from going offline */ +static struct crypto_acomp_ctx *acomp_ctx_get_cpu(struct crypto_acomp_ctx __percpu *acomp_ctx) +{ + migrate_disable(); + return raw_cpu_ptr(acomp_ctx); +} + +static void acomp_ctx_put_cpu(void) +{ + migrate_enable(); +} + static bool zswap_compress(struct page *page, struct zswap_entry *entry, struct zswap_pool *pool) { @@ -893,8 +905,7 @@ static bool zswap_compress(struct page * gfp_t gfp; u8 *dst;
- acomp_ctx = raw_cpu_ptr(pool->acomp_ctx); - + acomp_ctx = acomp_ctx_get_cpu(pool->acomp_ctx); mutex_lock(&acomp_ctx->mutex);
dst = acomp_ctx->buffer; @@ -950,6 +961,7 @@ unlock: zswap_reject_alloc_fail++;
mutex_unlock(&acomp_ctx->mutex); + acomp_ctx_put_cpu(); return comp_ret == 0 && alloc_ret == 0; }
@@ -960,7 +972,7 @@ static void zswap_decompress(struct zswa 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); @@ -990,6 +1002,7 @@ static void zswap_decompress(struct zswa
if (src != acomp_ctx->buffer) zpool_unmap_handle(zpool, entry->handle); + acomp_ctx_put_cpu(); }
/********************************* _
Patches currently in -mm which might be from yosryahmed@google.com are
revert-mm-zswap-fix-race-between-compression-and-cpu-hotunplug.patch mm-zswap-disable-migration-while-using-per-cpu-acomp_ctx.patch