Allocation requests with __GFP_DMA/__GFP_DMA32 or
SLAB_CACHE_DMA/SLAB_CACHE_DMA32 cannot be fulfilled by KFENCE, because
they must reside in low memory, whereas KFENCE memory pool is located in
high memory.

Skip such allocations to avoid crashes where DMAable memory is expected.

Fixes: 0ce20dd84089 ("mm: add Kernel Electric-Fence infrastructure")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org # 5.12+
Signed-off-by: Alexander Potapenko <glider@google.com>

---

v2:
 - added parentheses around the GFP clause, as requested by Marco
---
 mm/kfence/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 4d21ac44d5d35..f7ce3d876bc9e 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c

@@ -760,6 +760,14 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
        if (size > PAGE_SIZE)
                return NULL;

+       /*
+        * Skip DMA allocations. These must reside in the low memory, which we
+        * cannot guarantee.
+        */
+       if ((flags & (__GFP_DMA | __GFP_DMA32)) ||
+           (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32)))
+               return NULL;
+