On Wed, Jun 30, 2021 at 9:02 AM Souptick Joarder [off-list ref] wrote:

On Tue, Jun 29, 2021 at 9:47 PM Alexander Potapenko [off-list ref] wrote:

quoted

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;
+

I prefer to move this check at the top of the function.
Although it won't make much difference except avoiding atomic operations
in case this condition is true.

Agreed, we probably shouldn't be expecting a constant flow of
allocations from these zones that will be slowed down by this check.
On a related note, Marco suggested moving the PAGE_SIZE check to the
top of the function as well.

It will also make sense to check for GFP_ZONEMASK instead of just GFP DMA flags.
I couldn't see anyone passing e.g. __GFP_HIGHMEM or __GFP_MOVABLE to
kmem_cache_alloc(), but according to mm/slab.c it is possible, so just
to be on the safe side we'd better ignore them as well.

quoted

        return kfence_guarded_alloc(s, size, flags);
 }

--
2.32.0.93.g670b81a890-goog

-- 
Alexander Potapenko
Software Engineer

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