KHO allocates metadata for its preserved memory map using the slab
allocator via kzalloc(). This metadata is temporary and is used by the
next kernel during early boot to find preserved memory.

A problem arises when KFENCE is enabled. kzalloc() calls can be
randomly intercepted by kfence_alloc(), which services the allocation
from a dedicated KFENCE memory pool. This pool is allocated early in
boot via memblock.

When booting via KHO, the memblock allocator is restricted to a "scratch
area", forcing the KFENCE pool to be allocated within it. This creates a
conflict, as the scratch area is expected to be ephemeral and
overwriteable by a subsequent kexec. If KHO metadata is placed in this
KFENCE pool, it leads to memory corruption when the next kernel is
loaded.

To fix this, modify KHO to allocate its metadata directly from the buddy
allocator instead of slab.

Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
---
 include/linux/gfp.h     | 3 +++
 kernel/kexec_handover.c | 6 +++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 0ceb4e09306c..623bee335383 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -7,6 +7,7 @@
 #include <linux/mmzone.h>
 #include <linux/topology.h>
 #include <linux/alloc_tag.h>
+#include <linux/cleanup.h>
 #include <linux/sched.h>
 
 struct vm_area_struct;
@@ -463,4 +464,6 @@ static inline struct folio *folio_alloc_gigantic_noprof(int order, gfp_t gfp,
 /* This should be paired with folio_put() rather than free_contig_range(). */
 #define folio_alloc_gigantic(...) alloc_hooks(folio_alloc_gigantic_noprof(__VA_ARGS__))
 
+DEFINE_FREE(free_page, void *, free_page((unsigned long)_T))
+
 #endif /* __LINUX_GFP_H */
diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
index e5b91761fbfe..de4466b47455 100644
--- a/kernel/kexec_handover.c
+++ b/kernel/kexec_handover.c
@@ -142,7 +142,7 @@ static void *xa_load_or_alloc(struct xarray *xa, unsigned long index)
 	if (res)
 		return res;
 
-	void *elm __free(kfree) = kzalloc(PAGE_SIZE, GFP_KERNEL);
+	void *elm __free(free_page) = (void *)get_zeroed_page(GFP_KERNEL);
 
 	if (!elm)
 		return ERR_PTR(-ENOMEM);
@@ -348,9 +348,9 @@ static_assert(sizeof(struct khoser_mem_chunk) == PAGE_SIZE);
 static struct khoser_mem_chunk *new_chunk(struct khoser_mem_chunk *cur_chunk,
 					  unsigned long order)
 {
-	struct khoser_mem_chunk *chunk __free(kfree) = NULL;
+	struct khoser_mem_chunk *chunk __free(free_page) = NULL;
 
-	chunk = kzalloc(PAGE_SIZE, GFP_KERNEL);
+	chunk = (void *)get_zeroed_page(GFP_KERNEL);
 	if (!chunk)
 		return ERR_PTR(-ENOMEM);
 
-- 
2.51.0.869.ge66316f041-goog

On Mon, Oct 20, 2025 at 5:09 PM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> KHO allocates metadata for its preserved memory map using the slab
> allocator via kzalloc(). This metadata is temporary and is used by the
> next kernel during early boot to find preserved memory.
>
> A problem arises when KFENCE is enabled. kzalloc() calls can be
> randomly intercepted by kfence_alloc(), which services the allocation
> from a dedicated KFENCE memory pool. This pool is allocated early in
> boot via memblock.
>
> When booting via KHO, the memblock allocator is restricted to a "scratch
> area", forcing the KFENCE pool to be allocated within it. This creates a
> conflict, as the scratch area is expected to be ephemeral and
> overwriteable by a subsequent kexec. If KHO metadata is placed in this
> KFENCE pool, it leads to memory corruption when the next kernel is
> loaded.
>
> To fix this, modify KHO to allocate its metadata directly from the buddy
> allocator instead of slab.
>
> Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Reviewed-by: Pratyush Yadav <pratyush@kernel.org>

Reviewed-by: David Matlack <dmatlack@google.com>

On Mon, Oct 27, 2025 at 7:05 PM David Matlack <dmatlack@google.com> wrote:
>
> On Mon, Oct 20, 2025 at 5:09 PM Pasha Tatashin
> <pasha.tatashin@soleen.com> wrote:
> >
> > KHO allocates metadata for its preserved memory map using the slab
> > allocator via kzalloc(). This metadata is temporary and is used by the
> > next kernel during early boot to find preserved memory.
> >
> > A problem arises when KFENCE is enabled. kzalloc() calls can be
> > randomly intercepted by kfence_alloc(), which services the allocation
> > from a dedicated KFENCE memory pool. This pool is allocated early in
> > boot via memblock.
> >
> > When booting via KHO, the memblock allocator is restricted to a "scratch
> > area", forcing the KFENCE pool to be allocated within it. This creates a
> > conflict, as the scratch area is expected to be ephemeral and
> > overwriteable by a subsequent kexec. If KHO metadata is placed in this
> > KFENCE pool, it leads to memory corruption when the next kernel is
> > loaded.
> >
> > To fix this, modify KHO to allocate its metadata directly from the buddy
> > allocator instead of slab.
> >
> > Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> > Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
>
> Reviewed-by: David Matlack <dmatlack@google.com>

Thank you,
Pasha