[PATCH v3 1/3] mm/usercopy: Check kmap addresses properly

Matthew Wilcox (Oracle)
2021-12-13T14:28:21Z

If you are copying to an address in the kmap region, you may not copy
across a page boundary, no matter what the size of the underlying
allocation. You can't kmap() a slab page because slab pages always
come from low memory.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/highmem.h | 1 +
include/linux/highmem-internal.h | 10 ++++++++++
mm/usercopy.c | 16 ++++++++++------
3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/highmem.h b/arch/x86/include/asm/highmem.h
index 032e020853aa..731ee7cc40a5 100644
--- a/arch/x86/include/asm/highmem.h
+++ b/arch/x86/include/asm/highmem.h
@@ -26,6 +26,7 @@
#include <asm/tlbflush.h>
#include <asm/paravirt.h>
#include <asm/fixmap.h>
+#include <asm/pgtable_areas.h>

/* declarations for highmem.c */
extern unsigned long highstart_pfn, highend_pfn;
diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 0a0b2b09b1b8..01fb76d101b0 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -149,6 +149,11 @@ static inline void totalhigh_pages_add(long count)
atomic_long_add(count, &_totalhigh_pages);
}

+static inline bool is_kmap_addr(const void *x)
+{
+ unsigned long addr = (unsigned long)x;
+ return addr >= PKMAP_ADDR(0) && addr < PKMAP_ADDR(LAST_PKMAP);
+}
#else /* CONFIG_HIGHMEM */

static inline struct page *kmap_to_page(void *addr)
@@ -234,6 +239,11 @@ static inline void __kunmap_atomic(void *addr)
static inline unsigned int nr_free_highpages(void) { return 0; }
static inline unsigned long totalhigh_pages(void) { return 0UL; }

+static inline bool is_kmap_addr(const void *x)
+{
+ return false;
+}
+
#endif /* CONFIG_HIGHMEM */

/*
diff --git a/mm/usercopy.c b/mm/usercopy.c
index b3de3c4eefba..8c039302465f 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -228,12 +228,16 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
if (!virt_addr_valid(ptr))
return;

- /*
- * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
- * highmem page or fallback to virt_to_page(). The following
- * is effectively a highmem-aware virt_to_head_page().
- */
- page = compound_head(kmap_to_page((void *)ptr));
+ if (is_kmap_addr(ptr)) {
+ unsigned long page_end = (unsigned long)ptr | (PAGE_SIZE - 1);
+
+ if ((unsigned long)ptr + n - 1 > page_end)
+ usercopy_abort("kmap", NULL, to_user,
+ offset_in_page(ptr), n);
+ return;
+ }
+
+ page = virt_to_head_page(ptr);

if (PageSlab(page)) {
/* Check slab allocator for flags and size. */
--
2.33.0