[PATCH v2] mm/vmalloc: reject overflowing vm_map_ram page counts

[Shubhang Kaushik (Ampere)]

vm_map_ram() accepts a page count and must either map that many pages or
fail. It currently converts the count to a byte size with:

        (unsigned long)count << PAGE_SHIFT

On 32-bit kernels, this conversion overflows when count is greater than
ULONG_MAX >> PAGE_SHIFT. vm_map_ram() then allocates and maps only the
wrapped byte size, but returns a non-NULL address to its caller.

For example, with 4 KiB pages, a count of 1048577 wraps to PAGE_SIZE.
vm_map_ram() therefore returns a mapping containing one page instead of
the requested 1048577 pages.

If a caller uses the successful result as a mapping of all requested
pages, it can access beyond the allocated mapping. Such an access can
fault. If the accessed virtual address is occupied by another vmalloc
mapping, it can instead access and corrupt unrelated kernel memory. In a
userspace-reachable call path, this can result in a failed operation, a
kernel oops or panic, or the effects of corrupted kernel data.

Return NULL before allocating or mapping anything when count * PAGE_SIZE
cannot be represented in an unsigned long.

Fixes: db64fe02258f ("mm: rewrite vmap layer")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>
---
Changes in v2:
  - Use check_mul_overflow() to validate the page-count-to-byte conversion.
  - Describe the short-mapping failure mode and its worst-case effects.

Link to v1: https://lore.kernel.org/r/20261022-vmalloc-reject-overflow-v1-1-5fde0042ffb9@xxxxxxxxxx
---
 mm/vmalloc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index bea9f76ed7e74247d4ce919250c2f02dd5c36483..8a056e424a4f2c037c10f736002839014f1f57dc 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3083,10 +3083,13 @@ EXPORT_SYMBOL(vm_unmap_ram);
  */
 void *vm_map_ram(struct page **pages, unsigned int count, int node)
 {
-	unsigned long size = (unsigned long)count << PAGE_SHIFT;
+	unsigned long size;
 	unsigned long addr;
 	void *mem;
 
+	if (check_mul_overflow((unsigned long)count, PAGE_SIZE, &size))
+		return NULL;
+
 	if (likely(count <= VMAP_MAX_ALLOC)) {
 		mem = vb_alloc(size, GFP_KERNEL);
 		if (IS_ERR(mem))

---
base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
change-id: 20261022-vmalloc-reject-overflow-e532c556f3fa

Best regards,
-- 
Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>