[PATCH] mm/execmem: only free ROX cache vm_struct when fully unused

[Arnav Kansal <rnv@google.com>]

execmem_cache_clean() currently uses:

    IS_ALIGNED(size, PMD_SIZE) && IS_ALIGNED(mas.index, PMD_SIZE)

as a proxy to determine whether a free range in execmem_cache.free_areas
spans an entire unused vm_struct that can be returned to vmalloc.

This heuristic is broken in both directions:

1. Premature vfree() of multi-PMD vm_structs (Use-After-Free):
   When execmem_cache_populate_alloc() services a request larger than
   PMD_SIZE (for example, a module with > 6 MiB of .text),
   alloc_size = round_up(size, PMD_SIZE) creates a single multi-PMD
   vm_struct (e.g., 8 MiB) whose remaining tail space is handed out to
   subsequent module allocations (e.g., kvm.ko). When a module inside
   that vm_struct unloads and leaves a PMD_SIZE-aligned free sub-range
   (such as a 2 MiB or 4 MiB chunk at the start of the 8 MiB vm_struct),
   execmem_cache_clean() passes the IS_ALIGNED() check and calls
   vfree(area), which unmaps and frees the entire enclosing multi-PMD
   vm_struct while sibling modules are still live:

     BUG: unable to handle page fault for address: ffffffffc07dc030
     #PF: supervisor instruction fetch in kernel mode
     #PF: error_code(0x0010) - not-present page
     PGD 2c33067 P4D 2c33067 PUD 2c35067 PMD 0
     Oops: Oops: 0010 [#1] SMP NOPTI
     RIP: 0010:dummy_exit+0x0/0xfd0 [tailm]

2. Failure to reclaim fallback vm_structs:
   After commit 3bd4e0ac61b2 ("execmem: add fallback for failures in
   vmalloc(VM_ALLOW_HUGE_VMAP)") and commit 4afb85f5e359 ("mm: remove
   PMD alignment constraint in execmem_vmalloc()"), ROX cache vm_structs
   allocated via the base-page or exact-size fallback paths are only
   PAGE_SIZE-aligned in address and/or size, so they never satisfy the
   PMD_SIZE alignment check even when completely unused.

Because vmalloc allocations include a trailing guard page, separate
vm_struct allocations can never coalesce into a single contiguous entry
in execmem_cache.free_areas. Replace the PMD_SIZE alignment check with
an explicit check that the free range starts at vm->addr and spans the
entire get_vm_area_size(vm).

Fixes: 2e45474ab14f ("execmem: add support for cache of large ROX pages")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Co-developed-by: Rohan Kumar <rohakum@google.com>
Signed-off-by: Rohan Kumar <rohakum@google.com>
Signed-off-by: Arnav Kansal <rnv@google.com>
---
 mm/execmem.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/execmem.c b/mm/execmem.c
index 74a178a87e75..3a1da0498fe2 100644
--- a/mm/execmem.c
+++ b/mm/execmem.c
@@ -166,11 +166,9 @@ static void execmem_cache_clean(struct work_struct *work)
 	mutex_lock(mutex);
 	mas_for_each(&mas, area, ULONG_MAX) {
 		size_t size = mas_range_len(&mas);
+		struct vm_struct *vm = find_vm_area(area);
 
-		if (IS_ALIGNED(size, PMD_SIZE) &&
-		    IS_ALIGNED(mas.index, PMD_SIZE)) {
-			struct vm_struct *vm = find_vm_area(area);
-
+		if (vm->addr == area && get_vm_area_size(vm) == size) {
 			execmem_set_direct_map_valid(vm, true);
 			mas_store_gfp(&mas, NULL, GFP_KERNEL);
 			vfree(area);
-- 
2.47.0