perf/aux: Fix page UAF in map_range()

map_range() reads rb->aux_pages[], rb->aux_nr_pages and rb->aux_pgoff via
perf_mmap_to_page() while holding only event->mmap_mutex. Those fields are
serialized by rb->aux_mutex, and mmap_mutex is per event.

Thus, two events sharing one rb via PERF_EVENT_IOC_SET_OUTPUT can race
rb_alloc_aux() with map_range(), leading to a page-UAF scenario as follows:

  CPU 0                           CPU 1
  =====                           =====
  rb_alloc_aux()                  map_range()
  [1]: allocate rb->aux_pages[0]
  [2]: rb->aux_nr_pages++
                                  [3]: perf_mmap_to_page()
                                         returns rb->aux_pages[0]
                                  [4]: map it as VM_PFNMAP
  [5]: rb->aux_pgoff = 1

  munmap the page
  [6]: free rb->aux_pages[0]

Pages mapped as VM_PFNMAP have no refcount protection, so CPU 1 holds a
mapping to a freed physical frame.

Fix this by taking rb->aux_mutex across the page walk in map_range().

Fixes: b709eb872e ("perf: map pages in advance")
Signed-off-by: Lee Jia Jie <jiajie.lee@starlabs.sg>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>

kernel/events/core.c

@@ -7150,6 +7150,8 @@ static int map_range(struct perf_buffer *rb, struct vm_area_struct *vma)
 	int err = 0;
 	unsigned long pagenum;
 
+	guard(mutex)(&rb->aux_mutex);
+
 	/*
 	 * We map this as a VM_PFNMAP VMA.
 	 *