[PATCH] perf/core: Fix refcount bug and potential UAF in perf_mmap

[Haocheng Yu <yuhaocheng035@gmail.com>]

Syzkaller reported a refcount_t: addition on 0; use-after-free warning
in perf_mmap.

The issue is caused by a race condition between mmap() and event
teardown. In perf_mmap(), the ring_buffer (rb) is accessed via
map_range() after the mmap_mutex is released. If another thread
closes the event or detaches the buffer during this window, the
reference count of rb can drop to zero, leading to a UAF or
refcount saturation when map_range() or subsequent logic attempts
to use it.

Fix this by extending the scope of mmap_mutex to cover the entire
setup process, including map_range(), ensuring the buffer remains
valid until the mapping is complete.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602020208.m7KIjdzW-lkp@intel.com/
Signed-off-by: Haocheng Yu <yuhaocheng035@gmail.com>
---
 kernel/events/core.c | 38 +++++++++++++++++++-------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2c35acc2722b..abefd1213582 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7167,28 +7167,28 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 			ret = perf_mmap_aux(vma, event, nr_pages);
 		if (ret)
 			return ret;
-	}
 
-	/*
-	 * Since pinned accounting is per vm we cannot allow fork() to copy our
-	 * vma.
-	 */
-	vm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP);
-	vma->vm_ops = &perf_mmap_vmops;
+		/*
+		 * Since pinned accounting is per vm we cannot allow fork() to copy our
+		 * vma.
+		 */
+		vm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP);
+		vma->vm_ops = &perf_mmap_vmops;
 
-	mapped = get_mapped(event, event_mapped);
-	if (mapped)
-		mapped(event, vma->vm_mm);
+		mapped = get_mapped(event, event_mapped);
+		if (mapped)
+			mapped(event, vma->vm_mm);
 
-	/*
-	 * Try to map it into the page table. On fail, invoke
-	 * perf_mmap_close() to undo the above, as the callsite expects
-	 * full cleanup in this case and therefore does not invoke
-	 * vmops::close().
-	 */
-	ret = map_range(event->rb, vma);
-	if (ret)
-		perf_mmap_close(vma);
+		/*
+		 * Try to map it into the page table. On fail, invoke
+		 * perf_mmap_close() to undo the above, as the callsite expects
+		 * full cleanup in this case and therefore does not invoke
+		 * vmops::close().
+		 */
+		ret = map_range(event->rb, vma);
+		if (ret)
+			perf_mmap_close(vma);
+	}
 
 	return ret;
 }

base-commit: 7d0a66e4bb9081d75c82ec4957c50034cb0ea449
-- 
2.51.0