diff --git a/.gitea/workflows/tsan.yaml b/.gitea/workflows/tsan.yaml
index 53f81ad..7911d7d 100644
--- a/.gitea/workflows/tsan.yaml
+++ b/.gitea/workflows/tsan.yaml
@@ -18,6 +18,15 @@ jobs:
     runs-on: linux/amd64
     container:
       image: gitea.tourolle.paris/dtourolle/kpnpp-builder:latest
+      # This runner is Docker nested in an unprivileged LXC container, whose
+      # kernel randomizes mmap addresses beyond the range TSan's fixed shadow
+      # mapping expects, so TSan aborts at init with "unexpected memory
+      # mapping". The fix is to disable ASLR per-process with `setarch -R`
+      # (below), which needs the personality(2) syscall that Docker's default
+      # seccomp profile blocks. seccomp=unconfined permits it. Verified on the
+      # runner: setarch -R alone gets EPERM, seccomp alone still aborts, both
+      # together run clean. Scoped to this job, which runs only our own tests.
+      options: --security-opt seccomp=unconfined
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
@@ -54,13 +63,14 @@ jobs:
         # the full picture for lock-order issues.
         env:
           TSAN_OPTIONS: "halt_on_error=1 second_deadlock_stack=1"
-        run: ./build/tests/kpn_tests_stress
+        # setarch -R disables ASLR for this process; see the container comment.
+        run: setarch -R ./build/tests/kpn_tests_stress
 
       - name: Run unit tests under TSan
         working-directory: tsan-${{ github.run_id }}
         env:
           TSAN_OPTIONS: "halt_on_error=1 second_deadlock_stack=1"
-        run: ./build/tests/kpn_tests
+        run: setarch -R ./build/tests/kpn_tests
 
       - name: Cleanup
         if: always()