Description HLjksuga 2024-05-30 19:36:00 UTC

Description of problem:
tcp: Fix shift-out-of-bounds in dctcp_update_alpha().

In dctcp_update_alpha(), we use a module parameter dctcp_shift_g
    as follows:

      alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
      ...
      delivered_ce <<= (10 - dctcp_shift_g);

    It seems syzkaller started fuzzing module parameters and triggered
    shift-out-of-bounds [0] by setting 100 to dctcp_shift_g:

      memcpy((void*)0x20000080,
             "/sys/module/tcp_dctcp/parameters/dctcp_shift_g\000", 47);
      res = syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0x20000080ul,
                    /*flags=*/2ul, /*mode=*/0ul);
      memcpy((void*)0x20000000, "100\000", 4);
      syscall(__NR_write, /*fd=*/r[0], /*val=*/0x20000000ul, /*len=*/4ul);

    Let's limit the max value of dctcp_shift_g by param_set_uint_minmax().

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