Susctl CVE-2024-54507: A particularly 'sus' sysctl in the XNU kernel

148 points by jprx 5 months ago | 41 comments
  • kwitaszczyk 5 months ago
    This is a nice and easy to understand example of a memory-safety bug that CHERI [1] prevents (among other classes of vulnerabilities). Given that the SYSCTL_PROC() macro installs a pointer to an uint16_t value in the oid_arg1 field, a CHERI pure-capability kernel would construct a capability with bounds set to sizeof(uint16_t) and later the dereference of (int *)oidp->oid_arg1 in sysctl_udp_log_port() would trigger a capability bounds violation.

    `sysctl -a` would simply crash on CHERI allowing a developer to catch this without KASAN being involved.

    [1] http://cheri-cpu.org

    • pjmlp 5 months ago
      Not only, SPARC ADI and ARM MTE as well.
    • nixpulvis 5 months ago
      Seems like something to be integration tested in the future. Honestly surprised this slipped through.
      • 0x457 5 months ago
        I saw that types are different, but I was thinking "must be some weird C thing that I don't know about"
        • StressedDev 5 months ago
          I am not surprised. First, it's a subtle bug. Second, in C/C++. a lot of times you get unlucky when reading uninitialized memory. Basically, the bug does not occur when you test the code on your machine or when you run the automated tests.

          Another problem is writing good automated tests is hard and often skipped. Lots of software engineering teams talk about the wonders of automated tests. Unfortunately, many automated tests are not very good and either do not ensure the major functionality works or just do not test some of the code. There are also limits to how much time a software engineer has to test. No one can test everything.

          Basically, I am not surprised developers make mistakes and I am not surprised the tests either did not catch this mistake or even did not exist. Software is very hard and software engineers are far from perfect.

          • saagarjha 5 months ago
            Right, but this was caught basically instantly by Asan.
            • hulitu 5 months ago
              He had time to test it. /s
        • nxobject 5 months ago
          Did you get a bounty payout for this? I got the impression that Apple wasn't particularly on the ball with those.
          • ChocolateGod 5 months ago
            Is it even exploitable in the real world?

            Correct me if I'm wrong but you get 2 bytes of kernel data (potentially blank padding) and the same two bytes each time?

            • bean-weevil 5 months ago
              If the linker puts a pointer there, this would let you leak part of the pointer which could let you bypass kaslr. Not too likely for that to occur. If I were submitting this bug I would feel complete if they bought me a sandwich.
              • axoltl 5 months ago
                The bottom 2 bytes of a pointer contain two bits of the slide, assuming it's even a pointer into the kernelcache itself.

                I'd take half a sandwich.

              • buzzergfxkjkl 5 months ago
                Good to find the bug regardless! I appreciated the succinct and not overly dramatic write-up. I don't think anything significant was claimed other than the fact that it is a kernel bug (which is significant in itself don't get me wrong).
                • duskwuff 5 months ago
                  You are correct. It's clearly a bug, but the impact in shipping kernels appears to be limited to "leaking" some non-sensitive data.
                • 5 months ago
                • inetknght 5 months ago

                      -   int new_value = *(int *)oidp->oid_arg1;
                      +   int new_value = *(uint16_t *)oidp->oid_arg1;
                  
                  Why not just have `uint16_t new_value = ...`?

                  Ahh, because `new_value` is being given to `sysctl_handle_int(..., &new_value, ...);` which of course expects an `int`. So then it begs the question: if the value is really a `uint16_t`, then why is it being handled through a plain `int`? It smells like there could easily be tons of other memory-safety and/or type confusion problems endemic to the sysctl API.

                  • aritashion 5 months ago
                    > So then it begs the question: if the value is really a `uint16_t`, then why is it being handled through a plain `int`?

                    I don't think it begs the question, but it does raise one!

                    • cbarrick 5 months ago
                      Nit: "begs the question" means "raises the question" in many contemporary colloquial contexts. It can _also_ refer to a type of logical fallacy in philosophical contexts.

                      The phrase can be confusing because of its overloaded definitions, so it's best to avoid it. But if you understood what someone meant when they used it, then... you understood it's meaning.

                      Remember to treat the study of language descriptively rather than prescriptively!

                      https://en.wiktionary.org/wiki/beg_the_question

                      • bmacho 5 months ago
                      • kccqzy 5 months ago
                        Well there's the so-called usual arithmetic conversions that will basically convert every uint16_t to an unsigned int. The C and C++ languages do a silent conversion on your back anyways so you might as well make it explicit.
                        • loeg 5 months ago
                          Usually promotions are to signed int, not unsigned. (With some exceptions. But every uint16_t value can fit in int.)
                          • Filligree 5 months ago
                            Unless int is 16-bit. Code like this is potentially UB; you should use int32_t as the target.
                          • inetknght 5 months ago
                            A well-configured C++ compiler will error-out on such a silent conversion.
                      • soheil 5 months ago
                        Leaking two random bytes and in some cases just padding bytes to user space is not the end of the world and I don't get why there are so many negative comments blaming Apple for not handing out a handsome bounty for this bug.
                        • StressedDev 5 months ago
                          It's still a security bug. Often, multiple bugs like this are chained together to create one very nasty exploit. I agree that this bug probably does not deserve a massive payout, but I think $3,000-5,000$ is appropriate.
                          • loeg 5 months ago
                            You're joking. This is a $10 bug.
                            • dizhn 5 months ago
                              Dammit I gave a kid $20 in amazon books credit for "your site has a phpinfo() page".
                          • 5 months ago
                          • rvz 5 months ago
                            [flagged]
                            • wilg 5 months ago
                              Is there any reason to assume a conspiracy and drama around the bounty here other that just being bored and cynical? Apple has a well known security bounty program https://security.apple.com/bounty/
                              • rvz 5 months ago
                                So the researcher didn't get a bounty from Apple then, no?
                                • wilg 5 months ago
                                  we don't know
                              • zx8080 5 months ago
                                It's not possible to apply human morale and principles to companies. It just does not work that way.

                                Why would they pay if no profit if pay and they are not forced to pay?

                                • readyplayernull 5 months ago
                                  Do not fall into the trap of anthropomorphising Apple
                                  • emptiestplace 5 months ago
                                    To inspire more of the same.
                                    • bramhaag 5 months ago
                                      Ideally Apple would incentivise people reporting security issues to them, instead of risking people selling them to someone else who pays more.
                                      • Kenji 5 months ago
                                        [dead]
                                      • 5 months ago
                                      • yapyap 5 months ago
                                        I appreciate the among us.
                                        • jfasi 5 months ago
                                          Interestingly, ChatGPT correctly points out this exact issue:

                                          https://chatgpt.com/share/6793a2d1-5f84-8006-8e78-16be4d4908...