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taviso's profile
Tavis Ormandy
Tavis Ormandy
Tavis Ormandy
Verified account
@taviso

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Tavis OrmandyVerified account

@taviso

Vulnerability researcher at Google. This is a personal stream, opinions expressed are mine.

California
taviso.decsystem.org
Joined April 2008

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    1. Rich Felker‏ @RichFelker 27 Aug 2019
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      Replying to @taviso

      Does it abort or silently fail? The latter is CVE-worthy.

      2 replies 0 retweets 7 likes
    2. Tavis Ormandy‏Verified account @taviso 27 Aug 2019
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      Replying to @RichFelker

      Silently fail. The docs say no errors defined, and my mental model was just a wrapper around pushad, etc, etc. I didn't expect it to go wrong, so tracking down the bug was a nightmare 😬

      5 replies 0 retweets 15 likes
    3. Michael Crosby‏ @crosbymichael 28 Aug 2019
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      Replying to @taviso @RichFelker

      Have you traced the syscall to see if it is failing or if libc is discarding the error? The default profile *should* return EPERM for anything blocked. rt_sigprocmask is allowed but it looks like sigprocmask is still defined on 32bit and not...

      1 reply 0 retweets 2 likes
    4. Rich Felker‏ @RichFelker 28 Aug 2019
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      Replying to @crosbymichael @taviso

      sigprocmask is an operation where there's likely to be no possibility of forward progress if it fails (assuming valid usage), so it doesn't make sense for libc or anything else to be checking for error. However..

      1 reply 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker 28 Aug 2019
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      Replying to @RichFelker @crosbymichael @taviso

      If it's the old non-"rt_" sigprocmask syscall that's failing, this whole thing is likely a non-issue on Docker's side, because that's from the libc5 days and not usable (not compatible with modern sigset_t ABI). It's a glibc bug if glibc is ever making that syscall.

      1 reply 0 retweets 0 likes
    6. Tavis Ormandy‏Verified account @taviso 28 Aug 2019
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      Replying to @RichFelker @crosbymichael

      It's the old sigprocmask, but I disagree it's okay to break getcontext(). I guess I need to detect docker and use my own implementation, even if glibc get's updated nobody is going to have the patch for years 😠 https://github.com/bminor/glibc/blob/master/sysdeps/unix/sysv/linux/i386/getcontext.S#L70 …

      2 replies 0 retweets 4 likes
    7. Rich Felker‏ @RichFelker 28 Aug 2019
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      Replying to @taviso @crosbymichael

      OK, fair enough. But if glibc's getcontext() is wrongly calling the old sigprocmask syscall, it can't be working right to begin with.

      1 reply 0 retweets 1 like
    8. Rich Felker‏ @RichFelker 28 Aug 2019
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      Replying to @RichFelker @taviso @crosbymichael

      I just checked the source and indeed it is. This means it's leaving uninitialized junk in the upper 32 bits of the sigset_t, and this junk is possibly getting restored over the caller's signal mask later. Need to open a glibc bug for it.

      2 replies 0 retweets 5 likes
    9. Tavis Ormandy‏Verified account @taviso 28 Aug 2019
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      Replying to @RichFelker @crosbymichael

      I feel dirty just considering this, but it seems like it does all the work before the syscall.... so I can just ignore the return code and manage my own signal mask if I detect docker? 😬

      2 replies 0 retweets 4 likes
    10. Michael Crosby‏ @crosbymichael 28 Aug 2019
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      Replying to @taviso @RichFelker

      What about a patch to docker to enable sigprocmask on 32bit? Alt, if this is your own code, you can also provide a custom seccomp profile for your container.

      1 reply 0 retweets 0 likes
      Tavis Ormandy‏Verified account @taviso 28 Aug 2019
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      Replying to @crosbymichael @RichFelker

      I'm not a docker user, other people were reporting my code wasn't working and I couldn't reproduce...turns our they were using docker. If you think a patch to enable on x86 would be accepted, I'll try that.

      11:41 AM - 28 Aug 2019
      3 replies 0 retweets 0 likes
        1. Marcos Nils‏ @marcosnils 28 Aug 2019
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          Replying to @taviso @crosbymichael @RichFelker

          As a temporary fix you can tell people to grab this seccomp file (https://github.com/moby/moby/blob/master/profiles/seccomp/default.json …), add sigprocmask there and run containers with `docker run --security-opt seccomp=profile.json`

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        1. Michael Crosby‏ @crosbymichael 28 Aug 2019
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          Replying to @taviso @RichFelker

          I think it would be accepted, I'll LGTM it. I don't think there are any adverse issues by allowing this on x86.

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        1. Rich Felker‏ @RichFelker 28 Aug 2019
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          Replying to @taviso @crosbymichael

          Docker should be fixed. While glibc is wrong, the syscall shouldn't be failing, especially if current glibc is (even wrongly) using it.

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