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Don't return to parent after fork #886

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@bjorn3 bjorn3 commented Oct 21, 2024

This can cause closed files to be closed again, which will panic and is a violation of IO safety.

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bjorn3 commented Oct 21, 2024

Added a CI fix. See the commit message.

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squell commented Oct 21, 2024

Is it possible to write a regression test for this?

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bjorn3 commented Oct 21, 2024

If I recall correctly the original case where I found the panic is as follows:

  • su is missing setuid.
  • PAM is configured to unconditionally allows the user to authenticate as itself even when the program that runs PAM is not doesn't have euid set to 0.
  • The user runs su $(whoami) when not root.
  • Su considers the user authenticated and attempts to setuid and fails.

On Linux PAM will ask the user to authenticate even when the user wants to run su to become itself. On FreeBSD PAM will allow su to be used without authentication when the target is the same as the calling user. I'm not sure how to configure Linux PAM to get the FreeBSD behavior in a test.

@bjorn3 bjorn3 force-pushed the no_fork_return branch 2 times, most recently from 2b2bb1b to c9b5a80 Compare October 21, 2024 15:58
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squell commented Oct 21, 2024

It's OK to make an issue for this, so we can track the project. :)

@bjorn3 bjorn3 force-pushed the no_fork_return branch 3 times, most recently from 644e1d3 to 6ff7332 Compare October 25, 2024 07:44
@bjorn3 bjorn3 force-pushed the no_fork_return branch 2 times, most recently from f991c69 to ba7d2db Compare November 4, 2024 14:38
This can cause closed files to be closed again, which will panic and is
a violation of IO safety.
This makes it clearer that exec_monitor can only return when there is an
error.
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PAM behaviour differs between Linux/FreeBSD, causing fork-unsafety on BSD
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