Public bug reported:
When ssh-add is started without a controlling TTY
and with SIGTTOU/SIGTTIN set to SIG_IGN in the parent (e.g. a systemd
service unit, or a daemon that ignores job-control signals), readpassphrase()
spins with high CPU indefinitely and never prompts or exits.
Root cause: after installing its own handlers, readpassphrase() reads from
/dev/tty and receives SIGTTIN or SIGTTOU. The handler sets signo[i]=1 and
the read returns EINTR. readpassphrase then restores the original SIG_IGN
disposition and re-raises the signal via kill(getpid(), i). Because the
disposition is now SIG_IGN the kill() is a no-op, but need_restart was
already set to 1, so execution does "goto restart". The next iteration
re-opens /dev/tty, writes the prompt to stderr, hits SIGTTIN/SIGTTOU again,
and the loop repeats forever.
Fix: before setting need_restart for SIGTSTP/SIGTTIN/SIGTTOU, check
whether the saved (original) handler was SIG_IGN. If it was, skip the
restart -- readpassphrase falls through and returns NULL, letting the
caller handle the error.
See AI-aided patch attached.
** Affects: openssh (Ubuntu)
Importance: Undecided
Status: New
** Patch added: "Fix: before setting need_restart for SIGTSTP/SIGTTIN/SIGTTOU,
check whether the saved (original) handler was SIG_IGN. If it was, skip the
restart -- readpassphrase falls through and returns NULL, letting the caller
handle the error."
https://bugs.launchpad.net/bugs/2157917/+attachment/5978457/+files/patch.patch
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https://bugs.launchpad.net/bugs/2157917
Title:
readpassphrase() spins with high CPU indefinitely and never prompts or
exits.
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