Change vfork() to posix_spawn()?
Hello hackers, I'm looking through the Clang Analyzer scans on http://scan.freebsd.your.org/freebsd-head looking for false positives to report back to LLVM. There are quite a list of reports suggesting to change vfork() calls to posix_spawn(). Example from /bin/rpc: http://scan.freebsd.your.org/freebsd-head/bin.rcp/2012-09-12-amd64/report-nsOV80.html#EndPath I know nothing about this but I can see fork and posix_spawn have been discussed on this list previously. Is this a legitimate warning (in this case and in general in FreeBSD base)? Thanks, Erik___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: Change vfork() to posix_spawn()?
On 14/09/2012 09:49, Erik Cederstrand wrote: Hello hackers, I'm looking through the Clang Analyzer scans on http://scan.freebsd.your.org/freebsd-head looking for false positives to report back to LLVM. There are quite a list of reports suggesting to change vfork() calls to posix_spawn(). Example from /bin/rpc: http://scan.freebsd.your.org/freebsd-head/bin.rcp/2012-09-12-amd64/report-nsOV80.html#EndPath I know nothing about this but I can see fork and posix_spawn have been discussed on this list previously. Is this a legitimate warning (in this case and in general in FreeBSD base)? Currently (on 9-stable at least), posix_spawn() is implemented as a wrapper around vfork(), so I doubt replacing one with the other would do much. signature.asc Description: OpenPGP digital signature
Re: Change vfork() to posix_spawn()?
Den 14/09/2012 kl. 13.03 skrev Ivan Voras ivo...@freebsd.org: On 14/09/2012 09:49, Erik Cederstrand wrote: Hello hackers, I'm looking through the Clang Analyzer scans on http://scan.freebsd.your.org/freebsd-head looking for false positives to report back to LLVM. There are quite a list of reports suggesting to change vfork() calls to posix_spawn(). Example from /bin/rpc: http://scan.freebsd.your.org/freebsd-head/bin.rcp/2012-09-12-amd64/report-nsOV80.html#EndPath I know nothing about this but I can see fork and posix_spawn have been discussed on this list previously. Is this a legitimate warning (in this case and in general in FreeBSD base)? Currently (on 9-stable at least), posix_spawn() is implemented as a wrapper around vfork(), so I doubt replacing one with the other would do much. The analyzer added this warning in January. The release notes link to this explanation: https://www.securecoding.cert.org/confluence/display/seccode/POS33-C.+Do+not+use+vfork() I guess this is the important part: Because of the implementation of the vfork() function, the parent process is suspended while the child process executes. If a user sends a signal to the child process, delaying its execution, the parent process (which is privileged) is also blocked. This means that an unprivileged process can cause a privileged process to halt, which is a privilege inversion resulting in a denial of service. Erik___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: Change vfork() to posix_spawn()?
On Fri, Sep 14, 2012 at 4:45 AM, Erik Cederstrand e...@cederstrand.dk wrote: Den 14/09/2012 kl. 13.03 skrev Ivan Voras ivo...@freebsd.org: On 14/09/2012 09:49, Erik Cederstrand wrote: Hello hackers, I'm looking through the Clang Analyzer scans on http://scan.freebsd.your.org/freebsd-head looking for false positives to report back to LLVM. There are quite a list of reports suggesting to change vfork() calls to posix_spawn(). Example from /bin/rpc: http://scan.freebsd.your.org/freebsd-head/bin.rcp/2012-09-12-amd64/report-nsOV80.html#EndPath I know nothing about this but I can see fork and posix_spawn have been discussed on this list previously. Is this a legitimate warning (in this case and in general in FreeBSD base)? Currently (on 9-stable at least), posix_spawn() is implemented as a wrapper around vfork(), so I doubt replacing one with the other would do much. The analyzer added this warning in January. The release notes link to this explanation: https://www.securecoding.cert.org/confluence/display/seccode/POS33-C.+Do+not+use+vfork() I guess this is the important part: Because of the implementation of the vfork() function, the parent process is suspended while the child process executes. If a user sends a signal to the child process, delaying its execution, the parent process (which is privileged) is also blocked. This means that an unprivileged process can cause a privileged process to halt, which is a privilege inversion resulting in a denial of service. Isn't the important part the previous paragraph, which said that some older versions of Linux had the problem? The entire thing reads that the issue comes from an idiom of vfork(), setuid(), then exec, which is both undefined and would be specific to only some *uses* of vfork(), not the implementation. The whole thing isn't worded terribly usefully; e.g. it doesn't explain if it was only Linux that had an issue, which version of Linux is fixed, whether normal code that went straight from vfork() to exec() was fine, etc. Cheers, matthew ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: Change vfork() to posix_spawn()?
On Fri, Sep 14, 2012 at 01:45:49PM +0200, Erik Cederstrand wrote: Den 14/09/2012 kl. 13.03 skrev Ivan Voras ivo...@freebsd.org: On 14/09/2012 09:49, Erik Cederstrand wrote: I'm looking through the Clang Analyzer scans on http://scan.freebsd.your.org/freebsd-head looking for false positives to report back to LLVM. There are quite a list of reports suggesting to change vfork() calls to posix_spawn(). Example from /bin/rpc: http://scan.freebsd.your.org/freebsd-head/bin.rcp/2012-09-12-amd64/report-nsOV80.html#EndPath I know nothing about this but I can see fork and posix_spawn have been discussed on this list previously. Is this a legitimate warning (in this case and in general in FreeBSD base)? Currently (on 9-stable at least), posix_spawn() is implemented as a wrapper around vfork(), so I doubt replacing one with the other would do much. vfork() returns twice, possibly reanimating variables from the dead. Calling posix_spawn() limits this issue to the posix_spawn() implementation only. For example, in case of unwilling compiler developers, the optimization level for that file might be lowered or more volatile keywords might be added. I think it makes more sense to disable various optimizations in the compiler automatically in functions that call vfork(), longjmp() and similar functions, but I do not decide what compiler developers do. The analyzer added this warning in January. The release notes link to this explanation: https://www.securecoding.cert.org/confluence/display/seccode/POS33-C.+Do+not+use+vfork() I guess this is the important part: Because of the implementation of the vfork() function, the parent process is suspended while the child process executes. If a user sends a signal to the child process, delaying its execution, the parent process (which is privileged) is also blocked. This means that an unprivileged process can cause a privileged process to halt, which is a privilege inversion resulting in a denial of service. This problem only occurs if privileges are dropped between vfork and exec, which is uncommon. If no privileges are dropped, the user can affect the parent directly. Furthermore, this exact problem does not happen in FreeBSD because child processes between vfork and exec/exit are not affected by stop signals (this is stronger than the vfork(2) man page documents). However, related issues are still present. If there is a signal handler that blocks for a long time (many functions which do this are async-signal-safe) for a signal permitted by security.bsd.conservative_signals, an unprivileged user will be able to trigger it and delay the thread calling vfork(). A function may also be async-signal-safe but not suitable for a vforked child (for example, libthr makes many functions async-signal-safe by postponing signal handlers which is not good enough if a vforked child is SIGKILL'ed). An unprivileged user may also trigger priority inversion by lowering the priority of the child process and consuming CPU time at a higher priority. Obviously, the child process should not lower its priority voluntarily either. These problems can be fixed in various ways. The direct priority inversion problem can be fixed by using fork() instead in that case or by adding a priority inheritance scheme in the kernel for vforked children (but only for the static priority; the parent's dynamic priority will increase because it is sleeping). The privilege manipulation available via POSIX_SPAWN_RESETIDS seems safe enough. Since it only affects the effective UID/GID, it does not affect the ability to modify scheduling parameters (real UID) or to send signals (real or saved UID). Since the seteuid() call itself will set the issetugid flag to true if it changed anything, it does not affect the ability to debug before exec. More general privilege dropping frequently involves frameworks such as PAM which are not async-signal-safe and certainly not vfork-safe. -- Jilles Tjoelker ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
