per...@pluto.rain.com writes: > DarkSoul <darks...@darkbsd.org> wrote: >> Anthony Pankov wrote: >> > SGID/SUID bits don't work with shell scripts, do they? >> >> They don't. >> >> ... if they were applied, the following would occur : >> - execve() syscall reads your script's shebang line, and >> the script interpreter is executed, receiving the specified >> arguments along with the script name. >> - The interpreter then open()s the script file to read it, >> and run the code. >> >> The problem you then are faced with, is that you have a time >> frame defined by the moment between the aforementioned execve() >> and open(), during which it could be possible to unlink/move/ >> whatever the shell script the interpreter is going to open. >> >> You guess where this is going, you have no absolute way of >> guaranteeing you are executing the file you initially planned >> on opening because execution/opening/reading is not, and can't >> be done atomically for shell scripts. > > In principle, it should be possible to fix this exposure by > improving the interface between execve() and the interpreter: > > The execve() syscall already has the script file open to read the > shebang line. Leave it open, and ensure that the interpreter > receives the open descriptor as fd#3 just as 0, 1, and 2 are already > used for stdin, stdout, and stderr. An interpreter supporting this > approach would check whether stdscr (fd#3) is already open, and if > so read from it instead of open()ing the script file. This should > ensure that the script which gets executed is the same inode on > which execve() saw the SGID/SUID bits set, even if the filesystem > has been changed by the time the interpreter has gotten started. > It would be the responsibility of whomever decided to set the > SGID/SUID bits on a particular script to ensure that the interpreter > involved supports the mechanism. > > I vaguely recall having seen a similar (or even identical) approach > suggested some years ago. It may even have been implemented in some > variant of Un*x.
That's clever, but how would it work in practice, while common shells and scripting languages may not implement their side of it? _______________________________________________ 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"
