Re: [patch 7/8] fdmap v2 - implement sys_socket2
From: Kyle Moffett
Date: Sat Jun 09 2007 - 15:28:40 EST
On Jun 09, 2007, at 13:24:29, Al Viro wrote:
On Sat, Jun 09, 2007 at 10:08:59AM -0700, Ulrich Drepper wrote:
- - there are two interface to use: open + fcntl. This is racy.
And don't tell me this doesn't matter.
Racy with respect to what? Return-to-libc exploits from another
How about racy with respect to normal open or fork+exec from another
thread? Specifically there are cases where libc or other libraries
want to create a backend thread dealing with file descriptors in
response to the program's straightforward calls into that library
(Examples include using syslets or event-based polling threads).
Program Thread: Library Thread:
fd = socket(AF_*, SOCK_*, 0);
int x = FD_CLOEXEC;
fcntl(fd, F_SETFD, &x);
Whoops!!! Suddenly the user process executed by the (theoretically)
single-threaded program got a handle to a netlink socket affecting
some system resource!!!
Program Thread: Async libc getpwent()-cache syslet
fd = open("/etc/shadow");
Here we were trying to safely call into code which reads from stdin
and shouldn't be given privileged data, but the syslet makes the
common paradigm 'close(0); open("/dev/null");' horribly insecure.
If you extend all the FD syscalls to all take a "flags" parameter and
add the appropriate flags, then you can pass O_CLOEXEC|O_RANDFD to
whatever syscall you are using and both problems vanish.
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