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Date: Mon, 22 Aug 2011 14:32:04 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: fd 0-2 on SUID/SGID program startup
On Mon, Aug 22, 2011 at 09:07:54PM +0400, Solar Designer wrote:
> Rich,
>
> As you're probably aware, glibc makes sure that fd 0-2 are open on
> SUID/SGID program startup (opening them to /dev/null / /dev/full if
> they're not already open). This is needed to prevent misdirected
> reads/writes by programs that use those well-known fd's (in fact, even
> libc itself does) yet also open other files/sockets/whatever (so it may
> get opened on one of these special fd's if they're not already taken).
>
> I think musl must have the same countermeasure. I think it lacks it
> currently.
>
> Do you agree?
Indeed, this is useful, and POSIX explicitly allows that fd 0-2 might
be automatically opened for suid programs. I have an efficient test
using a single syscall:
struct pollfd pfd[3] = { { .fd = 0 }, { .fd = 1 }, { .fd = 2 } };
poll(pfd, 3, 0);
Then check each of pfd[0..2].revents for POLLNVAL:
for (i=0; i<3; i++)
if ((pfd[i].revents&POLLNVAL) && open("/dev/null", O_RDWR)<0)
*(volatile char *)0=0;
I assume crashing is the best action on failure to open, but I'd
welcome other ideas... perhaps raising SIGKILL? Plain _exit seems like
a really bad idea as it could be misinterpreted by the parent as a
normal exit.
Rich
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