A very important part of pledge's design is that a parent can be more restricted than its child. This isn't a sandboxing mechanism. It's intended to mitigate the dangers of some other vulnerability leading to remote code execution. With pledge, even if you get into the system you may not be able to make all the syscalls you need.
ISTM this will just slightly raise the bar so that attackers who get code execution have to force a call to execve.
On the other hand, it avoids needing to worry about all the setuid issues that Linux's seccomp avoids using PR_SET_NO_NEW_PRIVS.
In most cases your program doesn't need execve, so it can call pledge without "exec" promise.
Sometimes you will still be able to open some shell script and add your commands there or something like this, but without "wpath" promise it is impossible.
In fact, most programs will not need the exec promise in which case any attempt to call execv(3)..
Abort trap (core dumped)
If you think about the semantics of execv(3), it needs a path to an existing executable. So an attacker will need to know of one in advance, or write one out to disk beforehand.. oh but--
Abort trap (core dumped)
The exploited process pledged "stdio rpath proc exec" and cannot write to arbitrary files. Hmm. Bummer.
/bin/sh, perhaps? It's harder to pass in machine code, but I bet there's a way. /bin/sh -c "echo -e shellcode_here >/tmp/foo; chmod 700 /tmp/foo; /tmp/foo" seems plausible to me.
If the program did not pledge 'exec', then calling execve() will cause the program to be killed. Lots of programs don't need exec, so don't pledge it.
Source? I didn't see that in the man page.
> exec: Allows a process to call execve(2). Coupled with the proc promise, this allows a process to fork and execute another program. The new program starts running without pledge active and hopefully makes a new pledge().
$ cat testpledge.c
#include <unistd.h>
#include <stdio.h>
int main()
{
pledge("proc exec", NULL);
execl("/bin/echo", "echo", "asdf", NULL);
_exit(0);
}
$ cc testpledge.c
$ ./a.out
asdf