On Fri, 17 Apr 2015 12:48:44 +0200
David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Where's the problem? Just pass the read-only file-descriptor to your
> peers and make sure the access-mode of the memfd is 0600. No other
> user will be able to gain a writable file-descriptor, but you.
I see what you mean now, This does make sense. I started writing a test
and it seems like the write on a duplicated O_RDONLY fd does not fail
properly, and is causing a general protection error. Here is the output
and test code:
memfd: a dup test
expected EPERM on write(), but got 4: Operation not permitted
back in main thread
[ 8.563759] traps: memfd_test[548] general protection ip:b75b638c sp:bffdbbe0 error:0 in libc-2.20.so[b7589000+1ae000]
bash-4.3#
note that the return value 4 indicates successful write.
static void test_dup()
{
pid_t pid;
int status;
int fd_seal;
int fd_rdonly = 99;
fd_seal = mfd_assert_new("kern_memfd_seal_write",
MFD_DEF_SIZE,
MFD_CLOEXEC | MFD_ALLOW_SEALING);
fd_rdonly = dup3(fd_seal, fd_rdonly, O_RDONLY);
mfd_assert_add_seals(fd_seal, F_SEAL_SEAL);
if (fd_rdonly != 99) {
printf("dup3 error: %m\n");
abort();
}
pid = fork();
if (pid == 0)
{
int fd_peer = 97;
/*mfd_fail_write(fd_seal);*/
/* this does not fail properly? */
mfd_fail_write(fd_rdonly);
/* this will fail with, invalid argument */
/*fd_peer = dup3(fd_rdonly, fd_peer, O_RDWR);
if (fd_peer == -1) {
printf("dup3 error: %m\n");
abort();
}
mfd_fail_write(fd_peer);*/
printf("exiting normally\n");
exit(0);
}
usleep(100000);
printf("back in main thread\n");
mfd_assert_write(fd_seal);
/*mfd_fail_write(fd_rdonly);*/
usleep(1000000);
/* this seems to trigger general protection crash */
pid = waitpid(pid, &status, 0);
if (!WIFEXITED(status))
abort();
}
I don't have time right now to dig deep into this, but will look into it more
in the next few days, and report back.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>