[Bug 3430] 64 bit time and seccomp conflict

bugzilla-daemon at mindrot.org bugzilla-daemon at mindrot.org
Mon May 9 17:07:50 AEST 2022


--- Comment #4 from Lacky <alacki93 at gmail.com> ---
(In reply to Darren Tucker from comment #1)

> If you build an sshd against and older set of headers that does not
> define __NR_clock_gettime64 then it will not be included, but that's
> because sshd has no way of knowing about it or what the syscall
> number is at compile time.  I could imagine this biting you if you
> installed a new libc.so without recompiling sshd with the new
> headers.

Yes, but glibc doesn’t check it. They define syscalls in source code

Now what happens on described test environment. Every time when sshd
tries to use clock_gettime from glibc you can find in strace something
like this: 

4628  clock_gettime64(CLOCK_BOOTTIME, 0xbe8c1488) = -1 ENOSYS (Function
not implemented) 
4628  clock_gettime(CLOCK_BOOTTIME, {tv_sec=247, tv_nsec=653301168}) =

Glibc first tries to use clock_gettime for 64 bit time. This fails
because kernel 4.14 doesn’t have support for this syscall (ENOSYS).
Then glibc tries to use classic clock_gettime. Usually this is not a
problem, but sshd uses seccomp. As you said sshd doesn’t add
clock_gettime64 as an allowed syscall to seccomp because it cannot find
it in kernel headers. Now let’s look at strace log. This entry means
that seccomp was initialized successfully for process with PID 4637: 

4637  prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, {len=107,
filter=0x4ee324} <unfinished ...> 
4637  <... prctl resumed>)              = 0 

After initialization of seccomp, process 4637 is killed when it tries
use unallowed clock_gettime64: 

4637  clock_gettime64(CLOCK_BOOTTIME,  <unfinished ...> 
4637  <... clock_gettime64 resumed> <unfinished ...>) = ? 
4637  +++ killed by SIGSYS +++ 
4628  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=4637,
si_uid=101, si_status=SIGSYS, si_utime=4, si_stime=0} --- 
4628  kill(4637, SIGKILL) 

>From user perspective it looks like connection with server was closed
before any password prompt. 

In attachments you can find log from sshd (run with “-D –ddd") and
strace (run with “-f”). 

Temporary solution to fix this bugs are: 
-Building OpenSSH without seccomp
-Building kernel without seccomp 
-Patching glibc to remove clock_gettime64 syscall. 

Disabling seccomp is the easiest workaround, but it decreases security
of sshd.

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