Intercept /proc/sys/kernel/random/uuid

[sporksmith]

From proc(2):

       /proc/sys/kernel/random/uuid (since Linux 2.4)
              Each read from this read-only file returns a randomly generated 128-bit UUID, as a string in the standard UUID format.

I noticed the tor unit tests read this file; not sure whether tor reads it in production.

[sporksmith]

It looks like the tor processes in the tor-minimal test do access this during initialization, though I'm not sure yet what it's used for.

After enabling strace logging in the tor minimal test:

$ grep random/uuid build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/*/tor.*.strace 
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/4uthority/tor.1000.strace:00:00:01.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/4uthority/tor.1000.strace:00:00:01.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/4uthority/tor.1000.strace:00:00:01.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/4uthority/tor.1000.strace:00:00:01.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/4uthority/tor.1000.strace:00:00:01.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 12
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/4uthority/tor.1000.strace:00:00:01.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 12
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/exit2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/hiddenserver/tor.1001.strace:00:15:00.000000000 [tid 1001] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/hiddenserver/tor.1001.strace:00:15:00.000000000 [tid 1001] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/hiddenserver/tor.1001.strace:00:15:00.000000000 [tid 1001] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/hiddenserver/tor.1001.strace:00:15:00.000000000 [tid 1001] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/ptbridge/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/ptbridge/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/ptbridge/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/ptbridge/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/ptbridge/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 13
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/ptbridge/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 13
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay1/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay2/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay3/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay3/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay3/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay3/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay3/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay3/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay4/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay4/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay4/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay4/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay4/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/relay4/tor.1000.strace:00:01:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 11
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torhiddenclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torhiddenclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torhiddenclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torhiddenclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torptbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torptbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torptbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3
build/src/test/tor/minimal/tor-minimal-shadow.data/hosts/torptbridgeclient/tor.1000.strace:00:15:00.000000000 [tid 1000] open("/proc/sys/kernel/random/uuid", O_CLOEXEC, Mode(0x0)) = 3

[sporksmith]

Using strace -k on a native execution of tor, it looks like this is being used by libevent to initialize its RNG.

openat(AT_FDCWD, "/proc/sys/kernel/random/uuid", O_RDONLY|O_CLOEXEC) = 3
 > /usr/lib/x86_64-linux-gnu/libc.so.6(__open64+0x5b) [0x1146eb]
 > /usr/lib/x86_64-linux-gnu/libevent-2.1.so.7.0.1(evutil_open_closeonexec_+0x25) [0x1a575]
 > /usr/lib/x86_64-linux-gnu/libevent-2.1.so.7.0.1(evutil_make_internal_pipe_+0x5b0) [0x27420]
 > /usr/lib/x86_64-linux-gnu/libevent-2.1.so.7.0.1(evutil_secure_rng_init+0x23) [0x275b3]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(tor_init_libevent_rng+0x28) [0xe97f8]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(subsys_evloop_initialize+0x1d) [0xe9a1d]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(subsystems_init_upto+0x83) [0xbf7a3]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(tor_run_main+0x47) [0x69a67]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(tor_main+0x4d) [0x6615d]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(main+0x1d) [0x65d0d]
 > /usr/lib/x86_64-linux-gnu/libc.so.6(__libc_init_first+0x90) [0x29d90]
 > /usr/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x29e40]
 > /home/jnewsome/opt/tor-0.4.7.9/bin/tor(_start+0x25) [0x65d65]

[robgjansen]

Oh boy, that seems bad! Could be the last rng leak causing some non-determinism I've anecdotally noticed.

[sporksmith]

Yup, definitely seems worth fixing :)

It's a little more complex than e.g. /dev/random since it should produce valid UUID strings rather than just random bytes. i.e. probably needs its own FileType in regular_file.c

[sporksmith]

i.e. probably needs its own FileType

Actually maybe FILE_TYPE_IN_MEMORY would work, with a new file being created for each open operation on this path.

That just leaves generating valid UUID strings using the host's RNG

[stevenengler]

Actually maybe FILE_TYPE_IN_MEMORY would work, with a new file being created for each open operation on this path.

Might also be good to check if the random uuid is changed/regenerated when you seek to the start of the file, since some other files in /dev will sometimes do this and applications might rely on it if it does.