* Jamie Lokier <ja...@jlokier.co.uk> [2020-05-01 17:10:24 +0100]: > Hi Matt, > > > Not really sure of a good workaround. > > You can fchmod() or fchown() the pipe descriptor, with fchown() being more > secure. > > # echo hello | (ls -lL /proc/self/fd/0; sudo -u nobody cat > /proc/self/fd/0) > prw------- 1 root root 0 May 1 17:06 /proc/self/fd/0 > cat: /proc/self/fd/0: Permission denied > > # echo hello | (chmod a+rw /proc/self/fd/0; ls -lL /proc/self/fd/0; sudo > -u nobody cat /proc/self/fd/0) > prw-rw-rw- 1 root root 0 May 1 17:05 /proc/self/fd/0 > hello > > # echo hello | (chown nobody: /proc/self/fd/0; ls -lL /proc/self/fd/0; > sudo -u nobody cat /proc/self/fd/0) > prw------- 1 nobody nogroup 0 May 1 17:06 /proc/self/fd/0 > hello > > Best wishes, > - Jamie > > Matt Johnston wrote: > > Hi Szabolcs, > > > > Ah, that's a bit nasty. I guess the difference is that OpenSSH runs the > > daemon > > as the user, while Dropbear runs as root.
isn't it better security design to drop privs as soon as possible so everything in the process that's managing a user's session runs as that user and not as root? but i guess that requires more changes than fchown on 0/1/2 fds. i can rebuild dropbear so if the fchwon is a simple patch that works for me. thanks. > > > > The procfs manpage mentions the problem. [1]http://man7.org/linux/man-pages/ > > man5/proc.5.html > > > > Note that for file descriptors referring to inodes (pipes and > > sockets, see above), those inodes still have permission bits > > and ownership information distinct from those of the > > /proc/[pid]/fd entry, and that the owner may differ from the > > user and group IDs of the process. An unprivileged process > > may lack permissions to open them, as in this example: > > > > $ echo test | sudo -u nobody cat > > test > > $ echo test | sudo -u nobody cat /proc/self/fd/0 > > cat: /proc/self/fd/0: Permission denied > > > > Not really sure of a good workaround.
