SFTP subsystem and umask

[Rob C]

Hello,

I have noticed that the -u parameter to the sftp-server or internal-sftp subsystem is not working correctly.  For openssh-5.6p1 I believe that the problem lies in this code, starting at line 1414 in sftp-server.c:
----------------------------------------------------------
case 'u':
        mask = (mode_t)strtonum(optarg, 0, 0777, &errmsg);
        if (errmsg != NULL)
                fatal("Invalid umask \"%s\": %s",
                    optarg, errmsg);
        (void)umask(mask);
        break;
----------------------------------------------------------

I think that adherence to strtonum() in this instance causes unexpected results due to "mask" being set to decimal/base 10.

For example, say you had the following in sshd_config:
Subsystem       sftp    /usr/local/libexec/sftp-server -u 022

Then say you upload a file with permissions of 777 via sftp -p, you would expect the uploaded file to end up with permission of 755, right?  In this case you get a file with permissions of 751 or -rwxr-x--x.  Why?

I believe it is because decimal 22 == octal 026.  Further compounding the problem is that sftp-server doggedly insists upon accepting permissions from the sftp client before it applies umask.

To test, set the following in your sshd_config:
Subsystem       sftp    /your/path/to/sftp-server -u 18

Then kill -HUP sshd and create a file on the client with permissions of 777.  Upload the file via sftp -p and observe permissions of the file created by sftp-server. I ended up with a file with permissions of 755.  I think it's because decimal 18 == octal 022.....

Is the solution to use strtol() or strtoul()?  You would end up having to trust the user to use sane values in sshd_config, or trust that a user will run sftp-server with sane parameters.

I tested using RHEL4 and Ubuntu 10.04.

Best regards,

Rob Candland