Bug 922

Summary: The `sudo -e` command will empty files if the temp disk is out of space
Product: Sudo Reporter: Bailey Stoner <sudo>
Component: SudoAssignee: Todd C. Miller <Todd.Miller>
Status: RESOLVED FIXED    
Severity: high    
Priority: low    
Version: 1.8.31   
Hardware: PC   
OS: Linux   

Description Bailey Stoner 2020-04-12 16:05:31 MDT 
I went to edit a system configuration using `sudo -e /etc/nixos/configuration.nix`, but
editing the configuration file failed because the disk was full. It seems that `sudo` program went
ahead and emptied the original file, and now my system is broken. I assume that this is not
expected behavior, but maybe I am mistaken. Either way, I thought that I'd log the bug in the
case that it needed fixed.
Comment 1 Todd C. Miller 2020-04-12 16:34:52 MDT 
What editor did you use and do you have SELinux enabled?  I haven't been able to reproduce the problem locally.  Sudo itself checks for write errors when creating its temporary files.  However, if the disk filled up *after* the temp files were written successfully and your editor truncated the files sudo doesn't have a way to tell that something went wrong unless the editor exits with an error.
Comment 2 Bailey Stoner 2020-04-12 16:58:14 MDT 
Wow! Thanks for the quick followup.

My editor is neovim, no SE Linux. No fancy plugins that would change any dangerous buffer writing settings or anything.

There was a daemon writing that was writing files and I didn't realize it had filled the disk, so maybe it was the second issue that you mentioned. In the end, the temp file seemed to have been written to but became empty - so it seems sudo went ahead and moved the empty file over?

I wonder if sudo could have a safety check for the case of an empty file, but maybe that's more of a nuisance than a useful thing?
Comment 3 Todd C. Miller 2020-04-12 17:01:18 MDT 
Sudo could do an extra check when the edited file is zero length but the original was not and prompt the user for what to do.  Editing a file to be zero length might not always be wrong but it is suspicious.
Comment 4 Todd C. Miller 2020-07-22 10:33:44 MDT 
Added a prompt when the resulting file will become zero bytes:
https://www.sudo.ws/repos/sudo/rev/8bfaa57d5bd4
Comment 5 Todd C. Miller 2020-09-21 08:29:51 MDT 
In sudo 1.9.3, sudoedit will now prompt the user before overwriting an existing file with one that is zero-length after editing.