Bug 206552

Summary: [libc] Possible buffer overflow after flushing line-buffered files when only partial data was written
Product: Base System Reporter: mccoy
Component: kernAssignee: freebsd-bugs (Nobody) <bugs>
Status: New ---    
Severity: Affects Only Me CC: op
Priority: ---    
Version: 10.2-RELEASE   
Hardware: Any   
OS: Any   
Description Flags
A test program that demonstrates the buffer overflow none

Description mccoy 2016-01-24 01:20:13 UTC
Created attachment 166034 [details]
A test program that demonstrates the buffer overflow

Please see attached file evil.c for a possible scenario where it's possible to trigger buffer overflow.

It uses a somewhat contrived example of non-blocking pipes as an underlying file descriptor, mainly because it's easy to trigger (partially) failed writes.

The defect can be located in the code /usr/src/lib/libc/stdio/fflush.c and function __sflush. Line-buffered files where write(s) has partially succeeded will have their internal write pointer increased, but not getting a corresponding write space decrease.

(so, the defect is: if fp is a FILE *, then fp->_p is increased but fp->_w is NOT decreased in this situation)

Sample output on my FREEBSD 10.2-RELEASE-p7 amd64 machine:

zsh 1311 % cc evil.c -o evil && ./evil
rc from fread(1): 1
rc from fwrite(1): 1
rc from fwrite(1021): 1021
rc from fflush: -1
rc from fwrite(1): 1
rc from fwrite(4): 4
Canary overwritten: 97 65 98 66