Re: /proc filesystem allows bypassing directory permissions on Linux

[Marco Verschuur <marco@xxxxxx>]

My buy.. :-( I persumed a re-use of the read-only FD, but that's not  
the case.

I replayed it on a test-box and did some strace meanwhile and also  
took a look
at the sourcecode of kernel/fs/proc.

It seems that the /proc filedescriptor is directly referring the file  
inode
When creating this proc-entry the user guest did have access to the  
file and the path via tmp,
therefore a successfull filedescriptor straight to the file inode is  
being created, while checking
th entire path towards the file.

Although closing the path to the file, the actual file is made world  
writable due to the file permissions being 666.
When guest does the "echo got you > /proc/self/fd/3" the /proc  
filedescriptor (which directly refers the file inode)
is opened in O_WRONLY. So user guest is able to write the file.

IMHO; no bug or security issue, just a misunderstanding of the  
mechanism...

Best regards,
        Marco



On 27 okt 2009, at 13:56, psz@xxxxxxxxxxxxxxxxx wrote:

> Marco Verschuur <marco@xxxxxx> wrote:
>
> > And due to the actual file permissions the read-only fd can easily
> > changed to read-write.
>
> How would you do that? Cannot use fcntl() as that would not let you.
>
> Cheers, Paul
>
> Paul Szabo   psz@xxxxxxxxxxxxxxxxx   http://www.maths.usyd.edu.au/u/psz/
> School of Mathematics and Statistics   University of Sydney     
> Australia