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    SubjectRe: symlinks with permissions
    On Mon, Oct 26, 2009 at 06:46:31PM +0100, Jan Kara wrote:
    > On Mon 26-10-09 13:36:29, J. Bruce Fields wrote:
    > > On Mon, Oct 26, 2009 at 11:57:29AM -0500, Serge E. Hallyn wrote:
    > > > Quoting Jan Kara (jack@suse.cz):
    > > > > Hi,
    > > > >
    > > > > On Sun 25-10-09 07:29:53, Pavel Machek wrote:
    > > > > > ...yes, they do exist, in /proc/self/fd/* . Unfortunately, their
    > > > > > permissions are not actually checked during open, resulting in
    > > > > > (obscure) security hole: if you have fd open for reading, you can
    > > > > > reopen it for write, even through unix permissions would not allow
    > > > > > that.
    > > > > >
    > > > > > Now... I'd like to close the hole. One way would be to actually check
    > > > > > symlink permissions on open -- because those symlinks already have
    > > > > > correct permissions.
    > > > > Hmm, I'm not sure I understand the problem. Symlink is just a file
    > > > > containing a path. So if you try to open a symlink, you will actually open
    > > > > a file to which the path points. So what security problem is here? Either
    > > > > you can open the file symlink points to for writing or you cannot...
    > > > > Anyway, if you want to play with this,
    > > > > fs/proc/base.c:proc_pid_follow_link
    > > > > is probably the function you are interested in.
    > > >
    > > > The problem he's trying to address is that users may try to protect
    > > > a file by doing chmod 700 on the parent dir, but leave the file itself
    > > > accessible. They don't realize that merely having a task with an open
    > > > fd to that file gives other users another path to the file.
    > > >
    > > > Whether or not that's actually a problem is open to debate, but I think
    > > > he's right that many users aren't aware of it.
    > >
    > > If /proc/self/fd/23 is a symlink to /home/me/privatedir/secret, then an
    > > open("proc/self/fd/23",...) still traverses the whole /home/.../secret
    > > path, and needs appropriate permissions at each step, doesn't it?
    > >
    > > Probably I'm just terminally confused....
    > That's what I'd think as well but it does not as I've just learned and
    > tested :) proc_pid_follow_link actually directly gives a dentry of the
    > target file without checking permissions on the way.

    Got it, thanks.--b.


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