Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3

From: Cyrill Gorcunov
Date: Wed Mar 14 2012 - 01:47:30 EST


On Tue, Mar 13, 2012 at 06:41:06PM -0700, Matt Helsley wrote:
...
> > +
> > + exe_file = fget(fd);
> > + if (!exe_file)
> > + return -EBADF;
> > +
> > + dentry = exe_file->f_path.dentry;
> > +
> > + /*
> > + * Because the original mm->exe_file
> > + * points to executable file, make sure
> > + * this one is executable as well to not
> > + * break an overall picture.
> > + */
> > + err = -EACCES;
> > + if (!S_ISREG(dentry->d_inode->i_mode) ||
> > + exe_file->f_path.mnt->mnt_flags & MNT_NOEXEC)
> > + goto exit;
>
> You could factor out this portion of the access checking from open_exec()
> after the do_filp_open() in open_exec() and re-use it here. I know it's
> tiny helper but tying these two together might be good for
> maintenance later.
>

Matt, I really dont wanna touch code outside of prctl and this function
in particualar, at least in this patch, ie I can clean up and factor out
is on top of the patch, as a separate task.

> Should it check for some of the flags open_exec() uses? open_exec()
> passes:
>
> O_LARGEFILE|O_RDONLY|__FMODE_EXEC
>
> to do_filp_open(). I think a O_RDONLY check might be good. I don't
> think __FMODE_EXEC is something userspace can set so could be ignored.
> O_LARGEFILE might be important though.

Well, we're not going to read from this file, so it is not that important
at moment, so previously I've had

> + if ((exe_file->f_flags & O_ACCMODE) != O_RDONLY)
> + goto exit;

and Oleg pointed me

| But the O_RDONLY check looks strange. We are not going to write
| to this file, we only set the name (and that is why I think it
| should be mm->exe_path). What is the point to check that the file
| was opened without FMODE_WRITE? Even if there were any security
| risk the apllication can open this file again with the different
| flags.

so I dropped it. And I think the same applies to O_LARGEFILE. Sure
it's not a problem to bring it back but should we?

Cyrill
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