Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Konstantin Khlebnikov]

Cyrill Gorcunov wrote:
> On Sat, Jun 16, 2012 at 01:42:23PM +0400, Konstantin Khlebnikov wrote:
> > > Side note: there is a little nit with this patch actually,
> > > because while when we do c/r we do "right things" and unmap
> > > all vm-executable mappings before we set up new exe_file. But
> > > we can't guarantee that some brave soul would not setup
> > > new exe-file just for it's own, then what we migh have
> > >
> > >   - mm::exe_file set up and points to some file, moreover num_exe_file_vmas might be>   1
> > >   - application calls for prctl_set_mm_exe_file
> > >   - set_mm_exe_file(mm, exe_file) called, and it drops num_exe_file_vmas to 0
> > >   - finally application might call for removed_exe_file_vma
> > >
> > > void removed_exe_file_vma(struct mm_struct *mm)
> > > {
> > > 	mm->num_exe_file_vmas--;
> > > 	if ((mm->num_exe_file_vmas == 0)&&   mm->exe_file) {
> > > 		fput(mm->exe_file);
> > > 		mm->exe_file = NULL;
> > > 	}
> > >
> > > }
> > >
> > > and it does _not_ test for num_exe_file_vmas being 0 before doing decrement,
> > > thus we get inconsistency in counter.
> >
> > No, removed_exe_file_vma() is called only for vma with VM_EXECUTABLE flag,
> > there no way to get such vma other than sys_execve().
> > And this brave soul cannot call prctl_set_mm_exe_file() successfully,
> > just because for vma with VM_EXECUTABLE flag vma->vm_file == mm->exe_file.
> >
> > Anyway, I plan to get rid of mm->num_exe_file_vmas and VM_EXECUTABLE.
>
> Yeah, you've changed !path_equal to path_equal. And yes, getting rid of
> num_exe_file_vmas is good idea. Btw, Konstantin, why do we need to
> call for path_equal? Maybe we can simply test for mm->exe_file == NULL,
> and refuse to change anything if it's not nil value? This will simplify
> the code.

After removing VM_EXECUTABLE and mm->num_exe_file_vmas mm->exe_file
will never becomes NULL automatically. Patch for this not commited yet,
but I hope it will be in 3.6.

> 	Cyrill

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