Re: [PATCH 03/10] x86/fpu: Use validate_xstate_header() to validate the xstate_header in sanitize_restored_xstate()

[Eric Biggers <ebiggers3@gmail.com>]

On Mon, Sep 25, 2017 at 08:14:45AM +0200, Ingo Molnar wrote:
> > > 
> > > Could you please just send the delta patch against the whole tree to fix the bug? 
> > > I'll worry about the patch dependencies and back-merge it to the proper place.
> > > 
> > 
> > The following diff against tip/master fixes the bug.  Note: we *could* check
> > 'use_xsave()' instead of 'state_size > offsetof(struct xregs_state, header)',
> > but that might be confusing in the case where we couldn't find the xstate
> > information in the memory layout and only copy the fxregs_state, since then we'd
> > actually be validating the xsave_header which was already there, which shouldn't
> > ever fail.
> > 
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index afe54247cf27..fb639e70048f 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -331,7 +331,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> >  			err = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
> >  		} else {
> >  			err = __copy_from_user(&fpu->state.xsave, buf_fx, state_size);
> > -			if (!err)
> > +
> > +			if (!err && state_size > offsetof(struct xregs_state, header))
> >  				err = validate_xstate_header(&fpu->state.xsave.header);
> >  		}
> 
> I.e. a better check would be to check that the whole header can be accessed:
> 
> 	state_size >= offsetof(struct xregs_state, header) + sizeof(struct xstate_header)
> 
> Not that there should ever be a 'state_size' that points inside the header - so in 
> the end I back-merged your original (and tested ...) version.
> 

Well, actually we'd need to validate the header if userspace overwrote any part
of it.

But more importantly, I think the state_size check needs to go into the first
patch (the one that's Cc'ed to stable as it fixes the real bug), since
->xcomp_bv is part of the xstate_header.  So *before* we switch to
validate_xstate_header() in this patch, the code should already be:

		if (using_compacted_format()) {
			err = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
		} else {
			err = __copy_from_user(&fpu->state.xsave, buf_fx, state_size);

			/* xcomp_bv must be 0 when using uncompacted format */
			if (!err &&
			    state_size > offsetof(struct xregs_state, header) &&
			    fpu->state.xsave.header.xcomp_bv)
				err = -EINVAL;
		}

Also can you please fix the commit title and message of this patch?  It should
say "__fpu__restore_sig()", not "sanitize_restored_xstate()".

Thanks,

Eric