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

[Ingo Molnar]

* Eric Biggers <ebiggers3@xxxxxxxxx> wrote:

> 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;
> 		}

Note that I think the whole series is more robust if it goes to -stable as-is, as 
the ABI aspect should not be underestimated either. Nevertheless I've backmerged 
the fix further to the original commit, to maintain bisectability.

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

Indeed - and I fixed that too. I have pushed out the latest tip:WIP.x86/fpu - no 
change in the end result tree, but different inner structure.

Thanks,

	Ingo