linux-kernel - Re: [PATCH 03/10] x86/fpu: Use validate_xstate_header() to validate the xstate_header in sanitize_restored

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Date:   Mon, 25 Sep 2017 08:14:45 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 03/10] x86/fpu: Use validate_xstate_header() to validate
 the xstate_header in sanitize_restored_xstate()


* Eric Biggers <ebiggers3@...il.com> wrote:

> On Sun, Sep 24, 2017 at 09:02:42PM +0200, Ingo Molnar wrote:
> > 
> > * Eric Biggers <ebiggers3@...il.com> wrote:
> > 
> > > On Sun, Sep 24, 2017 at 12:59:06PM +0200, Ingo Molnar wrote:
> > > > @@ -328,10 +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);
> > > > -
> > > > -			/* xcomp_bv must be 0 when using uncompacted format */
> > > > -			if (!err && fpu->state.xsave.header.xcomp_bv)
> > > > -				err = -EINVAL;
> > > > +			if (!err)
> > > > +				err = validate_xstate_header(&fpu->state.xsave.header);
> > > >  		}
> > > >  
> > > 
> > > Sorry, this is the buggy part.  The problem is that this code runs even if XSAVE
> > > isn't being used --- and in that case the state size is 512 bytes or less, so
> > > the state doesn't actually include the xstate_header.  So
> > > validate_xstate_header() was reading out of bounds and seeing invalid values.
> > > 
> > > So I think we need to check use_xsave() here, but it really needs to be in the
> > > earlier patch which added the check for just ->xcomp_bv ("x86/fpu: Don't let
> > > userspace set bogus xcomp_bv"), not in this one.
> > > 
> > > As far the split of patch 2/3 into these 10 patches, it looks fine (though it
> > > suddenly became a *lot* of patches!).  One nit: the subject of this one really
> > > should say "__fpu__restore_sig()", not "sanitize_restored_xstate()".
> > > 
> > > I can send a fixed series when I have a chance.
> > 
> > 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.

Thanks,

	Ingo