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

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Message-ID: <20170924185147.GA26260@zzz.localdomain>
Date:   Sun, 24 Sep 2017 11:51:47 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
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()

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.

Eric