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From:   "Li, Xin3" <xin3.li@intel.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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        "x86@kernel.org" <x86@kernel.org>,
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        "brgerst@gmail.com" <brgerst@gmail.com>
Subject: RE: [PATCH] x86/ia32: Do not modify the DPL bits for a null selector
Thread-Topic: [PATCH] x86/ia32: Do not modify the DPL bits for a null selector
Thread-Index: AQHZr83AFvT4TTcrIkq3d3Abr5XF0q+sxk7ngAC/myCAAC0zwIAA3mCQgABOVsA=
Date:   Fri, 7 Jul 2023 22:17:36 +0000
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> Perhaps something like patch below to make it clear that we are
> normalizing the segment values and forcing that normalization.

"Normalizing" sounds a good term.

> I am a bit confused why this code is not the same for ia32 and
> ia32_emulation.  I would think the normalization at least should apply
> to the 32bit case as well.
>=20
> Eric
>=20
> diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
> index 9027fc088f97..e5f3978388fd 100644
> --- a/arch/x86/kernel/signal_32.c
> +++ b/arch/x86/kernel/signal_32.c
> @@ -36,22 +36,47 @@
>  #ifdef CONFIG_IA32_EMULATION
>  #include <asm/ia32_unistd.h>
>=20
> +static inline unsigned int normalize_seg_index(unsigned int index)

normalize_usrseg_index?

> +{
> +	/*
> +	 * Convert the segment index into normalized form.
> +	 *
> +	 * For the indexes 0,1,2,3 always use the value of 0, as IRET
> +	 * forces this form for the nul segment.
> +	 *
> +	 * Otherwise set both DPL bits to force it to be a userspace
> +	 * ring 3 segment index.
> +	 */
> +	return (index < 3) ? 0 : index | 3;

s/</<=3D

no?

With this patch it looks that 1,2,3 are no longer valid selector values
in Linux, which seems the right thing to do but I don't know if there is
any side effect.

> +}
> +
>  static inline void reload_segments(struct sigcontext_32 *sc)
>  {
> -	unsigned int cur;
> +	unsigned int new, cur;
>=20
> +	new =3D normalize_seg_index(sc->gs);
>  	savesegment(gs, cur);
> -	if ((sc->gs | 0x03) !=3D cur)
> -		load_gs_index(sc->gs | 0x03);
> +	cur =3D normalize_seg_index(cur);
> +	if (new !=3D cur)
> +		load_gs_index(new);
> +
> +	new =3D normalize_seg_index(sc->fs);
>  	savesegment(fs, cur);
> -	if ((sc->fs | 0x03) !=3D cur)
> -		loadsegment(fs, sc->fs | 0x03);
> +	cur =3D normalize_seg_index(cur);
> +	if (new !=3D cur)
> +		loadsegment(fs, new);
> +
> +	new =3D normalize_seg_index(sc->ds);
>  	savesegment(ds, cur);
> -	if ((sc->ds | 0x03) !=3D cur)
> -		loadsegment(ds, sc->ds | 0x03);
> +	cur =3D normalize_seg_index(cur);
> +	if (new !=3D cur)
> +		loadsegment(ds, new);
> +
> +	new =3D normalize_seg_index(sc->es);
>  	savesegment(es, cur);
> -	if ((sc->es | 0x03) !=3D cur)
> -		loadsegment(es, sc->es | 0x03);
> +	cur =3D normalize_seg_index(cur);
> +	if (new !=3D cur)
> +		loadsegment(es, new);
>  }
>=20
>  #define sigset32_t			compat_sigset_t