linux-kernel - RE: [PATCH 3/3] x86/sigreturn: Reject system segements

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Message-ID: <SA1PR11MB6734EA01DE0AED77E1A49623A890A@SA1PR11MB6734.namprd11.prod.outlook.com>
Date: Mon, 18 Dec 2023 08:31:50 +0000
From: "Li, Xin3" <xin3.li@...el.com>
To: "H. Peter Anvin" <hpa@...or.com>, Linus Torvalds
	<torvalds@...uxfoundation.org>
CC: Brian Gerst <brgerst@...il.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, "Ingo
 Molnar" <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, "Borislav
 Petkov" <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, Michal Luczaj
	<mhal@...x.co>
Subject: RE: [PATCH 3/3] x86/sigreturn: Reject system segements



> -----Original Message-----
> From: H. Peter Anvin <hpa@...or.com>
> Sent: Sunday, December 17, 2023 1:46 PM
> To: Linus Torvalds <torvalds@...uxfoundation.org>
> Cc: Brian Gerst <brgerst@...il.com>; linux-kernel@...r.kernel.org;
> x86@...nel.org; Ingo Molnar <mingo@...nel.org>; Thomas Gleixner
> <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Peter Zijlstra
> <peterz@...radead.org>; Michal Luczaj <mhal@...x.co>
> Subject: Re: [PATCH 3/3] x86/sigreturn: Reject system segements
> 
> On December 17, 2023 1:40:53 PM PST, Linus Torvalds
> <torvalds@...uxfoundation.org> wrote:
> >On Sun, 17 Dec 2023 at 13:08, H. Peter Anvin <hpa@...or.com> wrote:
> >>
> >> On December 13, 2023 10:54:00 AM PST, Linus Torvalds
> <torvalds@...uxfoundation.org> wrote:
> >]> >Side note: the SS/CS checks could be stricter than the usual selector tests.
> >> >
> >> >In particular, normal segments can be Null segments. But CS/SS must not be.
> >> >
> >> >Also, since you're now checking the validity, maybe we shouldn't do
> >> >the "force cpl3" any more, and just make it an error to try to load
> >> >a
> >> >non-cpl3 segment at sigreturn..
> >> >
> >> >That forcing was literally just because we weren't checking it for sanity...
> >> >
> >> >           Linus
> >>
> >> Not to mention that changing a null descriptor to 3 is wrong.
> >
> >I don't think it is. All of 0-3 are "Null selectors". The RPL of the
> >selector simply doesn't matter when the index is zero, afaik.
> >
> >But we obviously only do this for CS/SS, which can't be (any kind of)
> >Null selector and iret will GP on them regardless of the RPL in the
> >selector.
> >
> >              Linus
> 
> Of course not for CS/SS, but I would agree that if the selector is 0 before the
> signal it shouldn't mysteriously and asynchronously become 3.

Unfortunately reload_segments() _always_ sets DPL bits of GS/FS/DS/ES
to 3, even for 0. And IRET clears DPL bits when loading a NULL selector
into GS/FS/DS/ES:
https://lore.kernel.org/lkml/20230706052231.2183-1-xin3.li@intel.com/

Thanks!
    Xin