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

[Li, Xin3]

> -----Original Message-----
> From: H. Peter Anvin <hpa@xxxxxxxxx>
> Sent: Sunday, December 17, 2023 1:46 PM
> To: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
> Cc: Brian Gerst <brgerst@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> x86@xxxxxxxxxx; Ingo Molnar <mingo@xxxxxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Peter Zijlstra
> <peterz@xxxxxxxxxxxxx>; Michal Luczaj <mhal@xxxxxxx>
> Subject: Re: [PATCH 3/3] x86/sigreturn: Reject system segements
> 
> On December 17, 2023 1:40:53 PM PST, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxx> wrote:
> >On Sun, 17 Dec 2023 at 13:08, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> >>
> >> On December 13, 2023 10:54:00 AM PST, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx/

Thanks!
    Xin