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

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Date: Sun, 17 Dec 2023 13:45:50 -0800
From: "H. Peter Anvin" <hpa@...or.com>
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.