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Date: Sun, 17 Dec 2023 13:45:50 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linuxfoundation.org>
CC: Brian Gerst <brgerst@gmail.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>, Michal Luczaj <mhal@rbox.co>
Subject: Re: [PATCH 3/3] x86/sigreturn: Reject system segements
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On December 17, 2023 1:40:53 PM PST, Linus Torvalds <torvalds@linuxfoundati=
on=2Eorg> wrote:
>On Sun, 17 Dec 2023 at 13:08, H=2E Peter Anvin <hpa@zytor=2Ecom> wrote:
>>
>> On December 13, 2023 10:54:00 AM PST, Linus Torvalds <torvalds@linuxfou=
ndation=2Eorg> wrote:
>]> >Side note: the SS/CS checks could be stricter than the usual selector=
 tests=2E
>> >
>> >In particular, normal segments can be Null segments=2E But CS/SS must =
not be=2E
>> >
>> >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=2E=2E
>> >
>> >That forcing was literally just because we weren't checking it for san=
ity=2E=2E=2E
>> >
>> >           Linus
>>
>> Not to mention that changing a null descriptor to 3 is wrong=2E
>
>I don't think it is=2E All of 0-3 are "Null selectors"=2E The RPL of the
>selector simply doesn't matter when the index is zero, afaik=2E
>
>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=2E
>
>              Linus

Of course not for CS/SS, but I would agree that if the selector is 0 befor=
e the signal it shouldn't mysteriously and asynchronously become 3=2E