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From:   Linus Torvalds <torvalds@linuxfoundation.org>
Date:   Wed, 13 Dec 2023 10:54:00 -0800
Message-ID: <CAHk-=whXt7QZV+HDA8=PN1pTBSNTL0ZJrcqtc4af=FtJpjPQeQ@mail.gmail.com>
Subject: Re: [PATCH 3/3] x86/sigreturn: Reject system segements
To:     Brian Gerst <brgerst@gmail.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Luczaj <mhal@rbox.co>
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On Wed, 13 Dec 2023 at 08:34, Brian Gerst <brgerst@gmail.com> wrote:
>
> @@ -98,7 +98,11 @@ static bool ia32_restore_sigcontext(struct pt_regs *regs,
>
>         /* Get CS/SS and force CPL3 */
>         regs->cs = sc.cs | 0x03;
> +       if (!valid_user_selector(regs->cs))
> +               return false;
>         regs->ss = sc.ss | 0x03;
> +       if (!valid_user_selector(regs->ss))
> +               return false;

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