linux-kernel - Re: [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken

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Message-ID: <CAHk-=wgqRgk0hjvpjHNixK7xSOS_F3fpt3bL9ZUJVhCL3oGgyw@mail.gmail.com>
Date:   Sun, 31 Jan 2021 15:35:22 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kyle Huey <me@...ehuey.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        open list <linux-kernel@...r.kernel.org>,
        "Robert O'Callahan" <rocallahan@...il.com>
Subject: Re: [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken

On Sun, Jan 31, 2021 at 3:18 PM Kyle Huey <me@...ehuey.com> wrote:
>
> The key to triggering this bug is to enter a ptrace syscall stop and
> then use PTRACE_SINGLESTEP to exit it. On a good kernel this will not
> result in any userspace code execution in the tracee because on the
> way out of the kernel's syscall handling path the singlestep trap will
> be raised immediately. On a bad kernel that stop will not be raised,
> and in the example below, the program will crash.

Thanks, great explanation, and I can certainly see the behavior you mention.

I wonder if the simple solution is to just

 (a) always set one of the SYSCALL_WORK_EXIT bits on the child in
ptrace (exactly to catch the child on system call exit)

 (b) basically revert 299155244770 ("entry: Drop usage of TIF flags in
the generic syscall code") and have the syscall exit code check the
TIF_SINGLESTEP flag

Hmm?

        Linus