linux-kernel - Re: [PATCH v2 2/2] ARM: ptrace: Restore syscall skipping for tracers

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Message-Id: <7e137ddd-a954-4a2e-89ac-5d2cb59775de@app.fastmail.com>
Date:   Thu, 10 Aug 2023 22:11:08 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Kees Cook" <keescook@...omium.org>,
        "Russell King" <linux@...linux.org.uk>
Cc:     "Arnd Bergmann" <arnd@...nel.org>,
        "Lecopzer Chen" <lecopzer.chen@...iatek.com>,
        "Oleg Nesterov" <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        "Matthias Brugger" <matthias.bgg@...il.com>,
        "AngeloGioacchino Del Regno" 
        <angelogioacchino.delregno@...labora.com>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Wolfram Sang" <wsa@...nel.org>,
        "Dan Williams" <dan.j.williams@...el.com>,
        "Russell King" <rmk+kernel@...linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 2/2] ARM: ptrace: Restore syscall skipping for tracers

On Thu, Aug 10, 2023, at 21:54, Kees Cook wrote:
> Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
> thread_info->abi_syscall"), the seccomp selftests "syscall_errno"
> and "syscall_faked" have been broken. Both seccomp and PTRACE depend
> on using the special value of "-1" for skipping syscalls. This value
> wasn't working because it was getting masked by __NR_SYSCALL_MASK in
> both PTRACE_SET_SYSCALL and get_syscall_nr().
>
> Explicitly test for -1 in PTRACE_SET_SYSCALL and get_syscall_nr(),
> leaving it exposed when present, allowing tracers to skip syscalls
> again.
>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Arnd Bergmann <arnd@...nel.org>
> Cc: Lecopzer Chen <lecopzer.chen@...iatek.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Fixes: 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store 
> thread_info->abi_syscall")
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Arnd Bergmann <arnd@...db.de>