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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Kees Cook <keescook@chromium.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Robert O'Callahan <rocallahan@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@kernel.org>,
        linux-arch@vger.kernel.org, Will Deacon <will@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Mark Rutland <mark.rutland@arm.com>,
        Keno Fischer <keno@juliacomputing.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Kyle Huey <me@kylehuey.com>
Subject: Re: [REGRESSION] x86/entry: Tracer no longer has opportunity to change the syscall number at entry via orig_ax
In-Reply-To: <202009111609.61E7875B3@keescook>
References: <CAP045Arc1Vdh+n2j2ELE3q7XfagLjyqXji9ZD0jqwVB-yuzq-g@mail.gmail.com> <87blj6ifo8.fsf@nanos.tec.linutronix.de> <87a6xzrr89.fsf@mpe.ellerman.id.au> <202009111609.61E7875B3@keescook>
Date:   Sun, 13 Sep 2020 17:44:09 +1000
Message-ID: <87d02qqfxy.fsf@mpe.ellerman.id.au>
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Kees Cook <keescook@chromium.org> writes:
> On Wed, Sep 09, 2020 at 11:53:42PM +1000, Michael Ellerman wrote:
>> I can observe the difference between v5.8 and mainline, using the
>> raw_syscall trace event and running the seccomp_bpf selftest which turns
>> a getpid (39) into a getppid (110).
>> 
>> With v5.8 we see getppid on entry and exit:
>> 
>>      seccomp_bpf-1307  [000] .... 22974.874393: sys_enter: NR 110 (7ffff22c46e0, 40a350, 4, fffffffffffff7ab, 7fa6ee0d4010, 0)
>>      seccomp_bpf-1307  [000] .N.. 22974.874401: sys_exit: NR 110 = 1304
>> 
>> Whereas on mainline we see an enter for getpid and an exit for getppid:
>> 
>>      seccomp_bpf-1030  [000] ....    21.806766: sys_enter: NR 39 (7ffe2f6d1ad0, 40a350, 7ffe2f6d1ad0, 0, 0, 407299)
>>      seccomp_bpf-1030  [000] ....    21.806767: sys_exit: NR 110 = 1027
>
> For my own notes, this is how I reproduced it:
>
> # ./perf-$VER record -e raw_syscalls:sys_enter -e raw_syscalls:sys_exit &
> # ./seccomp_bpf
> # fg
> ctrl-c
> # ./perf-$VER script | grep seccomp_bpf | awk '{print $7}' | sort | uniq -c > $VER.log
> *repeat*
> # diff -u old.log new.log
> ...
>
> (Is there an easier way to get those results?)

I did more or less the same thing, except I ran the trace event manually
(via debugfs), which is no better really.

I think the right way to test it would be to have a test that modifies
the syscall via seccomp and also monitors the trace event using perf
events. But that wouldn't be easier :)

> I will go see if I can figure out the best way to correct this.

I think this works?

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 18683598edbc..901361e2f8ea 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -60,13 +60,15 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
                        return ret;
        }
 
+       syscall = syscall_get_nr(current, regs);
+
        if (unlikely(ti_work & _TIF_SYSCALL_TRACEPOINT))
                trace_sys_enter(regs, syscall);
 
        syscall_enter_audit(regs, syscall);
 
        /* The above might have changed the syscall number */
-       return ret ? : syscall_get_nr(current, regs);
+       return ret ? : syscall;
 }
 
 static __always_inline long


cheers