linux-kernel - Re: [REGRESSION] x86/entry: Tracer no longer has opportunity to change the syscall number at entry via orig

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Message-ID: <87blj6ifo8.fsf@nanos.tec.linutronix.de>
Date:   Wed, 19 Aug 2020 21:44:39 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kyle Huey <me@...ehuey.com>, Kees Cook <keescook@...omium.org>
Cc:     Robert O'Callahan <rocallahan@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@...nel.org>,
        linux-arch@...r.kernel.org, Will Deacon <will@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        Keno Fischer <keno@...iacomputing.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [REGRESSION] x86/entry: Tracer no longer has opportunity to change the syscall number at entry via orig_ax

On Wed, Aug 19 2020 at 10:14, Kyle Huey wrote:
> tl;dr: after 27d6b4d14f5c3ab21c4aef87dd04055a2d7adf14 ptracer
> modifications to orig_ax in a syscall entry trace stop are not honored
> and this breaks our code.

My fault and I have no idead why none of the silly test cases
noticed. Fix below.

Thanks,

        tglx
---
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 9852e0d62d95..fcae019158ca 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -65,7 +65,8 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
 
 	syscall_enter_audit(regs, syscall);
 
-	return ret ? : syscall;
+	/* The above might have changed the syscall number */
+	return ret ? : syscall_get_nr(current, regs);
 }
 
 noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)