linux-kernel - Re: [PATCH v1 1/4] um: Fix ptrace GETREGS/SETREGS bugs

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Date:	Mon, 21 Dec 2015 10:00:45 +0100
From:	Richard Weinberger <richard@....at>
To:	Mickaël Salaün <mic@...ikod.net>,
	linux-kernel@...r.kernel.org
Cc:	Jonathan Corbet <corbet@....net>, Jeff Dike <jdike@...toit.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>,
	Shuah Khan <shuahkh@....samsung.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Hogan <james.hogan@...tec.com>,
	Thomas Meyer <thomas@...3r.de>,
	Nicolas Iooss <nicolas.iooss_linux@....org>,
	Anton Ivanov <aivanov@...cade.com>, linux-doc@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net,
	user-mode-linux-user@...ts.sourceforge.net,
	linux-api@...r.kernel.org,
	Meredydd Luff <meredydd@...atehouse.org>,
	David Drysdale <drysdale@...gle.com>
Subject: Re: [PATCH v1 1/4] um: Fix ptrace GETREGS/SETREGS bugs

[sending the mail again, Thunderbird crashed :-\]

Am 21.12.2015 um 09:49 schrieb Mickaël Salaün:
> 
> On 21/12/2015 01:20, Richard Weinberger wrote:
>> Am 21.12.2015 um 01:03 schrieb Mickaël Salaün:
>>> diff --git a/arch/um/kernel/skas/syscall.c b/arch/um/kernel/skas/syscall.c
>>> index 1683b8e..65f0d1a 100644
>>> --- a/arch/um/kernel/skas/syscall.c
>>> +++ b/arch/um/kernel/skas/syscall.c
>>> @@ -7,6 +7,7 @@
>>>  #include <linux/ptrace.h>
>>>  #include <kern_util.h>
>>>  #include <sysdep/ptrace.h>
>>> +#include <sysdep/ptrace_user.h>
>>>  #include <sysdep/syscalls.h>
>>>  #include <os.h>
>>>  
>>> @@ -16,12 +17,16 @@ void handle_syscall(struct uml_pt_regs *r)
>>>  	long result;
>>>  	int syscall;
>>>  
>>> +	/* Save the syscall register. */
>>> +	UPT_SYSCALL_NR(r) = PT_SYSCALL_NR(r->gp);
>>> +
>>>  	if (syscall_trace_enter(regs)) {
>>>  		result = -ENOSYS;
>>>  		goto out;
>>>  	}
>>>  
>>> -	syscall = get_syscall(r);
>>> +	/* Get the syscall after being potentially updated with ptrace. */
>>> +	syscall = UPT_SYSCALL_NR(r);
>>
>> Doesn't this break the support for changing syscall numbers using PTRACE_SETREGS?
> 
> The logic is unchanged except updating the UPT_SYSCALL_NR before syscall_trace_enter(). I did my last tests with the x86_32 subarchitecture and all tests (from selftest/seccomp), including PTRACE_SETREGS for syscall numbers tests, passed. However, 2 of this tests still fail for x86_64 (only).

No, the logic is different.
syscall_trace_enter(regs) enters the ptrace() path and here registers can be changed.
Hence "syscall = UPT_SYSCALL_NR(r);" will see the old syscall number.
UPT_SYSCALL_NR() returns the syscall number before the ptrace() path...

Thanks,
//richard
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