linux-kernel - Re: [PATCH] sh: fix syscall tracing

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Date:   Thu, 3 Sep 2020 12:14:43 +0200
From:   John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To:     Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org
Cc:     Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>,
        linux-kernel@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH] sh: fix syscall tracing

Hi Rich!

On 9/3/20 7:48 AM, Rich Felker wrote:
> Addition of SECCOMP_FILTER exposed a longstanding bug in
> do_syscall_trace_enter, whereby r0 (the 5th argument register) was
> mistakenly used where r3 (syscall_nr) was intended. By overwriting r0
> rather than r3 with -1 when attempting to block a syscall, the
> existing code would instead have caused the syscall to execute with an
> argument clobbered.
> 
> Commit 0bb605c2c7f2b4b3 then introduced skipping of the syscall when
> do_syscall_trace_enter returns -1, so that the return value set by
> seccomp filters would not be clobbered by -ENOSYS. This eliminated the
> clobbering of the 5th argument register, but instead caused syscalls
> made with a 5th argument of -1 to be misinterpreted as a request by
> do_syscall_trace_enter to suppress the syscall.
> 
> Fixes: 0bb605c2c7f2b4b3 ("sh: Add SECCOMP_FILTER")
> Fixes: ab99c733ae73cce3 ("sh: Make syscall tracer use tracehook notifiers, add TIF_NOTIFY_RESUME.")
> Signed-off-by: Rich Felker <dalias@...c.org>
> ---
>  arch/sh/kernel/entry-common.S |  1 -
>  arch/sh/kernel/ptrace_32.c    | 15 +++++----------
>  2 files changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/sh/kernel/entry-common.S b/arch/sh/kernel/entry-common.S
> index ad963104d22d..91ab2607a1ff 100644
> --- a/arch/sh/kernel/entry-common.S
> +++ b/arch/sh/kernel/entry-common.S
> @@ -370,7 +370,6 @@ syscall_trace_entry:
>  	 nop
>  	cmp/eq	#-1, r0
>  	bt	syscall_exit
> -	mov.l	r0, @(OFF_R0,r15)	! Save return value
>  	!			Reload R0-R4 from kernel stack, where the
>  	!   	    	    	parent may have modified them using
>  	!   	    	    	ptrace(POKEUSR).  (Note that R0-R2 are
> diff --git a/arch/sh/kernel/ptrace_32.c b/arch/sh/kernel/ptrace_32.c
> index b05bf92f9c32..5281685f6ad1 100644
> --- a/arch/sh/kernel/ptrace_32.c
> +++ b/arch/sh/kernel/ptrace_32.c
> @@ -455,16 +455,11 @@ long arch_ptrace(struct task_struct *child, long request,
>  
>  asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
>  {
> -	long ret = 0;
> -
>  	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
> -	    tracehook_report_syscall_entry(regs))
> -		/*
> -		 * Tracing decided this syscall should not happen.
> -		 * We'll return a bogus call number to get an ENOSYS
> -		 * error, but leave the original number in regs->regs[0].
> -		 */
> -		ret = -1L;
> +	    tracehook_report_syscall_entry(regs)) {
> +		regs->regs[0] = -ENOSYS;
> +		return -1;
> +	}
>  
>  	if (secure_computing() == -1)
>  		return -1;
> @@ -475,7 +470,7 @@ asmlinkage long do_syscall_trace_enter(struct pt_regs *regs)
>  	audit_syscall_entry(regs->regs[3], regs->regs[4], regs->regs[5],
>  			    regs->regs[6], regs->regs[7]);
>  
> -	return ret ?: regs->regs[0];
> +	return 0;
>  }
>  
>  asmlinkage void do_syscall_trace_leave(struct pt_regs *regs)
> 

I can confirm that this patch fixes both strace for me and does not break libseccomp,
I have run the libseccomp testsuite with my patch for SuperH support applied on top
of a rebased libseccomp with the 32-bit fixes. Attaching the testsuite log.

Tested-by: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz@...ian.org
`. `'   Freie Universitaet Berlin - glaubitz@...sik.fu-berlin.de
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913

View attachment "seccomp-live-test.log" of type "text/x-log" (7172 bytes)