Re: [PATCH v2] parisc: Fix ptrace syscall number modification

[Helge Deller]

On 16.02.19 14:10, Dmitry V. Levin wrote:
> Commit 910cd32e552e ("parisc: Fix and enable seccomp filter support")
> introduced a regression in ptrace-based syscall tampering: when tracer
> changes syscall number to -1, the kernel fails to initialize %r28 with
> -ENOSYS and subsequently fails to return the error code of the failed
> syscall to userspace.
> 
> This erroneous behaviour could be observed with a simple strace syscall
> fault injection command which is expected to print something like this:
> 
> $ strace -a0 -ewrite -einject=write:error=enospc echo hello
> write(1, "hello\n", 6) = -1 ENOSPC (No space left on device) (INJECTED)
> write(2, "echo: ", 6) = -1 ENOSPC (No space left on device) (INJECTED)
> write(2, "write error", 11) = -1 ENOSPC (No space left on device) (INJECTED)
> write(2, "\n", 1) = -1 ENOSPC (No space left on device) (INJECTED)
> +++ exited with 1 +++
> 
> After commit 910cd32e552ea09caa89cdbe328e468979b030dd it loops printing
> something like this instead:
> 
> write(1, "hello\n", 6../strace: Failed to tamper with process 12345: unexpectedly got no error (return value 0, error 0)
> ) = 0 (INJECTED)
> 
> This bug was found by strace test suite.
> 
> Fixes: 910cd32e552e ("parisc: Fix and enable seccomp filter support")
> Cc: stable@xxxxxxxxxxxxxxx # v4.5+
> Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxxxxx>

Thanks, the patch works as expected.
You may add:
Tested-by: Helge Deller <deller@xxxxxx>

There is an "out" label a few lines below, which should be removed as well.
Otherwise you get this warning:
arch/parisc/kernel/ptrace.c: In function âdo_syscall_trace_enterâ:
arch/parisc/kernel/ptrace.c:357:1: warning: label âoutâ defined but not used [-Wunused-label]

I've fixed it up locally and added the patch to my for-next tree.
If it's ok for you, I'll push it through the parisc tree.

Helge

> ---
> 
>  v2: Updated comments.
>      Set gr[28] to -ENOSYS after tracehook_report_syscall_entry() invocation
>      because setting of syscall return code by tracer on entering syscall
>      shall not affect the syscall return code.
> 
>  N.B. I have no parisc box to test the patch.
> 
>  arch/parisc/kernel/ptrace.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/parisc/kernel/ptrace.c b/arch/parisc/kernel/ptrace.c
> index 2582df1c529b..be4d6a279b12 100644
> --- a/arch/parisc/kernel/ptrace.c
> +++ b/arch/parisc/kernel/ptrace.c
> @@ -308,15 +308,29 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
>  
>  long do_syscall_trace_enter(struct pt_regs *regs)
>  {
> -	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
> -	    tracehook_report_syscall_entry(regs)) {
> +	if (test_thread_flag(TIF_SYSCALL_TRACE)) {
> +		int rc = tracehook_report_syscall_entry(regs);
> +
>  		/*
> -		 * Tracing decided this syscall should not happen or the
> -		 * debugger stored an invalid system call number. Skip
> -		 * the system call and the system call restart handling.
> +		 * As tracesys_next does not set %r28 to -ENOSYS
> +		 * when %r20 is set to -1, initialize it here.
>  		 */
> -		regs->gr[20] = -1UL;
> -		goto out;
> +		regs->gr[28] = -ENOSYS;
> +
> +		if (rc) {
> +			/*
> +			 * A nonzero return code from
> +			 * tracehook_report_syscall_entry() tells us
> +			 * to prevent the syscall execution.  Skip
> +			 * the syscall call and the syscall restart handling.
> +			 *
> +			 * Note that the tracer may also just change
> +			 * regs->gr[20] to an invalid syscall number,
> +			 * that is handled by tracesys_next.
> +			 */
> +			regs->gr[20] = -1UL;
> +			return -1;
> +		}
>  	}
>  
>  	/* Do the secure computing check after ptrace. */
>