Re: kprobes string reading broken on s390

[Masami Hiramatsu]

Hi Sven,

On Fri, 5 Jun 2020 15:25:41 +0200
Christoph Hellwig <hch@xxxxxx> wrote:

> Yes, this looks correct.  You probably want to write a small changelog
> and add a Fixes tag, though.
> 
> On Fri, Jun 05, 2020 at 01:05:34PM +0200, Sven Schnelle wrote:
> > Hi Christoph,
> > 
> > with the latest linux-next i noticed that some tests in the
> > ftrace test suites are failing on s390, namely:
> > 
> > [FAIL] Kprobe event symbol argument
> > [FAIL] Kprobe event with comm arguments
> > 
> > The following doesn't work anymore:
> > 
> > cd /sys/kernel/tracing
> > echo 'p:testprobe _do_fork comm=$comm ' >kprobe_events
> > echo 1 >/sys/kernel/tracing/events/kprobes/testprobe/enable
> > cat /sys/kernel/tracing/trace
> > 
> > it will just show
> > 
> > test.sh-519   [012] ....    18.580625: testprobe: (_do_fork+0x0/0x3c8) comm=(fault)
> > 
> > Looking at d411a9c4e95a ("tracing/kprobes: handle mixed kernel/userspace probes
> > better") i see that there are two helpers for reading strings:
> > 
> > fetch_store_string_user() -> read string from user space
> > fetch_store_string() -> read string from kernel space(?)
> > 
> > but in the end both are using strncpy_from_user_nofault(), but i would
> > think that fetch_store_string() should use strncpy_from_kernel_nofault().
> > However, i'm not sure about the exact semantics of fetch_store_string(),
> > as there where a lot of wrong assumptions in the past, especially since
> > on x86 you usually don't fail if you use the same function for accessing kernel
> > and userspace although it's technically wrong.

Thanks for fixing!
This report can be a good changelog.
Please resend it with Fixed tag as Christoph said.

Christoph, it seems your series in the akpm tree, so this also should
be sent to Andrew?

Thank you,

> > 
> > Regards,
> > Sven
> > 
> > commit 81408eab8fcc79dc0871a95462b13176d3446f5e
> > Author: Sven Schnelle <svens@xxxxxxxxxxxxx>
> > Date:   Fri Jun 5 13:01:24 2020 +0200
> > 
> >     kprobes: use strncpy_from_kernel_nofault() in fetch_store_string()
> > 
> >     Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
> > 
> > diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> > index b1f21d558e45..ea8d0b094f1b 100644
> > --- a/kernel/trace/trace_kprobe.c
> > +++ b/kernel/trace/trace_kprobe.c
> > @@ -1278,7 +1278,7 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
> >  	 * Try to get string again, since the string can be changed while
> >  	 * probing.
> >  	 */
> > -	ret = strncpy_from_user_nofault(__dest, (void *)addr, maxlen);
> > +	ret = strncpy_from_kernel_nofault(__dest, (void *)addr, maxlen);
> >  	if (ret >= 0)
> >  		*(u32 *)dest = make_data_loc(ret, __dest - base);
> >  
> ---end quoted text---


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>