Date:   Thu, 7 Sep 2023 10:10:57 +0000
From:   Tze-nan Wu (吳澤南) 
        <Tze-nan.Wu@...iatek.com>
To:     "rostedt@...dmis.org" <rostedt@...dmis.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-trace-kernel@...r.kernel.org" 
        <linux-trace-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        Cheng-Jui Wang (王正睿) 
        <Cheng-Jui.Wang@...iatek.com>,
        wsd_upstream <wsd_upstream@...iatek.com>,
        Bobule Chang (張弘義) 
        <bobule.chang@...iatek.com>,
        Tze-nan Wu (吳澤南) 
        <Tze-nan.Wu@...iatek.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH] ring-buffer: Do not read at &event->array[0] if it across
 the page

On Thu, 2023-09-07 at 17:42 +0800, Tze-nan Wu wrote:

More information added.
> On Tue, 2023-09-05 at 12:39 -0400, Steven Rostedt wrote:
> >  	 
> > External email : Please do not click links or open attachments
> > until
> > you have verified the sender or the content.
> >  On Tue, 5 Sep 2023 22:12:45 +0800
> > Tze-nan Wu <Tze-nan.Wu@...iatek.com> wrote:
> > 
> > > While reading from the tracing/trace, the ftrace reader rarely
> > 
> > encounters
> > > a KASAN invalid access issue.
> > > It is likely that the writer has disrupted the ring_buffer that
> > > the
> > 
> > reader
> > > is currently parsing. the kasan report is as below:
> > 
> > What kernel are you using? There's been recent fixes that handle
> > the
> > race
> > between reading the last entry and the writer adding to it. This
> > looks like
> > you may have hit that bug.
> > 
> 
> I am currently using kernel version 6.1.24, but I have applied the
> patch for the ringbuffer.c file up to kernel version 6.1.52. However,
> I
> am still able to reprduce the issue.
> 
> BTW, my decice have ARCH=ARM64 and PAGESIZE=4K
> 
> > > 
> > > [name:report&]BUG: KASAN: invalid-access in
> > 
> > rb_iter_head_event+0x27c/0x3d0
> > > [name:report&]Read of size 4 at addr 71ffff8111a18000 by task
> > > xxxx
> > > [name:report_sw_tags&]Pointer tag: [71], memory tag: [0f]
> > > [name:report&]
> > > CPU: 2 PID: 380 Comm: xxxx
> > > Call trace:
> > > dump_backtrace+0x168/0x1b0
> > > show_stack+0x2c/0x3c
> > > dump_stack_lvl+0xa4/0xd4
> > > print_report+0x268/0x9b0
> > > kasan_report+0xdc/0x148
> > > kasan_tag_mismatch+0x28/0x3c
> > > __hwasan_tag_mismatch+0x2c/0x58
> > > rb_event_length() [inline]
> > > rb_iter_head_event+0x27c/0x3d0
> > > ring_buffer_iter_peek+0x23c/0x6e0
> > > __find_next_entry+0x1ac/0x3d8
> > > s_next+0x1f0/0x310
> > > seq_read_iter+0x4e8/0x77c
> > > seq_read+0xf8/0x150
> > > vfs_read+0x1a8/0x4cc
> > > 
> > > In some edge cases, ftrace reader could access to an invalid
> > 
> > address,
> > > specifically when reading 4 bytes beyond the end of the currently
> > 
> > page.
> > > While issue happened, the dump of rb_iter_head_event is shown as
> > 
> > below:
> > > 
> > >     in function rb_iter_head_event:
> > >           - iter->head = 0xFEC
> > >           - iter->next_event = 0xFEC
> > >           - commit = 0xFF0
> > >           - read_stamp = 0x2955AC46DB0
> > >           - page_stamp = 0x2955AC2439A
> > >           - iter->head_page->page = 0x71FFFF8111A17000
> > >           - iter->head_page->time_stamp = 0x2956A142267
> > >           - iter->head_page->page->commit = 0xFF0
> > >           - the content in iter->head_page->page
> > >                 0x71FFFF8111A17FF0: 01010075 00002421 0A123B7C
> > 
> > FFFFFFC0
> > > 
> > > In rb_iter_head_event, reader will call rb_event_length with
> > 
> > argument
> > > (struct ring_buffer_event *event = 0x71FFFF8111A17FFC).
> > 
> > As the commit is considered the end of buffer, it should never read
> > past
> > that, which the above appears to be doing. That's why I think it
> > hit
> > that
> > race.
> > 
> 
> Same as my thought,
> since every time the reader try to access the address in next page,
> the below condition hold in rb_iter_head_event function:
> 
>         if (iter->page_stamp != iter_head_page->page->time_stamp ||
>                 commit > rb_page_commit(iter_head_page))
>                         goto reset;
> 
> I observe the result by the debug patch provided below:
> 
>         @@ -2378,6 +2378,19 @@ rb_iter_head_event(struct
> ring_buffer_iter *iter)
>                 commit = rb_page_commit(iter_head_page);
>                 smp_rmb();
>         +       if ((iter->head >= 0xFECUL) && commit == 0xFF0UL) {
>         +               pr_info("rbdbg: cpu = %d, event = 0x%lx,
> iter-
> > head = 0x%lx,\
> 
>         +                       commit = 0xFF0, type = 0x%x, ts =
> 0x%x,
> array addr = 0x%lx\n",\
>         +                       iter->cpu_buffer->cpu, (unsigned
> long)event, iter->head,\
>         +                       event->type_len, event->time_delta,
> (unsigned long)(&(event->array[0])));
>         +               mdelay(500);
>         +               pr_info("rbdbg2: cpu = %d, event = 0x%lx,
> iter-
> > head = 0x%lx,\
> 
>         +                       commit = 0xFF0, type = 0x%x, ts =
> 0x%x,
> array addr = 0x%lx\n",\
>         +                       iter->cpu_buffer->cpu, (unsigned
> long)event, iter->head,\
>         +                       event->type_len, event->time_delta,
> (unsigned long)(&(event->array[0])));
>         +               if (iter->page_stamp != iter_head_page->page-
> > time_stamp || commit > rb_page_commit(iter_head_page))
> 
>         +                       pr_info("rbdbg: writer corrupt
> reader\n");
>         +       }
>                 event = __rb_page_index(iter_head_page, iter->head);
>                 length = rb_event_length(event);
> 
> Note that the mdelay(500) in the debug patch can reproduce the issue
> easier with same test in my environmnet,
> I am now able to reproduce the issue within 15 minutes if the debug
> patch on.
> 

The debug patch may give something similar to the below log just before
the invalid access happened, for me it looks like the padding event had
just corrupted by the writer before the reader invoke rb_event_length
function on it.

[  338.156772] cat: [name:ring_buffer&]rbdbg: cpu = 0, event =
0x????????????dffc, iter->head = 0xfec, commit = 0xFF0, type = 0x1d, ts
= 0x0, array addr = 0x????????????e000
[  338.656796] cat: [name:ring_buffer&]rbdbg2: cpu = 0, event =
0x????????????dffc, iter->head = 0xfec, commit = 0xFF0, type = 0x0, ts
= 0x0, array addr = 0x????????????e000
[  338.656803] cat: [name:ring_buffer&]rbdbg: writer corrupt reader
[  338.656810] cat:
[name:report&]=========================================================
=========
[  338.656819] cat: [name:report&]BUG: KASAN: invalid-access in
rb_event_length


> > > Since the content data start at address 0x71FFFF8111A17FFC are
> > 
> > 0xFFFFFFC0.
> > > event->type will be interpret as 0x0, than the reader will try to
> > 
> > get the
> > > length by accessing event->array[0], which is an invalid address:
> > >     &event->array[0] = 0x71FFFF8111A18000
> > > 
> > > Signed-off-by: Tze-nan Wu <Tze-nan.Wu@...iatek.com>
> > > ---
> > > Following patch may not become a solution, it merely checks if
> > > the
> > 
> > address
> > > to be accessed is valid or not within the rb_event_length before
> > 
> > access.
> > > And not sure if there is any side-effect it can lead to.
> > > 
> > > I am curious about what a better solution for this issue would
> > > look
> > 
> > like.
> > > Should we address the problem from the writer or the reader?
> > > 
> > > Also I wonder if the cause of the issue is exactly as I
> > > suspected.
> > > Any Suggestion will be appreciated.
> > 
> > I guess this depends on if you have the fixes or not?
> > 
> 
> yes, I could try to pick the patches that only included in mainline
> but
> not in kernel-6.1.52 for ring_buffer.c file,
> and do the same test to see if the issue is still reproducible.
> 
> > > 
> > > Test below can reproduce the issue in 2 hours on kernel-6.1.24:
> > >     $cd /sys/kernel/tracing/
> > >     # make the reader and writer race more through resize the
> > 
> > buffer to 8kb
> > >     $echo 8 > buffer_size_kn
> > >     # enable all events
> > >     $echo 1 > event/enable
> > >     # enable trace
> > >     $echo 1 > tracing_on
> > >  
> > >     # write and run a script that keep reading trace
> > >     $./read_trace.sh
> > > 
> > >     ``` read_trace.sh
> > >        while :
> > >        do
> > >            cat /sys/kernel/tracing/trace > /dev/null
> > >        done
> > > 
> > >     ```
> > 
> > Thanks, I'll look at that when I finish debugging the eventfs bug.
> > 
> > -- Steve
> 
> Also thank for your reply,
> 

And the temporary fix patch in my first mail should have some modified
as shown below.
+               if (((unsigned long)event & 0xfffUL) >= PAGE_SIZE - 4)
                                             ^^^^^^
                                            PAGE_SIZE-1
+                       return -1;

> -- Tze-nan

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