linux-kernel - Re: [PATCH -next v1 3/3] kernel/events/uprobes: uprobe_write

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Message-ID: <03a7797a-75a0-4b45-b5b0-bf8bfb7f0955@redhat.com>
Date: Tue, 11 Mar 2025 21:02:49 +0100
From: David Hildenbrand <david@...hat.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-arm-kernel@...ts.infradead.org, linux-trace-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Russell King <linux@...linux.org.uk>,
 Masami Hiramatsu <mhiramat@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>,
 "Liang, Kan" <kan.liang@...ux.intel.com>,
 Tong Tiangen <tongtiangen@...wei.com>
Subject: Re: [PATCH -next v1 3/3] kernel/events/uprobes: uprobe_write_opcode()
 rewrite

On 11.03.25 13:32, Oleg Nesterov wrote:
> On 03/11, David Hildenbrand wrote:
>>
>> Regarding both questions, the code is fairly racy. Nothing would stop user
>> space from (a) modifying that memory
> 
> Yes, but we don't really care. uprobes.c assumes that user-space won't play
> with the probed memory.

Right, I primarily care about that if user space would do it, that we 
don't trigger unintended behavior (e.g., overwriting pagecache pages 
etc, WARN etc).

Likely the re-validating the page content is indeed something we can drop.

> 
> Note that if is_register is false, then vma can be even writable. Hmm, why?
> Perhaps valid_vma() should ignore is_register and nack VM_MAYWRITE ? But
> this doesn't really matter, say, gdb can change this memory anyway. Again,
> we don't really care.
> 
>>> do something like
>>>
>>> 	/* Walk the page tables again, to perform the actual update. */
>>> 	ret = -EAGAIN;
>>> 	folio = folio_walk_start(&fw, vma, vaddr, 0);
>>> 	if (folio) {
>>> 		if (fw.page == page) {
>>> 			WARN_ON(is_register && !folio_test_anon(folio));
>>
>> Yes, that would work (we could leave the WARN_ON in __uprobe_write_opcode),
>> but I am not sure if the end result is better better. No strong opinion on
>> the details though.
> 
> Will, this way __uprobe_write_opcode() will look a little bit simpler...
> 
> But I won't insist, please do what you think is better.

I'll take another look at this series probably next week (I'm on PTO 
this week) to then resend once adjusted + retested.

Thanks!

-- 
Cheers,

David / dhildenb