linux-kernel - Re: uprobes misses breakpoint insertion into VM

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Date:   Fri, 16 Mar 2018 17:52:39 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Erica Bugden <ebugden@...icios.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: uprobes misses breakpoint insertion into VM_WRITE mappings

On 03/15, Mathieu Desnoyers wrote:
>
> Hi,
>
> Erica has been working on extending test-cases for uprobes, and found
> something unexpected:
>
> Since commit e40cfce626a5 "uprobes: Restrict valid_vma(false) to skip VM_SHARED vmas"
> uprobes does not insert breakpoints into mappings mprotect'd as writeable.

Not really, VM_WRITE was illegal from the very beginning, this commit only
affects the "is_register == false" case.

> This issue can be reproduced by compiling a library without PIC (not using GOT),
> and then concurrently:
>
> A) Load the library (dynamic loader mprotect the code as writeable to do
>    the relocations, and then mprotect as executable),
>
> B) Enable a uprobe through perf.
>
> (it is a race window between the two mprotect syscalls)
>
> It appears that the following restriction in valid_vma() is responsible
> for this behavior:
>
>         if (is_register)
>                 flags |= VM_WRITE;
>
> I don't figure a clear explanation for this flag based on the function
> comment nor the commit changelog. Any idea on whether this is really
> needed ?

Because we do not want to modify the writable area. If nothing else, this
can break the application which writes to the page we are going to replace.

> Note that on uprobes unregister, it allows removing a breakpoint event
> on a writeable mapping,

Yes. Because a probed apllication can do mprotect() after the kernel installs
the breakpoint. And we have to remove this breakpoint in any case, even if
this is unsafe too.

Oleg.