linux-kernel - Re: [PATCH] x86/alternative: don't call text

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Date:   Thu, 22 Oct 2020 12:48:40 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/alternative: don't call text_poke() in lazy TLB mode

On 22.10.20 12:45, Peter Zijlstra wrote:
> On Thu, Oct 22, 2020 at 11:24:39AM +0200, Jürgen Groß wrote:
>> On 09.10.20 16:42, Juergen Gross wrote:
>>> When running in lazy TLB mode the currently active page tables might
>>> be the ones of a previous process, e.g. when running a kernel thread.
>>>
>>> This can be problematic in case kernel code is being modified via
>>> text_poke() in a kernel thread, and on another processor exit_mmap()
>>> is active for the process which was running on the first cpu before
>>> the kernel thread.
>>>
>>> As text_poke() is using a temporary address space and the former
>>> address space (obtained via cpu_tlbstate.loaded_mm) is restored
>>> afterwards, there is a race possible in case the cpu on which
>>> exit_mmap() is running wants to make sure there are no stale
>>> references to that address space on any cpu active (this e.g. is
>>> required when running as a Xen PV guest, where this problem has been
>>> observed and analyzed).
>>>
>>> In order to avoid that, drop off TLB lazy mode before switching to the
>>> temporary address space.
>>>
>>> Fixes: cefa929c034eb5d ("x86/mm: Introduce temporary mm structs")
>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>
>> Can anyone look at this, please? It is fixing a real problem which has
>> been seen several times.
> 
> As it happens I picked it up yesterday, just pushed it out for you.

Thank you very much!


Juergen