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

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Message-ID: <20201022104527.GI2594@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Oct 2020 12:45:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jürgen Groß <jgross@...e.com>
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 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.

Thanks!