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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@xxxxxxxx>

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!




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