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Date: Thu, 25 Apr 2019 21:22:37 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "peterz@...radead.org" <peterz@...radead.org>
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Subject: Re: [PATCH v4 16/23] vmalloc: Add flag for free of special
 permsissions

On Thu, 2019-04-25 at 22:38 +0200, Peter Zijlstra wrote:
> On Mon, Apr 22, 2019 at 11:57:58AM -0700, Rick Edgecombe wrote:
> > Add a new flag VM_FLUSH_RESET_PERMS, for enabling vfree operations to
> > immediately clear executable TLB entries before freeing pages, and handle
> > resetting permissions on the directmap. This flag is useful for any kind
> > of memory with elevated permissions, or where there can be related
> > permissions changes on the directmap. Today this is RO+X and RO memory.
> > 
> > Although this enables directly vfreeing non-writeable memory now,
> > non-writable memory cannot be freed in an interrupt because the allocation
> > itself is used as a node on deferred free list. So when RO memory needs to
> > be freed in an interrupt the code doing the vfree needs to have its own
> > work queue, as was the case before the deferred vfree list was added to
> > vmalloc.
> > 
> > For architectures with set_direct_map_ implementations this whole operation
> > can be done with one TLB flush when centralized like this. For others with
> > directmap permissions, currently only arm64, a backup method using
> > set_memory functions is used to reset the directmap. When arm64 adds
> > set_direct_map_ functions, this backup can be removed.
> > 
> > When the TLB is flushed to both remove TLB entries for the vmalloc range
> > mapping and the direct map permissions, the lazy purge operation could be
> > done to try to save a TLB flush later. However today vm_unmap_aliases
> > could flush a TLB range that does not include the directmap. So a helper
> > is added with extra parameters that can allow both the vmalloc address and
> > the direct mapping to be flushed during this operation. The behavior of the
> > normal vm_unmap_aliases function is unchanged.
> > +static inline void set_vm_flush_reset_perms(void *addr)
> > +{
> > +	struct vm_struct *vm = find_vm_area(addr);
> > +
> > +	if (vm)
> > +		vm->flags |= VM_FLUSH_RESET_PERMS;
> > +}
> 
> So, previously in the series we added NX to module_alloc() and fixed up
> all the usage site. And now we're going through those very same sites to
> add set_vm_flush_reset_perms().
> 
> Why isn't module_alloc() calling the above function and avoid sprinkling
> it all over the place again?

Yea, that could make it more automatic, but there are some advantages to how it
is currently.

One is that most arch's have their own module_alloc(), and so calling
set_vm_flush_reset_perms() in kernel/module.c catches all architectures.
Otherwise it would be added in each arch which would be more sites.

The other reason is that the flush isn't actually needed until after the memory
is made executable, so we don't bother flushing if the allocation never gets set
executable. When that happens is only known by the callers of module_alloc().

Thanks,

Rick

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