linux-kernel - Re: [PATCH v4 16/23] vmalloc: Add flag for free of special permsissions

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Date:   Thu, 25 Apr 2019 22:38:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, hpa@...or.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux_dti@...oud.com, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, akpm@...ux-foundation.org,
        kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        kristen@...ux.intel.com, deneen.t.dock@...el.com
Subject: Re: [PATCH v4 16/23] vmalloc: Add flag for free of special
 permsissions

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?