Re: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

From: Borislav Petkov <bp@alien8.de>
To: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
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	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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Subject: Re: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot
Date: Wed, 8 Mar 2023 10:23:24 +0100	[thread overview]
Message-ID: <ZAhUDE+6F71onz0W@zn.tnic> (raw)
In-Reply-To: <20230227222957.24501-28-rick.p.edgecombe@intel.com>

On Mon, Feb 27, 2023 at 02:29:43PM -0800, Rick Edgecombe wrote:
> When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
			   ^
			   in

> shadow stack memory. So for shadow stack memory this bit combination is
> valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
> write protected, the kernel has been taught to transition the Dirty=1
> bit to SavedDirty=1, to avoid inadvertently creating shadow stack
> memory. It does this inside pte_wrprotect() because it knows the PTE is
> not intended to be a writable shadow stack entry, it is supposed to be
> write protected.

> 
> However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
> can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
> distinguish between the caller intending to create a shadow stack PTE or
> needing the SavedDirty shift.
> 
> The kernel has been updated to not do this, and so Write=0,Dirty=1
> memory should only be created by the pte_mkfoo() helpers. Add a warning
> to make sure no new mk_pte() start doing this.

Might wanna add the note from below here:

"... start doing this, like, for example, set_memory_rox() did."

> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Kees Cook <keescook@chromium.org>
> Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> 
> ---
> v6:
>  - New patch (Note, this has already been a useful warning, it caught the
>    newly added set_memory_rox() doing this)

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette