linux-kernel - Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

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Date:   Thu, 22 Jun 2023 18:27:40 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

On Thu, 2023-06-22 at 19:21 +0100, Matthew Wilcox wrote:
> On Mon, Jun 12, 2023 at 05:10:42PM -0700, Rick Edgecombe wrote:
> > +++ b/include/linux/mm.h
> > @@ -342,7 +342,36 @@ extern unsigned int kobjsize(const void
> > *objp);
> >   #endif /* CONFIG_ARCH_HAS_PKEYS */
> >   
> >   #ifdef CONFIG_X86_USER_SHADOW_STACK
> > -# define VM_SHADOW_STACK       VM_HIGH_ARCH_5 /* Should not be set
> > with VM_SHARED */
> > +/*
> > + * This flag should not be set with VM_SHARED because of lack of
> > support
> > + * core mm. It will also get a guard page. This helps userspace
> > protect
> > + * itself from attacks. The reasoning is as follows:
> > + *
> > + * The shadow stack pointer(SSP) is moved by CALL, RET, and
> > INCSSPQ. The
> > + * INCSSP instruction can increment the shadow stack pointer. It
> > is the
> > + * shadow stack analog of an instruction like:
> > + *
> > + *   addq $0x80, %rsp
> > + *
> > + * However, there is one important difference between an ADD on
> > %rsp
> > + * and INCSSP. In addition to modifying SSP, INCSSP also reads
> > from the
> > + * memory of the first and last elements that were "popped". It
> > can be
> > + * thought of as acting like this:
> > + *
> > + * READ_ONCE(ssp);       // read+discard top element on stack
> > + * ssp += nr_to_pop * 8; // move the shadow stack
> > + * READ_ONCE(ssp-8);     // read+discard last popped stack element
> > + *
> > + * The maximum distance INCSSP can move the SSP is 2040 bytes,
> > before
> > + * it would read the memory. Therefore a single page gap will be
> > enough
> > + * to prevent any operation from shifting the SSP to an adjacent
> > stack,
> > + * since it would have to land in the gap at least once, causing a
> > + * fault.
> > + *
> > + * Prevent using INCSSP to move the SSP between shadow stacks by
> > + * having a PAGE_SIZE guard gap.
> > + */
> > +# define VM_SHADOW_STACK       VM_HIGH_ARCH_5
> >   #else
> >   # define VM_SHADOW_STACK      VM_NONE
> >   #endif
> 
> This is a lot of very x86-specific language in a generic header file.
> I'm sure there's a better place for all this text.

Yes, I couldn't find another place for it. This was the reasoning:
https://lore.kernel.org/lkml/07deaffc10b1b68721bbbce370e145d8fec2a494.camel@intel.com/

Did you have any particular place in mind?