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

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On Thu, Jun 22, 2023 at 06:27:40PM +0000, Edgecombe, Rick P wrote:
> 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@xxxxxxxxx/
> 
> Did you have any particular place in mind?

Since it's near CONFIG_X86_USER_SHADOW_STACK the comment in mm.h could be 

/*
 * VMA is used for shadow stack and implies guard pages.
 * See arch/x86/kernel/shstk.c for details
 */

and the long reasoning comment can be moved near alloc_shstk in
arch/x86/kernel/shstk.h

-- 
Sincerely yours,
Mike.



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