Commit 87f0df78 authored by Rick Edgecombe's avatar Rick Edgecombe Committed by Dave Hansen
Browse files

x86/shstk: Move arch detail comment out of core mm 



The comment around VM_SHADOW_STACK in mm.h refers to a lot of x86
specific details that don't belong in a cross arch file. Remove these
out of core mm, and just leave the non-arch details.

Since the comment includes some useful details that would be good to
retain in the source somewhere, put the arch specifics parts in
arch/x86/shstk.c near alloc_shstk(), where memory of this type is
allocated. Include a reference to the existence of the x86 details near
the VM_SHADOW_STACK definition mm.h.

Signed-off-by: default avatarRick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: default avatarMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/all/20230706233248.445713-1-rick.p.edgecombe%40intel.com
parent 67840ad0

arch/x86/kernel/shstk.c

+25 −0
Original line number Diff line number Diff line
@@ -72,6 +72,31 @@ static int create_rstor_token(unsigned long ssp, unsigned long *token_addr) 
	return 0;

}



/*

 * VM_SHADOW_STACK will have 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.

 */

static unsigned long alloc_shstk(unsigned long addr, unsigned long size,

				 unsigned long token_offset, bool set_res_tok)

{

include/linux/mm.h

+6 −26
Original line number Diff line number Diff line
@@ -343,33 +343,13 @@ extern unsigned int kobjsize(const void *objp); 


#ifdef CONFIG_X86_USER_SHADOW_STACK

/*

 * 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:

 * VM_SHADOW_STACK should not be set with VM_SHARED because of lack of

 * support core mm.

 *

 * 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.

 * These VMAs will get a single end guard page. This helps userspace protect

 * itself from attacks. A single page is enough for current shadow stack archs

 * (x86). See the comments near alloc_shstk() in arch/x86/kernel/shstk.c

 * for more details on the guard size.

 */

# define VM_SHADOW_STACK	VM_HIGH_ARCH_5

#else