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: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/all/20230706233248.445713-1-rick.p.edgecombe%40intel.com

arch/x86/kernel/shstk.c

@@ -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

@@ -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