parisc: Update comments in make_insert_tlb

The following testcase exposed a problem with our read access checks
in get_user() and raw_copy_from_user():

#include <stdint.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/mman.h>
#include <sys/types.h>

int main(int argc, char **argv)
{
  unsigned long page_size = sysconf(_SC_PAGESIZE);
  char *p = malloc(3 * page_size);
  char *p_aligned;

  /* initialize memory region. If not initialized, write syscall below will correctly return EFAULT. */
  if (1)
	memset(p, 'X', 3 * page_size);

  p_aligned = (char *) ((((uintptr_t) p) + (2*page_size - 1)) & ~(page_size - 1));
  /* Drop PROT_READ protection. Kernel and userspace should fault when accessing that memory region */
  mprotect(p_aligned, page_size, PROT_NONE);

  /* the following write() should return EFAULT, since PROT_READ was dropped by previous mprotect() */
  int ret = write(2, p_aligned, 1);
  if (!ret || errno != EFAULT)
	printf("\n FAILURE: write() did not returned expected EFAULT value\n");

  return 0;
}

Because of the way _PAGE_READ is handled, kernel code never generates
a read access fault when it access a page as the kernel privilege level
is always less than PL1 in the PTE.

This patch reworks the comments in the make_insert_tlb macro to try
to make this clearer.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.12+

arch/parisc/kernel/entry.S

@@ -499,6 +499,12 @@
 	 * this happens is quite subtle, read below */
 	.macro		make_insert_tlb	spc,pte,prot,tmp
 	space_to_prot   \spc \prot        /* create prot id from space */
+
+#if _PAGE_SPECIAL_BIT == _PAGE_DMB_BIT
+	/* need to drop DMB bit, as it's used as SPECIAL flag */
+	depi		0,_PAGE_SPECIAL_BIT,1,\pte
+#endif
+
 	/* The following is the real subtlety.  This is depositing
 	 * T <-> _PAGE_REFTRAP
 	 * D <-> _PAGE_DIRTY
@@ -511,17 +517,18 @@
 	 * Finally, _PAGE_READ goes in the top bit of PL1 (so we
 	 * trigger an access rights trap in user space if the user
 	 * tries to read an unreadable page */
-#if _PAGE_SPECIAL_BIT == _PAGE_DMB_BIT
-	/* need to drop DMB bit, as it's used as SPECIAL flag */
-	depi		0,_PAGE_SPECIAL_BIT,1,\pte
-#endif
 	depd            \pte,8,7,\prot
 
 	/* PAGE_USER indicates the page can be read with user privileges,
 	 * so deposit X1|11 to PL1|PL2 (remember the upper bit of PL1
-	 * contains _PAGE_READ) */
+	 * contains _PAGE_READ). While the kernel can't directly write
+	 * user pages which have _PAGE_WRITE zero, it can read pages
+	 * which have _PAGE_READ zero (PL <= PL1). Thus, the kernel
+	 * exception fault handler doesn't trigger when reading pages
+	 * that aren't user read accessible */
 	extrd,u,*=      \pte,_PAGE_USER_BIT+32,1,%r0
 	depdi		7,11,3,\prot
+
 	/* If we're a gateway page, drop PL2 back to zero for promotion
 	 * to kernel privilege (so we can execute the page as kernel).
 	 * Any privilege promotion page always denys read and write */