parisc: Fix userspace graphics card breakage due to pgtable special bit

Commit df24e178 ("parisc: Add vDSO support") introduced the vDSO
support, for which a _PAGE_SPECIAL page table flag was needed.  Since we
wanted to keep every page table entry in 32-bits, this patch re-used the
existing - but yet unused - _PAGE_DMB flag (which triggers a hardware break
if a page is accessed) to store the special bit.

But when graphics card memory is mmapped into userspace, the kernel uses
vm_iomap_memory() which sets the the special flag. So, with the DMB bit
set, every access to the graphics memory now triggered a hardware
exception and segfaulted the userspace program.

Fix this breakage by dropping the DMB bit when writing the page
protection bits to the CPU TLB.

In addition this patch adds a small optimization: if huge pages aren't
configured (which is at least the case for 32-bit kernels), then the
special bit is stored in the hpage (HUGE PAGE) bit instead. That way we
can skip to reset the DMB bit.

Fixes: df24e178 ("parisc: Add vDSO support")
Cc: <stable@vger.kernel.org> # 5.18+
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/include/asm/pgtable.h

@@ -192,6 +192,11 @@ extern void __update_cache(pte_t pte);
 #define _PAGE_PRESENT_BIT  22   /* (0x200) Software: translation valid */
 #define _PAGE_HPAGE_BIT    21   /* (0x400) Software: Huge Page */
 #define _PAGE_USER_BIT     20   /* (0x800) Software: User accessible page */
+#ifdef CONFIG_HUGETLB_PAGE
+#define _PAGE_SPECIAL_BIT  _PAGE_DMB_BIT  /* DMB feature is currently unused */
+#else
+#define _PAGE_SPECIAL_BIT  _PAGE_HPAGE_BIT /* use unused HUGE PAGE bit */
+#endif
 
 /* N.B. The bits are defined in terms of a 32 bit word above, so the */
 /*      following macro is ok for both 32 and 64 bit.                */
@@ -219,7 +224,7 @@ extern void __update_cache(pte_t pte);
 #define _PAGE_PRESENT  (1 << xlate_pabit(_PAGE_PRESENT_BIT))
 #define _PAGE_HUGE     (1 << xlate_pabit(_PAGE_HPAGE_BIT))
 #define _PAGE_USER     (1 << xlate_pabit(_PAGE_USER_BIT))
-#define _PAGE_SPECIAL  (_PAGE_DMB)
+#define _PAGE_SPECIAL  (1 << xlate_pabit(_PAGE_SPECIAL_BIT))
 
 #define _PAGE_TABLE	(_PAGE_PRESENT | _PAGE_READ | _PAGE_WRITE | _PAGE_DIRTY | _PAGE_ACCESSED)
 #define _PAGE_CHG_MASK	(PAGE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY | _PAGE_SPECIAL)

arch/parisc/kernel/entry.S

@@ -499,6 +499,10 @@
 	 * 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,
@@ -529,6 +533,10 @@
 	 * makes the tlb entry for the differently formatted pa11
 	 * insertion instructions */
 	.macro		make_insert_tlb_11	spc,pte,prot
+#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
 	zdep		\spc,30,15,\prot
 	dep		\pte,8,7,\prot
 	extru,=		\pte,_PAGE_NO_CACHE_BIT,1,%r0