[POWERPC] Provide a way to protect 4k subpages when using 64k pages

Using 64k pages on 64-bit PowerPC systems makes life difficult for
emulators that are trying to emulate an ISA, such as x86, which use a
smaller page size, since the emulator can no longer use the MMU and
the normal system calls for controlling page protections.  Of course,
the emulator can emulate the MMU by checking and possibly remapping
the address for each memory access in software, but that is pretty
slow.

This provides a facility for such programs to control the access
permissions on individual 4k sub-pages of 64k pages.  The idea is
that the emulator supplies an array of protection masks to apply to a
specified range of virtual addresses.  These masks are applied at the
level where hardware PTEs are inserted into the hardware page table
based on the Linux PTEs, so the Linux PTEs are not affected.  Note
that this new mechanism does not allow any access that would otherwise
be prohibited; it can only prohibit accesses that would otherwise be
allowed.  This new facility is only available on 64-bit PowerPC and
only when the kernel is configured for 64k pages.

The masks are supplied using a new subpage_prot system call, which
takes a starting virtual address and length, and a pointer to an array
of protection masks in memory.  The array has a 32-bit word per 64k
page to be protected; each 32-bit word consists of 16 2-bit fields,
for which 0 allows any access (that is otherwise allowed), 1 prevents
write accesses, and 2 or 3 prevent any access.

Implicit in this is that the regions of the address space that are
protected are switched to use 4k hardware pages rather than 64k
hardware pages (on machines with hardware 64k page support).  In fact
the whole process is switched to use 4k hardware pages when the
subpage_prot system call is used, but this could be improved in future
to switch only the affected segments.

The subpage protection bits are stored in a 3 level tree akin to the
page table tree.  The top level of this tree is stored in a structure
that is appended to the top level of the page table tree, i.e., the
pgd array.  Since it will often only be 32-bit addresses (below 4GB)
that are protected, the pointers to the first four bottom level pages
are also stored in this structure (each bottom level page contains the
protection bits for 1GB of address space), so the protection bits for
addresses below 4GB can be accessed with one fewer loads than those
for higher addresses.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 69 insertions, 14 deletions

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index 9326a69..7b4cacb 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -637,7 +637,7 @@ unsigned int hash_page_do_lazy_icache(unsigned int pp, pte_t pte, int trap)
  * For now this makes the whole process use 4k pages.
  */
 #ifdef CONFIG_PPC_64K_PAGES
-static void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
+void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
 {
 	if (mm->context.user_psize == MMU_PAGE_4K)
 		return;
@@ -645,13 +645,62 @@ static void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
 #ifdef CONFIG_SPU_BASE
 	spu_flush_all_slbs(mm);
 #endif
+	if (get_paca()->context.user_psize != MMU_PAGE_4K) {
+		get_paca()->context = mm->context;
+		slb_flush_and_rebolt();
+	}
 }
 #endif /* CONFIG_PPC_64K_PAGES */
 
+#ifdef CONFIG_PPC_SUBPAGE_PROT
+/*
+ * This looks up a 2-bit protection code for a 4k subpage of a 64k page.
+ * Userspace sets the subpage permissions using the subpage_prot system call.
+ *
+ * Result is 0: full permissions, _PAGE_RW: read-only,
+ * _PAGE_USER or _PAGE_USER|_PAGE_RW: no access.
+ */
+static int subpage_protection(pgd_t *pgdir, unsigned long ea)
+{
+	struct subpage_prot_table *spt = pgd_subpage_prot(pgdir);
+	u32 spp = 0;
+	u32 **sbpm, *sbpp;
+
+	if (ea >= spt->maxaddr)
+		return 0;
+	if (ea < 0x100000000) {
+		/* addresses below 4GB use spt->low_prot */
+		sbpm = spt->low_prot;
+	} else {
+		sbpm = spt->protptrs[ea >> SBP_L3_SHIFT];
+		if (!sbpm)
+			return 0;
+	}
+	sbpp = sbpm[(ea >> SBP_L2_SHIFT) & (SBP_L2_COUNT - 1)];
+	if (!sbpp)
+		return 0;
+	spp = sbpp[(ea >> PAGE_SHIFT) & (SBP_L1_COUNT - 1)];
+
+	/* extract 2-bit bitfield for this 4k subpage */
+	spp >>= 30 - 2 * ((ea >> 12) & 0xf);
+
+	/* turn 0,1,2,3 into combination of _PAGE_USER and _PAGE_RW */
+	spp = ((spp & 2) ? _PAGE_USER : 0) | ((spp & 1) ? _PAGE_RW : 0);
+	return spp;
+}
+
+#else /* CONFIG_PPC_SUBPAGE_PROT */
+static inline int subpage_protection(pgd_t *pgdir, unsigned long ea)
+{
+	return 0;
+}
+#endif
+
 /* Result code is:
  *  0 - handled
  *  1 - normal page fault
  * -1 - critical hash insertion error
+ * -2 - access not permitted by subpage protection mechanism
  */
 int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
 {
@@ -802,7 +851,14 @@ int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
 		rc = __hash_page_64K(ea, access, vsid, ptep, trap, local, ssize);
 	else
 #endif /* CONFIG_PPC_HAS_HASH_64K */
-		rc = __hash_page_4K(ea, access, vsid, ptep, trap, local, ssize);
+	{
+		int spp = subpage_protection(pgdir, ea);
+		if (access & spp)
+			rc = -2;
+		else
+			rc = __hash_page_4K(ea, access, vsid, ptep, trap,
+					    local, ssize, spp);
+	}
 
 #ifndef CONFIG_PPC_64K_PAGES
 	DBG_LOW(" o-pte: %016lx\n", pte_val(*ptep));
@@ -874,7 +930,8 @@ void hash_preload(struct mm_struct *mm, unsigned long ea,
 		__hash_page_64K(ea, access, vsid, ptep, trap, local, ssize);
 	else
 #endif /* CONFIG_PPC_HAS_HASH_64K */
-		__hash_page_4K(ea, access, vsid, ptep, trap, local, ssize);
+		__hash_page_4K(ea, access, vsid, ptep, trap, local, ssize,
+			       subpage_protection(pgdir, ea));
 
 	local_irq_restore(flags);
 }
@@ -919,19 +976,17 @@ void flush_hash_range(unsigned long number, int local)
  * low_hash_fault is called when we the low level hash code failed
  * to instert a PTE due to an hypervisor error
  */
-void low_hash_fault(struct pt_regs *regs, unsigned long address)
+void low_hash_fault(struct pt_regs *regs, unsigned long address, int rc)
 {
 	if (user_mode(regs)) {
-		siginfo_t info;
-
-		info.si_signo = SIGBUS;
-		info.si_errno = 0;
-		info.si_code = BUS_ADRERR;
-		info.si_addr = (void __user *)address;
-		force_sig_info(SIGBUS, &info, current);
-		return;
-	}
-	bad_page_fault(regs, address, SIGBUS);
+#ifdef CONFIG_PPC_SUBPAGE_PROT
+		if (rc == -2)
+			_exception(SIGSEGV, regs, SEGV_ACCERR, address);
+		else
+#endif
+			_exception(SIGBUS, regs, BUS_ADRERR, address);
+	} else
+		bad_page_fault(regs, address, SIGBUS);
 }
 
 #ifdef CONFIG_DEBUG_PAGEALLOC