x86/mm/pat: Fix BUG_ON() in mmap_mem() on QEMU/i386

The following BUG_ON() crash was reported on QEMU/i386:

  kernel BUG at arch/x86/mm/physaddr.c:79!
  Call Trace:
  phys_mem_access_prot_allowed
  mmap_mem
  ? mmap_region
  mmap_region
  do_mmap
  vm_mmap_pgoff
  SyS_mmap_pgoff
  do_int80_syscall_32
  entry_INT80_32

after commit:

  edfe63ec97ed ("x86/mtrr: Fix Xorg crashes in Qemu sessions")

PAT is now set to disabled state when MTRRs are disabled.
Thus, reactivating the __pa(high_memory) check in
phys_mem_access_prot_allowed().

When CONFIG_DEBUG_VIRTUAL is set, __pa() calls __phys_addr(),
which in turn calls slow_virt_to_phys() for 'high_memory'.
Because 'high_memory' is set to (the max direct mapped virt
addr + 1), it is not a valid virtual address.  Hence,
slow_virt_to_phys() returns 0 and hit the BUG_ON.  Using
__pa_nodebug() instead of __pa() will fix this BUG_ON.

However, this code block, originally written for Pentiums and
earlier, is no longer adequate since a 32-bit Xen guest has
MTRRs disabled and supports ZONE_HIGHMEM.  In this setup,
this code sets UC attribute for accessing RAM in high memory
range.

Delete this code block as it has been unused for a long time.

Reported-by: kernel test robot <ying.huang@linux.intel.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/1460403360-25441-1-git-send-email-toshi.kani@hpe.com
Link: https://lkml.org/lkml/2016/4/1/608
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index c4c3ddcc9069d2cac227e6a8100bbf26333c6c2d..fb0604f11eec268a2cc69b1d47e3885b073c6cf1 100644 (file)
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -778,25 +778,6 @@ int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
        if (file->f_flags & O_DSYNC)
                pcm = _PAGE_CACHE_MODE_UC_MINUS;
 
-#ifdef CONFIG_X86_32
-       /*
-        * On the PPro and successors, the MTRRs are used to set
-        * memory types for physical addresses outside main memory,
-        * so blindly setting UC or PWT on those pages is wrong.
-        * For Pentiums and earlier, the surround logic should disable
-        * caching for the high addresses through the KEN pin, but
-        * we maintain the tradition of paranoia in this code.
-        */
-       if (!pat_enabled() &&
-           !(boot_cpu_has(X86_FEATURE_MTRR) ||
-             boot_cpu_has(X86_FEATURE_K6_MTRR) ||
-             boot_cpu_has(X86_FEATURE_CYRIX_ARR) ||
-             boot_cpu_has(X86_FEATURE_CENTAUR_MCR)) &&
-           (pfn << PAGE_SHIFT) >= __pa(high_memory)) {
-               pcm = _PAGE_CACHE_MODE_UC;
-       }
-#endif
-
        *vma_prot = __pgprot((pgprot_val(*vma_prot) & ~_PAGE_CACHE_MASK) |
                             cachemode2protval(pcm));
        return 1;