x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling

All copy_to_user() implementations need to be prepared to handle faults
accessing userspace. The __memcpy_mcsafe() implementation handles both
mmu-faults on the user destination and machine-check-exceptions on the
source buffer. However, the memcpy_mcsafe() wrapper may silently
fallback to memcpy() depending on build options and cpu-capabilities.

Force copy_to_user_mcsafe() to always use __memcpy_mcsafe() when
available, and otherwise disable all of the copy_to_user_mcsafe()
infrastructure when __memcpy_mcsafe() is not available, i.e.
CONFIG_X86_MCE=n.

This fixes crashes of the form:
    run fstests generic/323 at 2018-07-02 12:46:23
    BUG: unable to handle kernel paging request at 00007f0d50001000
    RIP: 0010:__memcpy+0x12/0x20
    [..]
    Call Trace:
     copyout_mcsafe+0x3a/0x50
     _copy_to_iter_mcsafe+0xa1/0x4a0
     ? dax_alive+0x30/0x50
     dax_iomap_actor+0x1f9/0x280
     ? dax_iomap_rw+0x100/0x100
     iomap_apply+0xba/0x130
     ? dax_iomap_rw+0x100/0x100
     dax_iomap_rw+0x95/0x100
     ? dax_iomap_rw+0x100/0x100
     xfs_file_dax_read+0x7b/0x1d0 [xfs]
     xfs_file_read_iter+0xa7/0xc0 [xfs]
     aio_read+0x11c/0x1a0

Reported-by: Ross Zwisler <[email protected]>
Tested-by: Ross Zwisler <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tony Luck <[email protected]>
Fixes: 8780356 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Link: http://lkml.kernel.org/r/153108277790.37979.1486841789275803399.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/Kconfig

@@ -63,7 +63,7 @@ config X86
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_REFCOUNT
 	select ARCH_HAS_UACCESS_FLUSHCACHE	if X86_64
-	select ARCH_HAS_UACCESS_MCSAFE		if X86_64
+	select ARCH_HAS_UACCESS_MCSAFE		if X86_64 && X86_MCE
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAS_STRICT_KERNEL_RWX

arch/x86/include/asm/uaccess_64.h

@@ -52,7 +52,12 @@ copy_to_user_mcsafe(void *to, const void *from, unsigned len)
 	unsigned long ret;
 
 	__uaccess_begin();
-	ret = memcpy_mcsafe(to, from, len);
+	/*
+	 * Note, __memcpy_mcsafe() is explicitly used since it can
+	 * handle exceptions / faults.  memcpy_mcsafe() may fall back to
+	 * memcpy() which lacks this handling.
+	 */
+	ret = __memcpy_mcsafe(to, from, len);
 	__uaccess_end();
 	return ret;
 }