x86/fpu: Use fault_in_pages_writeable() for pre-faulting

Since commit

   d9c9ce3 ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")

get_user_pages_unlocked() pre-faults user's memory if a write generates
a page fault while the handler is disabled.

This works in general and uncovered a bug as reported by Mike
Rapoport¹. It has been pointed out that this function may be fragile
and a simple pre-fault as in fault_in_pages_writeable() would be a
better solution. Better as in taste and simplicity: that write (as
performed by the alternative function) performs exactly the same
faulting of memory as before. This was suggested by Hugh Dickins and
Andrew Morton.

Use fault_in_pages_writeable() for pre-faulting user's stack.

  [ bigeasy: Write commit message. ]
  [ bp: Massage some. ]

¹ https://lkml.kernel.org/r/[email protected]

Fixes: d9c9ce3 ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")
Suggested-by: Andrew Morton <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Tested-by: Chris Wilson <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: linux-mm <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]

Author: Hugh Dickins

arch/x86/kernel/fpu/signal.c

@@ -5,6 +5,7 @@
 
 #include <linux/compat.h>
 #include <linux/cpu.h>
+#include <linux/pagemap.h>
 
 #include <asm/fpu/internal.h>
 #include <asm/fpu/signal.h>
@@ -189,15 +190,7 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 	fpregs_unlock();
 
 	if (ret) {
-		int aligned_size;
-		int nr_pages;
-
-		aligned_size = offset_in_page(buf_fx) + fpu_user_xstate_size;
-		nr_pages = DIV_ROUND_UP(aligned_size, PAGE_SIZE);
-
-		ret = get_user_pages_unlocked((unsigned long)buf_fx, nr_pages,
-					      NULL, FOLL_WRITE);
-		if (ret == nr_pages)
+		if (!fault_in_pages_writeable(buf_fx, fpu_user_xstate_size))
 			goto retry;
 		return -EFAULT;
 	}