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

[Sebastian Andrzej Siewior]

From: Hugh Dickins <hughd@xxxxxxxxxx>

Since commit

   d9c9ce34ed5c8 ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")

we use get_user_pages_unlocked() to pre-faulting 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 Rapoport.

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
that we had before. This was suggested by Hugh Dickins and Andrew
Morton.

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

Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/alpine.LSU.2.11.1905251033230.1112@xxxxxxxxxxxx
[bigeasy: patch description]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 arch/x86/kernel/fpu/signal.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 5a8d118bc423e..060d6188b4533 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/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;
 	}
-- 
2.20.1