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

[tip-bot for Hugh Dickins]

Commit-ID:  b81ff1013eb8eef2934ca7e8cf53d553c1029e84
Gitweb:     https://git.kernel.org/tip/b81ff1013eb8eef2934ca7e8cf53d553c1029e84
Author:     Hugh Dickins <hughd@xxxxxxxxxx>
AuthorDate: Wed, 29 May 2019 09:25:40 +0200
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Thu, 6 Jun 2019 19:15:17 +0200

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

Since commit

   d9c9ce34ed5c8 ("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
RapoportÂ. 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. ]

 https://lkml.kernel.org/r/1557844195-18882-1-git-send-email-rppt@xxxxxxxxxxxxx

Fixes: d9c9ce34ed5c8 ("x86/fpu: Fault-in user stack if copy_fpstate_to_sigframe() fails")
Suggested-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Tested-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: linux-mm <linux-mm@xxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190529072540.g46j4kfeae37a3iu@xxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/1557844195-18882-1-git-send-email-rppt@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 5a8d118bc423..060d6188b453 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 @@ retry:
 	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;
 	}