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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Fri, 2 Jan 2015 01:05:09 +0000 (UTC)
To: src-committers@freebsd.org, svn-src-all@freebsd.org,
 svn-src-head@freebsd.org
Subject: svn commit: r276523 - head/sys/amd64/amd64
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Author: kib
Date: Fri Jan  2 01:05:08 2015
New Revision: 276523
URL: https://svnweb.freebsd.org/changeset/base/276523

Log:
  Callers of pmap_kextract() cannot distinguish between failure and
  physical address zero.  Assume that the lowest page is always mapped
  by direct map.
  
  This restores access to the page at zero through /dev/mem after
  r263475.
  
  Reported and tested by:	neel
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week

Modified:
  head/sys/amd64/amd64/mem.c

Modified: head/sys/amd64/amd64/mem.c
==============================================================================
--- head/sys/amd64/amd64/mem.c	Fri Jan  2 01:00:38 2015	(r276522)
+++ head/sys/amd64/amd64/mem.c	Fri Jan  2 01:05:08 2015	(r276523)
@@ -77,7 +77,7 @@ int
 memrw(struct cdev *dev, struct uio *uio, int flags)
 {
 	struct iovec *iov;
-	u_long c, v;
+	u_long c, v, vd;
 	int error, o, sflags;
 	vm_offset_t addr, eaddr;
 
@@ -98,15 +98,15 @@ memrw(struct cdev *dev, struct uio *uio,
 kmemphys:
 			o = v & PAGE_MASK;
 			c = min(uio->uio_resid, (u_int)(PAGE_SIZE - o));
-			v = PHYS_TO_DMAP(v);
-			if (v < DMAP_MIN_ADDRESS ||
-			    (v > DMAP_MIN_ADDRESS + dmaplimit &&
-			    v <= DMAP_MAX_ADDRESS) ||
-			    pmap_kextract(v) == 0) {
+			vd = PHYS_TO_DMAP(v);
+			if (vd < DMAP_MIN_ADDRESS ||
+			    (vd > DMAP_MIN_ADDRESS + dmaplimit &&
+			    vd <= DMAP_MAX_ADDRESS) ||
+			    (pmap_kextract(vd) == 0 && (v & PG_FRAME) != 0)) {
 				error = EFAULT;
 				goto ret;
 			}
-			error = uiomove((void *)v, (int)c, uio);
+			error = uiomove((void *)vd, (int)c, uio);
 			continue;
 		} else if (dev2unit(dev) == CDEV_MINOR_KMEM) {
 			v = uio->uio_offset;