Add comment describing why we need to clear the buffer if uiomove()

fails, adapted from FreeBSD.  Also avoid clearing the buffer if it
was cleared when allocated.  OK deraadt@ otto@

diff --git a/sys/ufs/ffs/ffs_vnops.c b/sys/ufs/ffs/ffs_vnops.c
index 87cd37d..0d14649 100644 (file)
--- a/sys/ufs/ffs/ffs_vnops.c
+++ b/sys/ufs/ffs/ffs_vnops.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: ffs_vnops.c,v 1.89 2017/12/30 20:47:00 guenther Exp $ */
+/*     $OpenBSD: ffs_vnops.c,v 1.90 2018/01/13 15:56:02 millert Exp $  */
 /*     $NetBSD: ffs_vnops.c,v 1.7 1996/05/11 18:27:24 mycroft Exp $    */
 
 /*
@@ -357,8 +357,17 @@ ffs_write(void *v)
                        xfersize = size;
 
                error = uiomove(bp->b_data + blkoffset, xfersize, uio);
-
-               if (error != 0)
+               /*
+                * If the buffer is not already filled and we encounter an
+                * error while trying to fill it, we have to clear out any
+                * garbage data from the pages instantiated for the buffer.
+                * If we do not, a failed uiomove() during a write can leave
+                * the prior contents of the pages exposed to a userland mmap.
+                *
+                * Note that we don't need to clear buffers that were
+                * allocated with the B_CLRBUF flag set.
+                 */
+               if (error != 0 && !(flags & B_CLRBUF))
                        memset(bp->b_data + blkoffset, 0, xfersize);
 
 #if 0