fs/xattr.c: zero out memory copied to userspace in getxattr

getxattr uses vmalloc to allocate memory if kzalloc fails. This is
filled by vfs_getxattr and then copied to the userspace. vmalloc,
however, doesn't zero out the memory so if the specific implementation
of the xattr handler is sloppy we can theoretically expose a kernel
memory. There is no real sign this is really the case but let's make
sure this will not happen and use vzalloc instead.

Fixes: 779302e67835 ("fs/xattr.c:getxattr(): improve handling of allocation failures")
Link: http://lkml.kernel.org/r/[e-mail]
Acked-by: Kees Cook <[e-mail]>
Reported-by: Vlastimil Babka <[e-mail]>
Signed-off-by: Michal Hocko <[e-mail]>
Cc: <[e-mail]> [3.6+]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/xattr.c

···
 			size = XATTR_SIZE_MAX;
 		kvalue = kzalloc(size, GFP_KERNEL | __GFP_NOWARN);
 		if (!kvalue) {
-			kvalue = vmalloc(size);
+			kvalue = vzalloc(size);
 			if (!kvalue)
 				return -ENOMEM;
 		}