[Qemu-devel] [PATCH RFC 20/36] 9pfs: local: readlink: don't follow symlinks

[Greg Kurz]

This fixes CVE-2016-9602 for all security models.

Signed-off-by: Greg Kurz <gr...@kaod.org>
---
 hw/9pfs/9p-local.c |   37 +++++++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c
index a1fff04c3219..1f9239de07e5 100644
--- a/hw/9pfs/9p-local.c
+++ b/hw/9pfs/9p-local.c
@@ -346,31 +346,48 @@ err:
     return -1;
 }
 
+static int readlink_nofollow(FsContext *fs_ctx, const char *path, char *buf,
+                             size_t bufsz)
+{
+    char *dirpath = local_dirname(path);
+    char *name = local_basename(path);
+    int dirfd, ret = -1;
+
+    dirfd = local_opendir_nofollow(fs_ctx, dirpath);
+    if (dirfd == -1) {
+        goto out;
+    }
+
+    ret = readlinkat(dirfd, name, buf, bufsz);
+    close_preserve_errno(dirfd);
+out:
+    g_free(name);
+    g_free(dirpath);
+    return ret;
+}
+
 static ssize_t local_readlink(FsContext *fs_ctx, V9fsPath *fs_path,
                               char *buf, size_t bufsz)
 {
-    ssize_t tsize = -1;
-    char *buffer;
-    char *path = fs_path->data;
+    ssize_t tsize;
 
     if ((fs_ctx->export_flags & V9FS_SM_MAPPED) ||
         (fs_ctx->export_flags & V9FS_SM_MAPPED_FILE)) {
         int fd;
-        buffer = rpath(fs_ctx, path);
-        fd = open(buffer, O_RDONLY | O_NOFOLLOW);
-        g_free(buffer);
+
+        fd = local_open_nofollow(fs_ctx, fs_path->data, O_RDONLY, 0);
         if (fd == -1) {
             return -1;
         }
         do {
             tsize = read(fd, (void *)buf, bufsz);
         } while (tsize == -1 && errno == EINTR);
-        close(fd);
+        close_preserve_errno(fd);
     } else if ((fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
                (fs_ctx->export_flags & V9FS_SM_NONE)) {
-        buffer = rpath(fs_ctx, path);
-        tsize = readlink(buffer, buf, bufsz);
-        g_free(buffer);
+        tsize = readlink_nofollow(fs_ctx, fs_path->data, buf, bufsz);
+    } else {
+        g_assert_not_reached();
     }
     return tsize;
 }