9pfs: local: lgetxattr: don't follow symlinks

The local_lgetxattr() callback is vulnerable to symlink attacks because
it calls lgetxattr() which follows symbolic links in all path elements but
the rightmost one.

This patch introduces a helper to emulate the non-existing fgetxattrat()
function: it is implemented with /proc/self/fd which provides a trusted
path that can be safely passed to lgetxattr().

local_lgetxattr() is converted to use this helper and opendir_nofollow().

This partly fixes CVE-2016-9602.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/hw/9pfs/9p-posix-acl.c b/hw/9pfs/9p-posix-acl.c
index ec003181cd33d6e3c87f5a9b448e3f99d43c770f..9435e27a368c5d09769bc30c0affacbc4c5ce1bc 100644 (file)
--- a/hw/9pfs/9p-posix-acl.c
+++ b/hw/9pfs/9p-posix-acl.c
@@ -25,13 +25,7 @@
 static ssize_t mp_pacl_getxattr(FsContext *ctx, const char *path,
                                 const char *name, void *value, size_t size)
 {
-    char *buffer;
-    ssize_t ret;
-
-    buffer = rpath(ctx, path);
-    ret = lgetxattr(buffer, MAP_ACL_ACCESS, value, size);
-    g_free(buffer);
-    return ret;
+    return local_getxattr_nofollow(ctx, path, MAP_ACL_ACCESS, value, size);
 }
 
 static ssize_t mp_pacl_listxattr(FsContext *ctx, const char *path,
@@ -89,13 +83,7 @@ static int mp_pacl_removexattr(FsContext *ctx,
 static ssize_t mp_dacl_getxattr(FsContext *ctx, const char *path,
                                 const char *name, void *value, size_t size)
 {
-    char *buffer;
-    ssize_t ret;
-
-    buffer = rpath(ctx, path);
-    ret = lgetxattr(buffer, MAP_ACL_DEFAULT, value, size);
-    g_free(buffer);
-    return ret;
+    return local_getxattr_nofollow(ctx, path, MAP_ACL_DEFAULT, value, size);
 }
 
 static ssize_t mp_dacl_listxattr(FsContext *ctx, const char *path,

diff --git a/hw/9pfs/9p-util.c b/hw/9pfs/9p-util.c
index 54134b0bb42439f86529fe4573a68e0f631993ae..fdb4d5737635ee1cb43f8ed4c233a3527077312c 100644 (file)
--- a/hw/9pfs/9p-util.c
+++ b/hw/9pfs/9p-util.c
@@ -11,6 +11,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/xattr.h"
 #include "9p-util.h"
 
 int relative_openat_nofollow(int dirfd, const char *path, int flags,
@@ -55,3 +56,14 @@ int relative_openat_nofollow(int dirfd, const char *path, int flags,
 
     return fd;
 }
+
+ssize_t fgetxattrat_nofollow(int dirfd, const char *filename, const char *name,
+                             void *value, size_t size)
+{
+    char *proc_path = g_strdup_printf("/proc/self/fd/%d/%s", dirfd, filename);
+    int ret;
+
+    ret = lgetxattr(proc_path, name, value, size);
+    g_free(proc_path);
+    return ret;
+}

diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index e80b5a551d0aaf9c2057d54004f296a2f6842b02..676641f7c0e0b09d952bb6cdbd3d3f0067426607 100644 (file)
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -46,5 +46,7 @@ static inline int openat_file(int dirfd, const char *name, int flags,
 
 int relative_openat_nofollow(int dirfd, const char *path, int flags,
                              mode_t mode);
+ssize_t fgetxattrat_nofollow(int dirfd, const char *path, const char *name,
+                             void *value, size_t size);
 
 #endif

diff --git a/hw/9pfs/9p-xattr-user.c b/hw/9pfs/9p-xattr-user.c
index f87530c8b5269f914ce0b0527acf6857aa69b596..4071fbc4c08699e650b7be7b0473627c9f844e73 100644 (file)
--- a/hw/9pfs/9p-xattr-user.c
+++ b/hw/9pfs/9p-xattr-user.c
@@ -20,9 +20,6 @@
 static ssize_t mp_user_getxattr(FsContext *ctx, const char *path,
                                 const char *name, void *value, size_t size)
 {
-    char *buffer;
-    ssize_t ret;
-
     if (strncmp(name, "user.virtfs.", 12) == 0) {
         /*
          * Don't allow fetch of user.virtfs namesapce
@@ -31,10 +28,7 @@ static ssize_t mp_user_getxattr(FsContext *ctx, const char *path,
         errno = ENOATTR;
         return -1;
     }
-    buffer = rpath(ctx, path);
-    ret = lgetxattr(buffer, name, value, size);
-    g_free(buffer);
-    return ret;
+    return local_getxattr_nofollow(ctx, path, name, value, size);
 }
 
 static ssize_t mp_user_listxattr(FsContext *ctx, const char *path,

diff --git a/hw/9pfs/9p-xattr.c b/hw/9pfs/9p-xattr.c
index 19a2daf02f5c08ebfc02cfa2c254cd252a37e534..aa4391e6b317959942b6f81c88c2a6ea7d1c6606 100644 (file)
--- a/hw/9pfs/9p-xattr.c
+++ b/hw/9pfs/9p-xattr.c
@@ -15,6 +15,8 @@
 #include "9p.h"
 #include "fsdev/file-op-9p.h"
 #include "9p-xattr.h"
+#include "9p-util.h"
+#include "9p-local.h"
 
 
 static XattrOperations *get_xattr_operations(XattrOperations **h,
@@ -143,18 +145,33 @@ int v9fs_remove_xattr(FsContext *ctx,
 
 }
 
-ssize_t pt_getxattr(FsContext *ctx, const char *path, const char *name,
-                    void *value, size_t size)
+ssize_t local_getxattr_nofollow(FsContext *ctx, const char *path,
+                                const char *name, void *value, size_t size)
 {
-    char *buffer;
-    ssize_t ret;
+    char *dirpath = g_path_get_dirname(path);
+    char *filename = g_path_get_basename(path);
+    int dirfd;
+    ssize_t ret = -1;
+
+    dirfd = local_opendir_nofollow(ctx, dirpath);
+    if (dirfd == -1) {
+        goto out;
+    }
 
-    buffer = rpath(ctx, path);
-    ret = lgetxattr(buffer, name, value, size);
-    g_free(buffer);
+    ret = fgetxattrat_nofollow(dirfd, filename, name, value, size);
+    close_preserve_errno(dirfd);
+out:
+    g_free(dirpath);
+    g_free(filename);
     return ret;
 }
 
+ssize_t pt_getxattr(FsContext *ctx, const char *path, const char *name,
+                    void *value, size_t size)
+{
+    return local_getxattr_nofollow(ctx, path, name, value, size);
+}
+
 int pt_setxattr(FsContext *ctx, const char *path, const char *name, void *value,
                 size_t size, int flags)
 {

diff --git a/hw/9pfs/9p-xattr.h b/hw/9pfs/9p-xattr.h
index 3f43f5153f3c2f1856650d7f3a67524cee5f9d1e..69a8b6b62e3c31f1fd75ae3e099b40e747761754 100644 (file)
--- a/hw/9pfs/9p-xattr.h
+++ b/hw/9pfs/9p-xattr.h
@@ -29,6 +29,8 @@ typedef struct xattr_operations
                        const char *path, const char *name);
 } XattrOperations;
 
+ssize_t local_getxattr_nofollow(FsContext *ctx, const char *path,
+                                const char *name, void *value, size_t size);
 
 extern XattrOperations mapped_user_xattr;
 extern XattrOperations passthrough_user_xattr;