[Trusty][SRU][PATCH 1/1] getxattr: use correct xattr length

Po-Hsu Lin po-hsu.lin at canonical.com
Wed Oct 17 04:17:44 UTC 2018

From: Christian Brauner <christian at brauner.io>

BugLink: https://bugs.launchpad.net/bugs/1798013

When running in a container with a user namespace, if you call getxattr
with name = "system.posix_acl_access" and size % 8 != 4, then getxattr
silently skips the user namespace fixup that it normally does resulting in
un-fixed-up data being returned.
This is caused by posix_acl_fix_xattr_to_user() being passed the total
buffer size and not the actual size of the xattr as returned by
This commit passes the actual length of the xattr as returned by
vfs_getxattr() down.

A reproducer for the issue is:

  touch acl_posix

  setfacl -m user:0:rwx acl_posix

and the compile:

  #define _GNU_SOURCE
  #include <errno.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <sys/types.h>
  #include <unistd.h>
  #include <attr/xattr.h>

  /* Run in user namespace with nsuid 0 mapped to uid != 0 on the host. */
  int main(int argc, void **argv)
          ssize_t ret1, ret2;
          char buf1[128], buf2[132];
          int fret = EXIT_SUCCESS;
          char *file;

          if (argc < 2) {
                          "Please specify a file with "
                          "\"system.posix_acl_access\" permissions set\n");
          file = argv[1];

          ret1 = getxattr(file, "system.posix_acl_access",
                          buf1, sizeof(buf1));
          if (ret1 < 0) {
                  fprintf(stderr, "%s - Failed to retrieve "
                                  "\"system.posix_acl_access\" "
                                  "from \"%s\"\n", strerror(errno), file);

          ret2 = getxattr(file, "system.posix_acl_access",
                          buf2, sizeof(buf2));
          if (ret2 < 0) {
                  fprintf(stderr, "%s - Failed to retrieve "
                                  "\"system.posix_acl_access\" "
                                  "from \"%s\"\n", strerror(errno), file);

          if (ret1 != ret2) {
                  fprintf(stderr, "The value of \"system.posix_acl_"
                                  "access\" for file \"%s\" changed "
                                  "between two successive calls\n", file);

          for (ssize_t i = 0; i < ret2; i++) {
                  if (buf1[i] == buf2[i])

                          "Unexpected different in byte %zd: "
                          "%02x != %02x\n", i, buf1[i], buf2[i]);
                  fret = EXIT_FAILURE;

          if (fret == EXIT_SUCCESS)
                  fprintf(stderr, "Test passed\n");
                  fprintf(stderr, "Test failed\n");

and run:

  ./tester acl_posix

On a non-fixed up kernel this should return something like:

  root at c1:/# ./t
  Unexpected different in byte 16: ffffffa0 != 00
  Unexpected different in byte 17: ffffff86 != 00
  Unexpected different in byte 18: 01 != 00

and on a fixed kernel:

  root at c1:~# ./t
  Test passed

Cc: stable at vger.kernel.org
Fixes: 2f6f0654ab61 ("userns: Convert vfs posix_acl support to use kuids and kgids")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=199945
Reported-by: Colin Watson <cjwatson at ubuntu.com>
Signed-off-by: Christian Brauner <christian at brauner.io>
Acked-by: Serge Hallyn <serge at hallyn.com>
Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
(cherry picked from commit 82c9a927bc5df6e06b72d206d24a9d10cced4eb5)
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
 fs/xattr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/xattr.c b/fs/xattr.c
index 3377dff..afc74ba 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -466,7 +466,7 @@ getxattr(struct dentry *d, const char __user *name, void __user *value,
 	if (error > 0) {
 		if ((strcmp(kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
 		    (strcmp(kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
-			posix_acl_fix_xattr_to_user(kvalue, size);
+			posix_acl_fix_xattr_to_user(kvalue, error);
 		if (size && copy_to_user(value, kvalue, error))
 			error = -EFAULT;
 	} else if (error == -ERANGE && size >= XATTR_SIZE_MAX) {

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