nptl: Zero-extend arguments to SETXID syscalls [BZ #26248]

nptl has

/* Opcodes and data types for communication with the signal handler to
   change user/group IDs.  */
struct xid_command
{
  int syscall_no;
  long int id[3];
  volatile int cntr;
  volatile int error;
};

 /* This must be last, otherwise the current thread might not have
     permissions to send SIGSETXID syscall to the other threads.  */
  result = INTERNAL_SYSCALL_NCS (cmdp->syscall_no, 3,
                                 cmdp->id[0], cmdp->id[1], cmdp->id[2]);

But the second argument of setgroups syscal is a pointer:

       int setgroups (size_t size, const gid_t *list);

But on x32, pointers passed to syscall must have pointer type so that
they will be zero-extended.  The kernel XID arguments are unsigned and
do not require sign extension.  Change xid_command to

struct xid_command
{
  int syscall_no;
  unsigned long int id[3];
  volatile int cntr;
  volatile int error;
};

so that all arguments are zero-extended.  A testcase is added for x32 and
setgroups returned with EFAULT when running as root without the fix.

1 files changed, 7 insertions, 1 deletions

diff --git a/nptl/descr.h b/nptl/descr.h
index 6a509b6..d8343ff 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -95,7 +95,13 @@ struct pthread_unwind_buf
 struct xid_command
 {
   int syscall_no;
-  long int id[3];
+  /* Enforce zero-extension for the pointer argument in
+
+     int setgroups (size_t size, const gid_t *list);
+
+     The kernel XID arguments are unsigned and do not require sign
+     extension.  */
+  unsigned long int id[3];
   volatile int cntr;
   volatile int error; /* -1: no call yet, 0: success seen, >0: error seen.  */
 };