x86/ioperm: Prevent a memory leak when fork fails

In the copy_process() routine called by _do_fork(), failure to allocate
a PID (or further along in the function) will trigger an invocation to
exit_thread(). This is done to clean up from an earlier call to
copy_thread_tls(). Naturally, the child task is passed into exit_thread(),
however during the process, io_bitmap_exit() nullifies the parent's
io_bitmap rather than the child's.

As copy_thread_tls() has been called ahead of the failure, the reference
count on the calling thread's io_bitmap is incremented as we would expect.
However, io_bitmap_exit() doesn't accept any arguments, and thus assumes
it should trash the current thread's io_bitmap reference rather than the
child's. This is pretty sneaky in practice, because in all instances but
this one, exit_thread() is called with respect to the current task and
everything works out.

A determined attacker can issue an appropriate ioctl (i.e. KDENABIO) to
get a bitmap allocated, and force a clone3() syscall to fail by passing
in a zeroed clone_args structure. The kernel handles the erroneous struct
and the buggy code path is followed, and even though the parent's reference
to the io_bitmap is trashed, the child still holds a reference and thus
the structure will never be freed.

Fix this by tweaking io_bitmap_exit() and its subroutines to accept a
task_struct argument which to operate on.

Fixes: ea5f1cd7ab49 ("x86/ioperm: Remove bitmap if all permissions dropped")
Signed-off-by: Jay Lang <jaytlang@mit.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable#@vger.kernel.org
Link: https://lkml.kernel.org/r/20200524162742.253727-1-jaytlang@mit.edu

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9da70b279dad..35638f1c5791 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -96,7 +96,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 }
 
 /*
- * Free current thread data structures etc..
+ * Free thread data structures etc..
  */
 void exit_thread(struct task_struct *tsk)
 {
@@ -104,7 +104,7 @@ void exit_thread(struct task_struct *tsk)
 	struct fpu *fpu = &t->fpu;
 
 	if (test_thread_flag(TIF_IO_BITMAP))
-		io_bitmap_exit();
+		io_bitmap_exit(tsk);
 
 	free_vm86(t);