Commit 4bfe6cce authored by Jay Lang's avatar Jay Lang Committed by Thomas Gleixner
Browse files

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: ea5f1cd7 ("x86/ioperm: Remove bitmap if all permissions dropped")
Signed-off-by: default avatarJay Lang <jaytlang@mit.edu>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: stable#@vger.kernel.org
Link: https://lkml.kernel.org/r/20200524162742.253727-1-jaytlang@mit.edu
parent 88743470

arch/x86/include/asm/io_bitmap.h

+2 −2
Original line number Diff line number Diff line
@@ -17,7 +17,7 @@ struct task_struct; 


#ifdef CONFIG_X86_IOPL_IOPERM

void io_bitmap_share(struct task_struct *tsk);

void io_bitmap_exit(void);

void io_bitmap_exit(struct task_struct *tsk);



void native_tss_update_io_bitmap(void);
@@ -29,7 +29,7 @@ void native_tss_update_io_bitmap(void); 


#else

static inline void io_bitmap_share(struct task_struct *tsk) { }

static inline void io_bitmap_exit(void) { }

static inline void io_bitmap_exit(struct task_struct *tsk) { }

static inline void tss_update_io_bitmap(void) { }

#endif

arch/x86/kernel/ioport.c

+11 −11
Original line number Diff line number Diff line
@@ -33,15 +33,15 @@ void io_bitmap_share(struct task_struct *tsk) 
	set_tsk_thread_flag(tsk, TIF_IO_BITMAP);

}



static void task_update_io_bitmap(void)

static void task_update_io_bitmap(struct task_struct *tsk)

{

	struct thread_struct *t = &current->thread;

	struct thread_struct *t = &tsk->thread;



	if (t->iopl_emul == 3 || t->io_bitmap) {

		/* TSS update is handled on exit to user space */

		set_thread_flag(TIF_IO_BITMAP);

		set_tsk_thread_flag(tsk, TIF_IO_BITMAP);

	} else {

		clear_thread_flag(TIF_IO_BITMAP);

		clear_tsk_thread_flag(tsk, TIF_IO_BITMAP);

		/* Invalidate TSS */

		preempt_disable();

		tss_update_io_bitmap();
@@ -49,12 +49,12 @@ static void task_update_io_bitmap(void) 
	}

}



void io_bitmap_exit(void)

void io_bitmap_exit(struct task_struct *tsk)

{

	struct io_bitmap *iobm = current->thread.io_bitmap;

	struct io_bitmap *iobm = tsk->thread.io_bitmap;



	current->thread.io_bitmap = NULL;

	task_update_io_bitmap();

	tsk->thread.io_bitmap = NULL;

	task_update_io_bitmap(tsk);

	if (iobm && refcount_dec_and_test(&iobm->refcnt))

		kfree(iobm);

}
@@ -102,7 +102,7 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on) 
		if (!iobm)

			return -ENOMEM;

		refcount_set(&iobm->refcnt, 1);

		io_bitmap_exit();

		io_bitmap_exit(current);

	}



	/*
@@ -134,7 +134,7 @@ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on) 
	}

	/* All permissions dropped? */

	if (max_long == UINT_MAX) {

		io_bitmap_exit();

		io_bitmap_exit(current);

		return 0;

	}
@@ -192,7 +192,7 @@ SYSCALL_DEFINE1(iopl, unsigned int, level) 
	}



	t->iopl_emul = level;

	task_update_io_bitmap();

	task_update_io_bitmap(current);



	return 0;

}

arch/x86/kernel/process.c

+2 −2
Original line number Diff line number Diff line
@@ -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);

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