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

diff --git a/arch/x86/include/asm/io_bitmap.h b/arch/x86/include/asm/io_bitmap.h
index 07344d82e88ee6b28e4bb2040932eb1135a3b4a3..ac1a99ffbd8d263e4dc2e8a6cba3b589a9913d58 100644 (file)
--- a/arch/x86/include/asm/io_bitmap.h
+++ b/arch/x86/include/asm/io_bitmap.h
@@ -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
 

diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c
index a53e7b4a741925ba9048fcbba3030bccc9f3766e..e2fab3ceb09fb7da1d307e69af014b24ea65735f 100644 (file)
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -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;
 }

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9da70b279dad8c36a68b6c4c5f2862ad394d849f..35638f1c5791f17247d9c046351435cb75f119f6 100644 (file)
--- 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);