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Subject[tip: x86/urgent] x86/ioperm: Prevent a memory leak when fork fails
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 4bfe6cce133cad82cea04490c308795275857782
Gitweb: https://git.kernel.org/tip/4bfe6cce133cad82cea04490c308795275857782
Author: Jay Lang <jaytlang@mit.edu>
AuthorDate: Sun, 24 May 2020 12:27:39 -04:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 28 May 2020 21:36:20 +02:00

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
---
arch/x86/include/asm/io_bitmap.h | 4 ++--
arch/x86/kernel/ioport.c | 22 +++++++++++-----------
arch/x86/kernel/process.c | 4 ++--
3 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/io_bitmap.h b/arch/x86/include/asm/io_bitmap.h
index 07344d8..ac1a99f 100644
--- 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 a53e7b4..e2fab3c 100644
--- 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 9da70b2..35638f1 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);

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