Re: [PATCH] s390: fix double free of GS and RI CBs on fork() failure

[Brian Foster <bfoster@redhat.com>]

On Wed, Aug 17, 2022 at 01:09:06PM +0200, Gerald Schaefer wrote:
> On Tue, 16 Aug 2022 11:54:07 -0400
> Brian Foster <bfoster@redhat.com> wrote:
> 
> > The pointers for guarded storage and runtime instrumentation control
> > blocks are stored in the thread_struct of the associated task. These
> > pointers are initially copied on fork() via arch_dup_task_struct()
> > and then cleared via copy_thread() before fork() returns. If fork()
> > happens to fail after the initial task dup and before copy_thread(),
> > the newly allocated task and associated thread_struct memory are
> > freed via free_task() -> arch_release_task_struct(). This results in
> > a double free of the guarded storage and runtime info structs
> > because the fields in the failed task still refer to memory
> > associated with the source task.
> > 
> > This problem can manifest as a BUG_ON() in set_freepointer() (with
> > CONFIG_SLAB_FREELIST_HARDENED enabled) or KASAN splat (if enabled)
> > when running trinity syscall fuzz tests on s390x. To avoid this
> > problem, clear the associated pointer fields in
> > arch_dup_task_struct() immediately after the new task is copied.
> > Note that the RI flag is still cleared in copy_thread() because it
> > resides in thread stack memory and that is where stack info is
> > copied.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> > 
> > Hi all,
> > 
> > Note that I'm not subscribed to the list so please CC on reply. Further,
> > I'm not terribly familiar with these associated features and so have not
> > run any kind of functional testing here. My testing was purely around
> > producing/preventing the double free issue. Any thoughts, reviews or
> > further testing is appreciated. Thanks.
> > 
> > Brian
> > 
> >  arch/s390/kernel/process.c | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/s390/kernel/process.c b/arch/s390/kernel/process.c
> > index 89949b9f3cf8..d5119e039d85 100644
> > --- a/arch/s390/kernel/process.c
> > +++ b/arch/s390/kernel/process.c
> > @@ -91,6 +91,18 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> >  
> >  	memcpy(dst, src, arch_task_struct_size);
> >  	dst->thread.fpu.regs = dst->thread.fpu.fprs;
> > +
> > +	/*
> > +	 * Don't transfer over the runtime instrumentation or the guarded
> > +	 * storage control block pointers. These fields are cleared here instead
> > +	 * of in copy_thread() to avoid premature freeing of associated memory
> > +	 * on fork() failure. Wait to clear the RI flag because ->stack still
> > +	 * refers to the source thread.
> > +	 */
> > +	dst->thread.ri_cb = NULL;
> > +	dst->thread.gs_cb = NULL;
> > +	dst->thread.gs_bc_cb = NULL;
> > +
> >  	return 0;
> >  }
> >  
> > @@ -150,13 +162,11 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
> >  	frame->childregs.flags = 0;
> >  	if (new_stackp)
> >  		frame->childregs.gprs[15] = new_stackp;
> > -
> > -	/* Don't copy runtime instrumentation info */
> > -	p->thread.ri_cb = NULL;
> > +	/*
> > +	 * Clear the runtime instrumentation flag after the above childregs
> > +	 * copy. The CB pointer was already cleared in arch_dup_task_struct().
> > +	 */
> >  	frame->childregs.psw.mask &= ~PSW_MASK_RI;
> > -	/* Don't copy guarded storage control block */
> > -	p->thread.gs_cb = NULL;
> > -	p->thread.gs_bc_cb = NULL;
> >  
> >  	/* Set a new TLS ?  */
> >  	if (clone_flags & CLONE_SETTLS) {
> 
> Thanks Brian, nice catch! Looks good to me. For completeness, we should
> add stable / Fixes tags, like this:
> 
> Fixes: 8d9047f8b967c ("s390/runtime instrumentation: simplify task exit handling")
> Fixes: 7b83c6297d2fc ("s390/guarded storage: simplify task exit handling")
> Cc: <stable@vger.kernel.org> # 4.15
> 
> Not 100% sure about the Fixes tags, Heiko should also have a look when
> he returns next week.
> 
> Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> 

Hi Gerald,

Sounds good, thanks for the review. I'll wait a bit so Heiko has a
chance to take a look and then follow up with the tag updates and any
other necessary changes..

Brian