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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-api <linux-api@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King <linux@arm.linux.org.uk>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
	Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Joel Fernandes <joelaf@google.com>
Subject: Re: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8)
Date: Tue, 16 Oct 2018 15:17:37 -0400 (EDT)
Message-ID: <1984292897.263.1539717457933.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20181016081029.GA30363@jagdpanzerIV>

----- On Oct 16, 2018, at 4:10 AM, Sergey Senozhatsky sergey.senozhatsky.work@gmail.com wrote:

> Hi Mathieu,
> 
> On (10/10/18 15:19), Mathieu Desnoyers wrote:
> [..]
>> +SYSCALL_DEFINE4(cpu_opv, struct cpu_op __user *, ucpuopv, int, cpuopcnt,
>> +		int, cpu, int, flags)
>> +{
> [..]
>> +again:
>> +	ret = cpu_opv_pin_pages(cpuopv, cpuopcnt, &vaddr_ptrs);
>> +	if (ret)
>> +		goto end;
>> +	ret = do_cpu_opv(cpuopv, cpuopcnt, &vaddr_ptrs, cpu);
>> +	if (ret == -EAGAIN)
>> +		retry = true;
>> +end:
>> +	for (i = 0; i < vaddr_ptrs.nr_vaddr; i++) {
>> +		struct vaddr *vaddr = &vaddr_ptrs.addr[i];
>> +		int j;
>> +
>> +		vm_unmap_user_ram((void *)vaddr->mem, vaddr->nr_pages);
> 
> A dumb question.
> 
> Both vm_unmap_user_ram() and vm_map_user_ram() can BUG_ON().
> So this is
>   userspace -> syscall -> cpu_opv() -> vm_unmap_user_ram() -> BUG_ON()
> 
> Any chance someone can exploit it?

Hi Sergey,

Let's look at vm_unmap_user_ram() and vm_map_user_ram() separately.

If we look at the input from vm_unmap_user_ram, it's called with the
following parameters by the cpu_opv system call:

        for (i = 0; i < vaddr_ptrs.nr_vaddr; i++) {
                struct vaddr *vaddr = &vaddr_ptrs.addr[i];
                int j;

                vm_unmap_user_ram((void *)vaddr->mem, vaddr->nr_pages);
[...]
        }

The vaddr_ptrs array content is filled by the call to cpu_opv_pin_pages above:

        ret = cpu_opv_pin_pages(cpuopv, cpuopcnt, &vaddr_ptrs);
        if (ret)
                goto end;

by passing the array to cpu_op_pin_pages(), which appends a virtual address at
the end of the array (on success) and increments nr_vaddr. Those virtual
addresses are returned by vm_map_user_ram(), so they are not user-controlled.

Therefore, only an internal kernel bug between vm_map_user_ram() and
vm_unmap_user_ram() should trigger the BUG_ON(). No user input is passed
to vm_unmap_user_ram().

Now, let's look at vm_map_user_ram(). It calls alloc_vmap_area(), which returns
a vmap_area. Then if vmap_page_range failed, vm_unmap_user_ram is called on the
memory that has just been returned by vm_map_user_ram. Again, only an internal
bug between map/unmap can trigger the BUG_ON() in vm_unmap_user_ram.

Is there another scenario I missed ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

  reply index

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-10 19:19 [RFC PATCH for 4.21 00/16] rseq updates, new cpu_opv system call Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 01/16] rseq/selftests: Add reference counter to coexist with glibc Mathieu Desnoyers
2018-10-11 10:37   ` Szabolcs Nagy
2018-10-11 15:13     ` Mathieu Desnoyers
2018-10-11 16:20       ` Szabolcs Nagy
2018-10-11 16:37         ` Mathieu Desnoyers
2018-10-11 17:04           ` Szabolcs Nagy
2018-10-11 19:42             ` Mathieu Desnoyers
2018-10-12  9:59               ` Szabolcs Nagy
2018-10-23 14:59                 ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 02/16] rseq/selftests: Adapt number of threads to the number of detected cpus Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 03/16] sched: Implement push_task_to_cpu (v2) Mathieu Desnoyers
2018-10-17  6:51   ` Srikar Dronamraju
2018-10-17 15:09     ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram, vm_unmap_user_ram Mathieu Desnoyers
2018-10-16 18:30   ` Steven Rostedt
2018-10-16 19:21     ` Mathieu Desnoyers
2018-10-16 19:40       ` Steven Rostedt
2018-10-17  0:27     ` Sergey Senozhatsky
2018-10-17 15:00       ` Mathieu Desnoyers
2018-10-17 15:04         ` Mathieu Desnoyers
2018-10-17 15:34           ` Sergey Senozhatsky
2018-10-10 19:19 ` [RFC PATCH for 4.21 05/16] mm: Provide is_vma_noncached Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8) Mathieu Desnoyers
2018-10-16  8:10   ` Sergey Senozhatsky
2018-10-16 19:17     ` Mathieu Desnoyers [this message]
2018-10-17  1:46       ` Sergey Senozhatsky
2018-10-17  7:19   ` Srikar Dronamraju
2018-10-17 15:11     ` Mathieu Desnoyers
2018-10-17 16:09       ` Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 07/16] cpu_opv: limit amount of virtual address space used by cpu_opv Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 08/16] x86: Wire up cpu_opv system call Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 09/16] powerpc: " Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 10/16] arm: " Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 11/16] cpu-opv/selftests: Provide cpu-op library Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 12/16] cpu-opv/selftests: Provide basic test Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 13/16] cpu-opv/selftests: Provide percpu_op API Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 14/16] cpu-opv/selftests: Provide basic percpu ops test Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 15/16] cpu-opv/selftests: Provide parametrized tests Mathieu Desnoyers
2018-10-10 19:19 ` [RFC PATCH for 4.21 16/16] cpu-opv/selftests: Provide Makefile, scripts, gitignore Mathieu Desnoyers

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