Messages in this thread | | | Date | Tue, 16 Oct 2018 15:17:37 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8) |
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----- 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
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