Re: [PATCH V7 2/7] vfio/type1: prevent underflow of locked_vm via exec()

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jan 03, 2023 at 01:12:53PM -0500, Steven Sistare wrote:
> On 1/3/2023 10:20 AM, Jason Gunthorpe wrote:
> > On Tue, Dec 20, 2022 at 12:39:20PM -0800, Steve Sistare wrote:
> >> When a vfio container is preserved across exec, the task does not change,
> >> but it gets a new mm with locked_vm=0, and loses the count from existing
> >> dma mappings.  If the user later unmaps a dma mapping, locked_vm underflows
> >> to a large unsigned value, and a subsequent dma map request fails with
> >> ENOMEM in __account_locked_vm.
> >>
> >> To avoid underflow, grab and save the mm at the time a dma is mapped.
> >> Use that mm when adjusting locked_vm, rather than re-acquiring the saved
> >> task's mm, which may have changed.  If the saved mm is dead, do nothing.
> >>
> >> locked_vm is incremented for existing mappings in a subsequent patch.
> >>
> >> Fixes: 73fa0d10d077 ("vfio: Type1 IOMMU implementation")
> >> Cc: stable@vger.kernel.org
> >> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> >> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> >> ---
> >>  drivers/vfio/vfio_iommu_type1.c | 27 +++++++++++----------------
> >>  1 file changed, 11 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 144f5bb..71f980b 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -100,6 +100,7 @@ struct vfio_dma {
> >>  	struct task_struct	*task;
> >>  	struct rb_root		pfn_list;	/* Ex-user pinned pfn list */
> >>  	unsigned long		*bitmap;
> >> +	struct mm_struct	*mm;
> >>  };
> >>  
> >>  struct vfio_batch {
> >> @@ -420,8 +421,8 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
> >>  	if (!npage)
> >>  		return 0;
> >>  
> >> -	mm = async ? get_task_mm(dma->task) : dma->task->mm;
> >> -	if (!mm)
> >> +	mm = dma->mm;
> >> +	if (async && !mmget_not_zero(mm))
> >>  		return -ESRCH; /* process exited */
> > 
> > Just delete the async, the lock_acct always acts on the dma which
> > always has a singular mm.
> > 
> > FIx the few callers that need it to do the mmget_no_zero() before
> > calling in.
> 
> Most of the callers pass async=true:
>   ret = vfio_lock_acct(dma, lock_acct, false);
>   vfio_lock_acct(dma, locked - unlocked, true);
>   ret = vfio_lock_acct(dma, 1, true);
>   vfio_lock_acct(dma, -unlocked, true);
>   vfio_lock_acct(dma, -1, true);
>   vfio_lock_acct(dma, -unlocked, true);
>   ret = mm_lock_acct(task, mm, lock_cap, npage, false);
>   mm_lock_acct(dma->task, dma->mm, dma->lock_cap, -npage, true);
>   vfio_lock_acct(dma, locked - unlocked, true);

Seems like if you make a lock_sub_acct() function that does the -1*
and does the mmget it will be OK?

Jason