Re: [PATCH v2 1/2] xen/gntdev: Prevent leaking grants

[Demi Marie Obenour <demi@invisiblethingslab.com>]

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On Sun, Oct 02, 2022 at 06:20:05PM -0400, M. Vefa Bicakci wrote:
> Prior to this commit, if a grant mapping operation failed partially,
> some of the entries in the map_ops array would be invalid, whereas all
> of the entries in the kmap_ops array would be valid. This in turn would
> cause the following logic in gntdev_map_grant_pages to become invalid:
> 
>   for (i = 0; i < map->count; i++) {
>     if (map->map_ops[i].status == GNTST_okay) {
>       map->unmap_ops[i].handle = map->map_ops[i].handle;
>       if (!use_ptemod)
>         alloced++;
>     }
>     if (use_ptemod) {
>       if (map->kmap_ops[i].status == GNTST_okay) {
>         if (map->map_ops[i].status == GNTST_okay)
>           alloced++;
>         map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>       }
>     }
>   }
>   ...
>   atomic_add(alloced, &map->live_grants);
> 
> Assume that use_ptemod is true (i.e., the domain mapping the granted
> pages is a paravirtualized domain). In the code excerpt above, note that
> the "alloced" variable is only incremented when both kmap_ops[i].status
> and map_ops[i].status are set to GNTST_okay (i.e., both mapping
> operations are successful).  However, as also noted above, there are
> cases where a grant mapping operation fails partially, breaking the
> assumption of the code excerpt above.
> 
> The aforementioned causes map->live_grants to be incorrectly set. In
> some cases, all of the map_ops mappings fail, but all of the kmap_ops
> mappings succeed, meaning that live_grants may remain zero. This in turn
> makes it impossible to unmap the successfully grant-mapped pages pointed
> to by kmap_ops, because unmap_grant_pages has the following snippet of
> code at its beginning:
> 
>   if (atomic_read(&map->live_grants) == 0)
>     return; /* Nothing to do */
> 
> In other cases where only some of the map_ops mappings fail but all
> kmap_ops mappings succeed, live_grants is made positive, but when the
> user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
> will then make map->live_grants negative, because the latter function
> does not check if all of the pages that were requested to be unmapped
> were actually unmapped, and the same function unconditionally subtracts
> "data->count" (i.e., a value that can be greater than map->live_grants)
> from map->live_grants. The side effects of a negative live_grants value
> have not been studied.
> 
> The net effect of all of this is that grant references are leaked in one
> of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
> mechanism extensively for X11 GUI isolation), this issue manifests
> itself with warning messages like the following to be printed out by the
> Linux kernel in the VM that had granted pages (that contain X11 GUI
> window data) to dom0: "g.e. 0x1234 still pending", especially after the
> user rapidly resizes GUI VM windows (causing some grant-mapping
> operations to partially or completely fail, due to the fact that the VM
> unshares some of the pages as part of the window resizing, making the
> pages impossible to grant-map from dom0).
> 
> The fix for this issue involves counting all successful map_ops and
> kmap_ops mappings separately, and then adding the sum to live_grants.
> During unmapping, only the number of successfully unmapped grants is
> subtracted from live_grants. The code is also modified to check for
> negative live_grants values after the subtraction and warn the user.
> 
> Link: https://github.com/QubesOS/qubes-issues/issues/7631
> Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")

Looks like this patch has been pretty buggy, sorry.  This is the second
time there has been a problem with it.  Thanks for the fix.

> Cc: stable@vger.kernel.org
> Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>
> ---
> 
> Changes since v1:
> - To determine which unmap operations were successful, the previous
>   version of this patch set the "unmap_ops[i].status" and
>   "kunmap_ops[i].status" fields to the value "1" prior to passing these
>   data structures to the hypervisor. Instead of doing that, the code now
>   checks whether the "handle" fields in the same data structures were
>   *not* set to "INVALID_GRANT_HANDLE". (Suggested by Juergen Gross.)
> ---
>  drivers/xen/gntdev.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 84b143eef395..eb0586b9767d 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -367,8 +367,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>  	for (i = 0; i < map->count; i++) {
>  		if (map->map_ops[i].status == GNTST_okay) {
>  			map->unmap_ops[i].handle = map->map_ops[i].handle;
> -			if (!use_ptemod)
> -				alloced++;
> +			alloced++;
>  		} else if (!err)
>  			err = -EINVAL;
>  
> @@ -377,8 +376,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>  
>  		if (use_ptemod) {
>  			if (map->kmap_ops[i].status == GNTST_okay) {
> -				if (map->map_ops[i].status == GNTST_okay)
> -					alloced++;
> +				alloced++;
>  				map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>  			} else if (!err)
>  				err = -EINVAL;
> @@ -394,8 +392,14 @@ static void __unmap_grant_pages_done(int result,
>  	unsigned int i;
>  	struct gntdev_grant_map *map = data->data;
>  	unsigned int offset = data->unmap_ops - map->unmap_ops;
> +	int successful_unmaps = 0;
> +	int live_grants;
>  
>  	for (i = 0; i < data->count; i++) {
> +		if (map->unmap_ops[offset + i].status == GNTST_okay &&
> +		    map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
> +			successful_unmaps++;
> +
>  		WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
>  			map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>  		pr_debug("unmap handle=%d st=%d\n",
> @@ -403,6 +407,10 @@ static void __unmap_grant_pages_done(int result,
>  			map->unmap_ops[offset+i].status);
>  		map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>  		if (use_ptemod) {
> +			if (map->kunmap_ops[offset + i].status == GNTST_okay &&
> +			    map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
> +				successful_unmaps++;
> +
>  			WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
>  				map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>  			pr_debug("kunmap handle=%u st=%d\n",
> @@ -411,11 +419,15 @@ static void __unmap_grant_pages_done(int result,
>  			map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>  		}
>  	}
> +
>  	/*
>  	 * Decrease the live-grant counter.  This must happen after the loop to
>  	 * prevent premature reuse of the grants by gnttab_mmap().
>  	 */
> -	atomic_sub(data->count, &map->live_grants);
> +	live_grants = atomic_sub_return(successful_unmaps, &map->live_grants);
> +	if (WARN_ON(live_grants < 0))
> +		pr_err("%s: live_grants became negative (%d) after unmapping %d pages!\n",
> +		       __func__, live_grants, successful_unmaps);
>  
>  	/* Release reference taken by __unmap_grant_pages */
>  	gntdev_put_map(NULL, map);
> -- 
> 2.37.3

Is there a possibility that live_grants could overflow, as it is now
set to a value twice as large as what it had been previously?

If not, you can add:

Acked-by: Demi Marie Obenour <demi@invisiblethingslab.com>
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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