Date:   Mon, 19 Sep 2022 11:52:23 +0200
From:   Juergen Gross <jgross@...e.com>
To:     "M. Vefa Bicakci" <m.v.b@...box.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Demi Marie Obenour <demi@...isiblethingslab.com>,
        Gerd Hoffmann <kraxel@...hat.com>
Subject: Re: [PATCH 1/2] xen/gntdev: Prevent leaking grants

On 12.09.22 06:00, 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. To determine which grants were successfully
> unmapped, their status fields are set to an arbitrary positive number
> (1), as was done in commit ebee0eab0859 ("Xen/gntdev: correct error
> checking in gntdev_map_grant_pages()"). 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()")
> Cc: stable@...r.kernel.org
> Signed-off-by: M. Vefa Bicakci <m.v.b@...box.com>
> ---
>   drivers/xen/gntdev.c | 32 +++++++++++++++++++++++++++-----
>   1 file changed, 27 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 84b143eef395..485fa9c630aa 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 +392,13 @@ 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)
> +			successful_unmaps++;

Shouldn't this test include "&& handle != INVALID_GRANT_HANDLE" ?

This should enable you to drop setting status to 1 below.

> +
>   		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 +406,9 @@ 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)
> +				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 +417,15 @@ 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);
> @@ -424,6 +434,8 @@ static void __unmap_grant_pages_done(int result,
>   static void __unmap_grant_pages(struct gntdev_grant_map *map, int offset,
>   			       int pages)
>   {
> +	int idx;
> +
>   	if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
>   		int pgno = (map->notify.addr >> PAGE_SHIFT);
>   
> @@ -436,6 +448,16 @@ static void __unmap_grant_pages(struct gntdev_grant_map *map, int offset,
>   		}
>   	}
>   
> +	/* Set all unmap/kunmap status fields to an arbitrary positive value,
> +	 * so that it is possible to determine which grants were successfully
> +	 * unmapped by inspecting the status fields.
> +	 */
> +	for (idx = offset; idx < offset + pages; idx++) {
> +		map->unmap_ops[idx].status = 1;
> +		if (use_ptemod)
> +			map->kunmap_ops[idx].status = 1;
> +	}
> +
>   	map->unmap_data.unmap_ops = map->unmap_ops + offset;
>   	map->unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
>   	map->unmap_data.pages = map->pages + offset;

Juergen

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