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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2 1/2] xen/gntdev: Prevent leaking grants
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@xxxxxxxxxxxxxxx
> Signed-off-by: M. Vefa Bicakci <m.v.b@xxxxxxxxxx>
> ---
>
> 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@xxxxxxxxxxxxxxxxxxxxxx>
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
Attachment:
signature.asc
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