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Message-ID: <1bc60f1c-2aa7-2d4a-a6a2-c8592ed83486@runbox.com>
Date: Thu, 22 Sep 2022 21:48:56 -0400
From: "M. Vefa Bicakci" <m.v.b@...box.com>
To: Juergen Gross <jgross@...e.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 2022-09-19 05:52, Juergen Gross wrote:
> 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++;
>
Hi,
Sorry for the delay, and thank you for reviewing my patches!
> Shouldn't this test include "&& handle != INVALID_GRANT_HANDLE" ?
>
> This should enable you to drop setting status to 1 below.
I had not thought of the approach you suggested. Just now, I applied
your suggestion to my local kernel tree, and I am building a new kernel
for testing now. I hope to publish a newer version of this patch set
over the weekend.
Thanks again,
Vefa
>
>> +
>> 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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