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xen/xenbus: don't let xenbus_grant_ring() remove grants in error case 

Letting xenbus_grant_ring() tear down grants in the error case is
problematic, as the other side could already have used these grants.
Calling gnttab_end_foreign_access_ref() without checking success is
resulting in an unclear situation for any caller of xenbus_grant_ring()
as in the error case the memory pages of the ring page might be
partially mapped. Freeing them would risk unwanted foreign access to
them, while not freeing them would leak memory.

In order to remove the need to undo any gnttab_grant_foreign_access()
calls, use gnttab_alloc_grant_references() to make sure no further
error can occur in the loop granting access to the ring pages.

It should be noted that this way of handling removes leaking of
grant entries in the error case, too.

This is CVE-2022-23040 / part of XSA-396.

Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
This commit is contained in:
Juergen Gross 2022-03-07 09:48:54 +01:00
parent ffb217a13a
commit 3777ea7bac
1 changed files with 11 additions and 13 deletions
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24
drivers/xen/xenbus/xenbus_client.c
View File

@ -379,7 +379,14 @@ int xenbus_grant_ring(struct xenbus_device *dev, void *vaddr,
unsigned int nr_pages, grant_ref_t *grefs)
{
int err;
int i, j;
unsigned int i;
grant_ref_t gref_head;
err = gnttab_alloc_grant_references(nr_pages, &gref_head);
if (err) {
xenbus_dev_fatal(dev, err, "granting access to ring page");
return err;
}
for (i = 0; i < nr_pages; i++) {
unsigned long gfn;
@ -389,23 +396,14 @@ int xenbus_grant_ring(struct xenbus_device *dev, void *vaddr,
else
gfn = virt_to_gfn(vaddr);
err = gnttab_grant_foreign_access(dev->otherend_id, gfn, 0);
if (err < 0) {
xenbus_dev_fatal(dev, err,
"granting access to ring page");
goto fail;
}
grefs[i] = err;
grefs[i] = gnttab_claim_grant_reference(&gref_head);
gnttab_grant_foreign_access_ref(grefs[i], dev->otherend_id,
gfn, 0);
vaddr = vaddr + XEN_PAGE_SIZE;
}
return 0;
fail:
for (j = 0; j < i; j++)
gnttab_end_foreign_access_ref(grefs[j], 0);
return err;
}
EXPORT_SYMBOL_GPL(xenbus_grant_ring);