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Re: [Xen-devel] [PATCH 2/2] xen/x86: hap: Clean-up and harden hap_enable()





On 04/02/2020 10:51, Roger Pau Monné wrote:
On Tue, Feb 04, 2020 at 09:34:11AM +0000, Julien Grall wrote:
From: Julien Grall <jgrall@xxxxxxxxxx>

Unlike shadow_enable(), hap_enable() can only be called once during
domain creation and with the mode equal to mode equal to
                                     ^ equals to

Will fix it.

PG_external | PG_translate | PG_refcounts.

If it were called twice, then we might have something interesting
                                               ^ a problem
problem as the p2m tables would be re-allocated (and therefore all the
mappings would be lost).

Add code to sanity check the mode and that the function is only called
once. Take the opportunity to an if checking that PG_translate is set.
                                 ^ add an if

Will fix it.


Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

---

It is not entirely clear when PG_translate was enforced.
---
  xen/arch/x86/mm/hap/hap.c | 18 +++++++++++-------
  1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index 31362a31b6..b734e2e6d3 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -445,6 +445,13 @@ int hap_enable(struct domain *d, u32 mode)
      unsigned int i;
      int rv = 0;
+ if ( mode != (PG_external | PG_translate | PG_refcounts) )
+        return -EINVAL;
+
+    /* The function can only be called once */
+    if ( d->arch.paging.mode != 0 )
+        return -EINVAL;

If you want to return EINVAL for both they can be merged into a single
if. Also note that this would usually be written as
if ( d->arch.paging.mode ) to keep it shorter.

To be honest, this is a matter of taste. There is also an argument that for MISRA, your suggestion is not compliant (see Rule 14.4).


Albeit I think you might want to return EEXIST instead of EINVAL if
mode is already set.

I am happy with that.


+
      domain_pause(d);
old_pages = d->arch.paging.hap.total_pages;
@@ -465,13 +472,10 @@ int hap_enable(struct domain *d, u32 mode)
      d->arch.paging.alloc_page = hap_alloc_p2m_page;
      d->arch.paging.free_page = hap_free_p2m_page;
- /* allocate P2m table */
-    if ( mode & PG_translate )
-    {
-        rv = p2m_alloc_table(p2m_get_hostp2m(d));
-        if ( rv != 0 )
-            goto out;
-    }
+    /* allocate P2M table */
+    rv = p2m_alloc_table(p2m_get_hostp2m(d));
+    if ( rv != 0 )

I would also avoid comparing against 0 here.

See above.

Cheers,

--
Julien Grall

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