[Xen-devel] [PATCH] tools/migrate: Prevent PTE truncation from being fatal duing the live phase

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

It is possible, when normalising a PV pagetable that the table has been freed
and reused for something else by the guest.

In such a case, data read might no longer be a pagetable, and fail the
truncation check.  However, this should only be fatal if we encounter such a
page in the paused phase.

This check is now consistent with all other checks in the same area.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>

This was hit by OSSTest:
http://logs.test-lab.xenproject.org/osstest/logs/100684/test-amd64-i386-xl-raw/16.ts-guest-localmigrate.log

The patch is untested, but the only plausible option given the circumstances.
---
 tools/libxc/xc_sr_save_x86_pv.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/tools/libxc/xc_sr_save_x86_pv.c b/tools/libxc/xc_sr_save_x86_pv.c
index 7043409..f218d17 100644
--- a/tools/libxc/xc_sr_save_x86_pv.c
+++ b/tools/libxc/xc_sr_save_x86_pv.c
@@ -949,9 +949,14 @@ static int normalise_pagetable(struct xc_sr_context *ctx, 
const uint64_t *src,
 #ifdef __i386__
             if ( mfn == INVALID_MFN )
             {
-                ERROR("PTE truncation detected.  L%lu[%u] = %016"PRIx64,
-                      type >> XEN_DOMCTL_PFINFO_LTAB_SHIFT, i, pte);
-                errno = E2BIG;
+                if ( !ctx->dominfo.paused )
+                    errno = EAGAIN;
+                else
+                {
+                    ERROR("PTE truncation detected.  L%lu[%u] = %016"PRIx64,
+                          type >> XEN_DOMCTL_PFINFO_LTAB_SHIFT, i, pte);
+                    errno = E2BIG;
+                }
                 return -1;
             }
 #endif
-- 
2.1.4


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