[Xen-devel] [PATCH] hvm/load: Correct length checks for zeroextended records

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

In the case that Xen is attempting to load a zeroextended HVM record where the
difference needing extending would overflow the data blob, _hvm_check_entry()
will incorrectly fail before working out that it would have been safe.

The "len + sizeof(*d)" check is wrong.  Consider zeroextending a 16 byte
record into a 32 byte structure.  "32 + hdr" will fail the overall context
length check even though the pre-extended record in the stream is 16 bytes.

The first condition is reduced to just a length check for hvm save header,
while the second condition is extended to include a check that the record in
the stream not exceeding the stream length.

The error messages are extended to include further useful information.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Tim Deegan <tim@xxxxxxx>
---
 xen/common/hvm/save.c |   17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/xen/common/hvm/save.c b/xen/common/hvm/save.c
index 6c16399..e20b1e6 100644
--- a/xen/common/hvm/save.c
+++ b/xen/common/hvm/save.c
@@ -292,19 +292,22 @@ int _hvm_check_entry(struct hvm_domain_context *h,
 {
     struct hvm_save_descriptor *d 
         = (struct hvm_save_descriptor *)&h->data[h->cur];
-    if ( len + sizeof (*d) > h->size - h->cur)
+    if ( sizeof(*d) > h->size - h->cur)
     {
         printk(XENLOG_G_WARNING
-               "HVM restore: not enough data left to read %u bytes "
-               "for type %u\n", len, type);
+               "HVM restore: not enough data left to read %zu bytes "
+               "for type %u header\n", sizeof(*d), type);
         return -1;
-    }    
+    }
     if ( (type != d->typecode) || (len < d->length) ||
-         (strict_length && (len != d->length)) )
+         (strict_length && (len != d->length)) ||
+         (d->length > (h->size - h->cur - sizeof(*d))) )
     {
         printk(XENLOG_G_WARNING
-               "HVM restore mismatch: expected type %u length %u, "
-               "saw type %u length %u\n", type, len, d->typecode, d->length);
+               "HVM restore mismatch: expected %s type %u length %u, "
+               "saw type %u length %u.  %zu bytes remaining\n",
+               strict_length ? "strict" : "zeroextended", type, len,
+               d->typecode, d->length, h->size - h->cur - sizeof(*d));
         return -1;
     }
     h->cur += sizeof(*d);
-- 
1.7.10.4


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