[Xen-devel] [PATCH] x86/microcode: Avoid undefined behaviour from signed integer overflow

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The checksums should be calculated using unsigned 32bit integers, as they are
intended to overflow and end at 0.  Replace some other signed integers with
unsigned ones, to avoid mixed-sign comparisons.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>

v2:
 * Include the extended checksum
 * Swap more integers to being unsigned
---
 xen/arch/x86/microcode_intel.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/microcode_intel.c b/xen/arch/x86/microcode_intel.c
index 6949c25..93d9d0f 100644
--- a/xen/arch/x86/microcode_intel.c
+++ b/xen/arch/x86/microcode_intel.c
@@ -143,7 +143,8 @@ static int microcode_sanity_check(void *mc)
     struct extended_sigtable *ext_header = NULL;
     struct extended_signature *ext_sig;
     unsigned long total_size, data_size, ext_table_size;
-    int sum, orig_sum, ext_sigcount = 0, i;
+    unsigned int ext_sigcount = 0, i;
+    uint32_t sum, orig_sum;
 
     total_size = get_totalsize(mc_header);
     data_size = get_datasize(mc_header);
@@ -183,8 +184,8 @@ static int microcode_sanity_check(void *mc)
     /* check extended table checksum */
     if ( ext_table_size )
     {
-        int ext_table_sum = 0;
-        int *ext_tablep = (int *)ext_header;
+        uint32_t ext_table_sum = 0;
+        uint32_t *ext_tablep = (uint32_t *)ext_header;
 
         i = ext_table_size / DWSIZE;
         while ( i-- )
@@ -201,7 +202,7 @@ static int microcode_sanity_check(void *mc)
     orig_sum = 0;
     i = (MC_HEADER_SIZE + data_size) / DWSIZE;
     while ( i-- )
-        orig_sum += ((int *)mc)[i];
+        orig_sum += ((uint32_t *)mc)[i];
     if ( orig_sum )
     {
         printk(KERN_ERR "microcode: aborting, bad checksum\n");
-- 
2.1.4


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