Re: [PATCH v2 2/3] megasas: avoid NULL pointer dereference

[P J P <ppandit@redhat.com>]

  Hello,

+-- On Thu, 21 May 2020, Paolo Bonzini wrote --+
| I think the code here was expecting frame_size_p to be 0 if cmd->frame is 
| NULL.  Can you check why this is not the case, or whether it ever was the 
| case?

static MegasasCmd *megasas_enqueue_frame(MegasasState *s, hwaddr frame,
    ...
    int frame_size = MEGASAS_MAX_SGE * sizeof(union mfi_sgl);                   
    hwaddr frame_size_p = frame_size;  <== = 128 * 16 = 2048

so 'frame_size_p' always starts with value '2048'

    ...
    cmd->frame = pci_dma_map(pcid, frame, &frame_size_p, 0);
     -> pci_dma_map
      -> dma_memory_map
       -> address_space_map
          mr = flatview_translate(fv, addr, &xlat, &l, is_write, attrs);
          ...
          if (atomic_xchg(&bounce.in_use, true)) {
              return NULL;    <== NULL is returned from here
          }

Later when address_space_map() returns 'NULL' above, '*plen' is not set to 
zero.

diff --git a/exec.c b/exec.c
index 5162f0d12f..4eea84bf66 100644
--- a/exec.c
+++ b/exec.c
@@ -3538,6 +3538,7 @@ void *address_space_map(AddressSpace *as,
 
     if (!memory_access_is_direct(mr, is_write)) {
         if (atomic_xchg(&bounce.in_use, true)) {
+            *plen = 0;
             return NULL;
         }

I'll send a revised patch above.

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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