Re: [PATCH v2 1/3] megasas: use unsigned type for reply_queue_head and check index

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

  Hello Darren,

+-- On Thu, 14 May 2020, Darren Kenny wrote --+
| > Update v1 -> v2: fix OOB access when index > MEGASAS_MAX_FRAMES(=2048)
| >  -> https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg03131.html
| >
| > diff --git a/hw/scsi/megasas.c b/hw/scsi/megasas.c
| > -    int reply_queue_head;
| > +    uint16_t reply_queue_head;
| > @@ -445,7 +445,7 @@ static MegasasCmd *megasas_lookup_frame(MegasasState *s,
| >  
| >      index = s->reply_queue_head;
| 
| While it is probably unlikely that it would cause an integer underflow
| here,

Yes, integer overflow is unlikely going from uint16_t -> to -> int type.

  s->reply_queue_head = ldl_le_pci_dma(pcid, s->producer_pa);
  s->reply_queue_head %= MEGASAS_MAX_FRAMES;

Also here 's->reply_queue_head' is restricted between 0...MEGASAS_MAX_FRAMES=2048

| -    while (num < s->fw_cmds) {
| +    while (num < s->fw_cmds && index < MEGASAS_MAX_FRAMES) {

And this patch would help keep 'index' within the same 0..MEGASAS_MAX_FRAMES 
range.

| for consistency, index probably should also be declared as unsigned, but 
| from what I can tell it is still an 'int'.

It did cross my mind, but it's generally advised to keep these fixes to 
minimum possible changes and specific to the issue they fix. Index being a 
local variable, changing it to an unsigned type wouldn't help much to fix the 
issue or otherwise I think.

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