Re: [PATCH] ati: mask x y display parameter values

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

+-- On Tue, 20 Oct 2020, BALATON Zoltan wrote --+
| The card has 32 bit registers with values in them interpreted differently for
| different regs. For dst_x|y lower 14 bits can be set and value should be
| interpreted as -8192:8191 according to docs. I've got this wrong because all
| guests I've tried did not actually use negative values. The Solaris driver I
| was recently shown not to work may use that so I plan to look at it and fix it
| when I'll have time. 
... 
| Docs aren't very clear on that but I think these cannot be negative so 
| 0:8191 is valid range because it mentions that also bits 0-13 (or maybe 
| 0-15, the docs have a lot of copy&paste errors) are valid but only 0-12 are 
| used for rectangles, 13-15 used only for trapezoids (which we don't 
| emulate). The docs are really bad so we have to guess and see what guest 
| drivers do most of the time.

* I see. Are the docs available/accessible online?

| >  dst_y(=4294950914(=-16382)) + s->regs.dst_height(=16383)) overflows to => 1
| > Ie. (dst_bits + dst_x(=0) + (1) * dst_stride >= end) returns false.
| 
| So maybe we should cast something (like dst_stride) to uint64_t here to
| promote everything to 64 bit and prevent it overflowing which then should
| catch this as well?
... 
| > +    if (dst_x > 0x3fff || dst_y > 0x3fff || dst_bits >= end
| > +        || dst_bits + dst_x + (dst_y + s->regs.dst_height)
| > +         * dst_stride >= end) {
| > ...
| > +        if (src_x > 0x3fff || src_y > 0x3ff || src_bits >= end
| > +            || src_bits + src_x + (src_y + s->regs.dst_height)
| > +             * src_stride >= end) {
| >             qemu_log_mask(LOG_UNIMP, "blt outside vram not implemented\n");
| 
| I can live with that until I have a chance to rewrite it but are you sure this
| will catch all possible overflows with all vram sizes that can be set with
| vgamem_mb property?

* Considering all fields are 'uint32_t' type; And majority of the values 
  s->regs.[src|dst]_[xy], s->regs.dst_height are masked with '0x3fff', it 
  should help to avoid overflows.

* Not sure about all vram sizes. What are possible/supported size options?

* Between casting expression to 64 bits & explicit src_[xy] > 0x3fff check, 
  I'd go with explicit check, as it's easy to follow.

  Will send a revised patch with src_[xy] > 0x3fff if it's okay with you.


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