Re: [SPAM][PATCH] iommu/mediatek: Validate number of phandles associated with "mediatek,larbs"

On Tue, 2021-12-14 at 07:02 -0800, Guenter Roeck wrote:

On 12/13/21 11:31 PM, Yong Wu wrote:

quoted

On Fri, 2021-12-10 at 12:57 -0800, Guenter Roeck wrote:

quoted

Since commit baf94e6ebff9 ("iommu/mediatek: Add device link for
smi-
common
and m4u"), the driver assumes that at least one phandle
associated
with
"mediatek,larbs" exists. If that is not the case, for example if
reason
"mediatek,larbs" is provided as boolean property, the code will
use
an
uninitialized pointer and may crash. To fix the problem, ensure
that
the
number of phandles associated with "mediatek,larbs" is at least 1
and
bail out immediately if that is not the case.

 From the dt-binding, "mediatek,larbs" always is a phandle-array. I
assumed the dts should conform to the dt-binding before. Then the
problem is that if we should cover the case that someone
abuses/attacks
the dts. Could you help add more comment in the commit message?
something like: this is for avoid abuse the dt-binding.

This doesn't have to be an abuse or attack. It can simply be an error
by the person who wrote the devicetree file. Sure, bugs or lack of

A minor question: If someone wrote error data that don't conform to the
dtbinding, the error result is expected. He should fix that problem,
right? If we could avoid abort and show error message at the beginning,
it's better of course.

error checking can often be used for attacks, but that doesn't mean
that all bad data is an exploit or attack.

quoted

quoted

Cc: Yong Wu <yong.wu@mediatek.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Fixes: baf94e6ebff9 ("iommu/mediatek: Add device link for smi-
common
and m4u")
Reported-by: kernel test robot <redacted>
Reported-by: Dan Carpenter <redacted>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
  drivers/iommu/mtk_iommu.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/drivers/iommu/mtk_iommu.c

b/drivers/iommu/mtk_iommu.c
index 25b834104790..0bbe32d0a2a6 100644

--- a/drivers/iommu/mtk_iommu.c
+++ b/drivers/iommu/mtk_iommu.c

@@ -828,6 +828,8 @@ static int mtk_iommu_probe(struct

platform_device
*pdev)
  					     "mediatek,larbs",
NULL);
  	if (larb_nr < 0)
  		return larb_nr;
+	if (larb_nr == 0)
+		return -EINVAL;

Just assigning the larbnode to NULL may be simpler. In this case,
it
won't enter the loop below, and return 0 in the
of_parse_phandle(larbnode, "mediatek,smi", 0).

-       struct device_node      *larbnode, *smicomm_node;
+       struct device_node      *larbnode = NULL, *smicomm_node;

It is an option, but it would need to be explained and would not be
as simple as it looks. And, yes, it would result in unnecessary code
execution.

Why does it need to be explained ? I spent quite some additional
time with the code trying to understand _why_ it works, and we should
make sure that others don't have to spend that time.

Anyway, that additional time made me find additional problems with
the code.

The for loop below assigns larbnode to the last node it finds.
However, that node can be disabled.

		if (!of_device_is_available(larbnode)) {
                         of_node_put(larbnode);
                         continue;
                 }

Is such a disabled larbnode, if it is the last one, the node to use
when looking for "mediatek,smi" ?

Also, there is

	ret = of_property_read_u32(larbnode, "mediatek,larb-id", &id);
         if (ret)/* The id is consecutive if there is no this
property */
                 id = i;

There are two problems with this code. First, neither i nor id are
range
checked, but used later in

	data->larb_imu[id].dev = &plarbdev->dev;

That means a devicetree with a bad value for "mediatek,larb-id"
or more than MTK_LARB_NR_MAX larb nodes will result in writes after
the end of struct mtk_iommu_data.

On top of that, the comment states that the nodes are consecutive if
there
is no "mediatek,larb-id". However, that isn't really the case if
there
are disabled nodes. If there are disabled nodes, there will be a gap
in
larb_imu[]. I don't know if that matters; if it doesn't, there should
be
a comment about it in the code.

Last but not least, it would probably make sense to explain what the
"last"
larb node is expected to be in more detail. It is the last larb node
in
the devicetree file, but not the one with the highest id, and not
(necessarily) an enabled one. For example, in
arch/arm64/boot/dts/mediatek/mt2712e.dtsi, the code would pick
<&smi_common0> even though <&smi_common1> is associated with a higher
larb id.

One could of course argue that this all doesn't matter because it
would
suggest that the devicetree data is bad, but it is common practice to
validate
devicetree data and not just blindly accept it. One could also argue
that such bad data would be an "attack", but, again, we don't know
that.

In summary,

Thanks very much for your time to check here. All the issues are
introduced by the values from dts are untrusted. The detail platform
informations are replied below.

- The check I introduced should probably be something like

	if (larb_nr == 0 || larb_nr > MTK_LARB_NR_MAX)
		return -EINVAL;

OK. Add a "else" to show it is a block with the "if" above?

        if (larb_nr < 0)
             return larb_nr;
        else if (larb_nr == 0 || larb_nr > MTK_LARB_NR_MAX)
             return -EINVAL;

- It needs to be clarified if larbnode to use for finding
"mediatek,smi"
   is indeed always the last one, even if it is disabled. If so, we 

We could find the "mediatek,smi" with any available larb. Of course it
should not be a disabled one. The code using the last larb is for
reusing the variable "larbnode".

should
   probably also handle the situation that of_node_put(larbnode); was
called
   on that larbnode. Alternatively, if the last larb node to use is
the last
   _active_ larb node, we'll probably need a separate variable to
save that
   larb node pointer for later use.

A new variable is ok.

- It needs to be clarified if larb_imu[] may have gaps if there are
disabled
   larb nodes and "mediatek,larb-id" is not specified. If so, there 

Yes. It may have gaps. the commit message of this patch may be helpful.

50fa3cd33f9d ("dt-bindings: mediatek: Add binding for mt2712 IOMMU and
SMI")

is still the
   problem that 'i' and a previous value of "mediatek,larb-id" may be
identical
   [ eg the first node provides mediatek,larb-id = <1> and the second
node
     doesn't provide "mediatek,larb-id" ]

This case did don't meet my expectation. OK, then we add a checking?
like:

   if (data->larb_imu[i].dev) {
         dev_err(dev, "the larb %d exist.", i);
         return -EEXIST;
   }         

- "id" should be range checked.

It should be [0, MTK_LARB_NR_MAX).

- The meaning of "last" larb node to use when looking for
mediatek,smi should
   be explained in more detail.

We could use any available larb node to find mediatek,smi.

Their "mediatek,smi" node must be the same. OK, In this case, they are
possible different. We should add a checking: return -EINVAL if they
are not same.

Once we have determined the correct handling of all those situations,
I'll
be happy to send another revision of this patch (or possibly multiple
patches).

Appreciate for help enhance the safe here. I will test it.

Thanks,
Guenter

quoted

quoted

  
  	for (i = 0; i < larb_nr; i++) {
  		u32 id;

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