Re: [PATCH] drm/i915/gt: Prevent possible NULL dereference in __caps_show()

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Hi,

On 06/02/2024 16:45, Nikita Zhandarovich wrote:
After falling through the switch statement to default case 'repr' is
initialized with NULL, which will lead to incorrect dereference of
'!repr[n]' in the following loop.

Fix it with the help of an additional check for NULL.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 4ec76dbeb62b ("drm/i915/gt: Expose engine properties via sysfs")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@xxxxxxxxxx>
---
P.S. The NULL-deref problem might be dealt with this way but I am
not certain that the rest of the __caps_show() behaviour remains
correct if we end up in default case. For instance, as far as I
can tell, buf might turn out to be w/o '\0'. I could use some
direction if this has to be addressed as well.

  drivers/gpu/drm/i915/gt/sysfs_engines.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/sysfs_engines.c b/drivers/gpu/drm/i915/gt/sysfs_engines.c
index 021f51d9b456..6b130b732867 100644
--- a/drivers/gpu/drm/i915/gt/sysfs_engines.c
+++ b/drivers/gpu/drm/i915/gt/sysfs_engines.c
@@ -105,7 +105,7 @@ __caps_show(struct intel_engine_cs *engine,
len = 0;
  	for_each_set_bit(n, &caps, show_unknown ? BITS_PER_LONG : count) {
-		if (n >= count || !repr[n]) {
+		if (n >= count || !repr || !repr[n]) {

There are two input combinations to this function when repr is NULL.

First is show_unknown=true and caps=0, which means the for_each_set_bit will not execute its body. (No bits set.)

Second is show_unknown=false and caps=~0, which means count is zero so for_each_set_bit will again not run. (Bitfield size input param is zero.)

So unless I am missing something I do not see the null pointer dereference.

What could theoretically happen is that a third input combination appears, where caps is not zero in the show_unknown=true case, either via a fully un-handled engine->class (switch), or a new capability bit not added to the static array a bit above.

That would assert during driver development here:

			if (GEM_WARN_ON(show_unknown))

Granted that could be after the dereference in "if (n >= count || !repr[n])", but would be caught in debug builds (CI) and therefore not be able to "ship" (get merge to the repo).

Your second question is about empty buffer returned i.e. len=0 at the end of the function? (Which is when the buffer will not be null terminated - or you see another option?)

That I think is safe too since it just results in a zero length read in sysfs.

Regards,

Tvrtko

  			if (GEM_WARN_ON(show_unknown))
  				len += sysfs_emit_at(buf, len, "[%x] ", n);
  		} else {



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