CVE-2025-71130: drm/i915/gem: Zero-initialize the eb.vma array in i915_gem_do_execbuffer

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/gem: Zero-initialize the eb.vma array in i915gemdoexecbuffer

Initialize the eb.vma array with values of 0 when the eb structure is first set up. In particular, this sets the eb->vma[i].vma pointers to NULL, simplifying cleanup and getting rid of the bug described below.

During the execution of eblookupvmas(), the eb->vma array is successively filled up with struct ebvma objects. This process includes calling ebaddvma(), which might fail; however, even in the event of failure, eb->vma[i].vma is set for the currently processed buffer.

If ebaddvma() fails, eblookupvmas() returns with an error, which prompts a call to ebreleasevmas() to clean up the mess. Since eblookupvmas() might fail during processing any (possibly not first) buffer, ebreleasevmas() checks whether a buffer's vma is NULL to know at what point did the lookup function fail.

In eblookupvmas(), eb->vma[i].vma is set to NULL if either the helper function eblookupvma() or ebvalidatevma() fails. eb->vma[i+1].vma is set to NULL in case i915gemobjectuserptrsubmitinit() fails; the current one needs to be cleaned up by ebreleasevmas() at this point, so the next one is set. If ebaddvma() fails, neither the current nor the next vma is set to NULL, which is a source of a NULL deref bug described in the issue linked in the Closes tag.

When entering eblookupvmas(), the vma pointers are set to the slab poison value, instead of NULL. This doesn't matter for the actual lookup, since it gets overwritten anyway, however the ebreleasevmas() function only recognizes NULL as the stopping value, hence the pointers are being set to NULL as they go in case of intermediate failure. This patch changes the approach to filling them all with NULL at the start instead, rather than handling that manually during failure.

(cherry picked from commit 08889b706d4f0b8d2352b7ca29c2d8df4d0787cd)