drm/i915: 2 GiB of relocations ought to be enough for anybody*

Kernel test robot reports i915 can hit a warn in kvmalloc_node which has
a purpose of dissalowing crazy size kernel allocations. This was added in
7661809d ("mm: don't allow oversized kvmalloc() calls"):

       /* Don't even allow crazy sizes */
       if (WARN_ON_ONCE(size > INT_MAX))
               return NULL;

This would be kind of okay since i915 at one point dropped the need for
making a shadow copy of the relocation list, but then it got re-added in
fd1500fc ("Revert "drm/i915/gem: Drop relocation slowpath".") a year
after Linus added the above warning.

It is plausible that the issue was not seen until now because to trigger
gem_exec_reloc test requires a combination of an relatively older
generation hardware but with at least 8GiB of RAM installed. Probably even
more depending on runtime checks.

Lets cap what we allow userspace to pass in using the matching limit.
There should be no issue for real userspace since we are talking about
"crazy" number of relocations which have no practical purpose.

*) Well IGT tests might get upset but they can be easily adjusted.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202405151008.6ddd1aaf-oliver.sang@intel.com
Cc: Kees Cook <keescook@chromium.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20240521101201.18978-1-tursulin@igalia.com

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -1533,7 +1533,7 @@ static int eb_relocate_vma(struct i915_execbuffer *eb, struct eb_vma *ev)
 		u64_to_user_ptr(entry->relocs_ptr);
 	unsigned long remain = entry->relocation_count;
 
-	if (unlikely(remain > N_RELOC(ULONG_MAX)))
+	if (unlikely(remain > N_RELOC(INT_MAX)))
 		return -EINVAL;
 
 	/*
@@ -1641,7 +1641,7 @@ static int check_relocations(const struct drm_i915_gem_exec_object2 *entry)
 	if (size == 0)
 		return 0;
 
-	if (size > N_RELOC(ULONG_MAX))
+	if (size > N_RELOC(INT_MAX))
 		return -EINVAL;
 
 	addr = u64_to_user_ptr(entry->relocs_ptr);