mm: kmsan: panic on failure to allocate early boot metadata

Given large enough allocations and a machine with low enough memory (i.e a
default QEMU VM), it's entirely possible that
kmsan_init_alloc_meta_for_range's shadow+origin allocation fails.

Instead of eating a NULL deref kernel oops, check explicitly for
memblock_alloc() failure and panic with a nice error message.

Alexander Potapenko said:

For posterity, it is generally quite important for the allocated shadow
and origin to be contiguous, otherwise an unaligned memory write may
result in memory corruption (the corresponding unaligned shadow write will
be assuming that shadow pages are adjacent).  So instead of panicking we
could have split the range into smaller ones until the allocation
succeeds, but that would've led to hard-to-debug problems in the future.

Link: https://lkml.kernel.org/r/20231016153446.132763-1-pedro.falcato@gmail.comSigned-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kmsan/shadow.c

@@ -285,12 +285,17 @@ void __init kmsan_init_alloc_meta_for_range(void *start, void *end)
 	size = PAGE_ALIGN((u64)end - (u64)start);
 	shadow = memblock_alloc(size, PAGE_SIZE);
 	origin = memblock_alloc(size, PAGE_SIZE);
+
+	if (!shadow || !origin)
+		panic("%s: Failed to allocate metadata memory for early boot range of size %llu",
+		      __func__, size);
+
 	for (u64 addr = 0; addr < size; addr += PAGE_SIZE) {
 		page = virt_to_page_or_null((char *)start + addr);
-		shadow_p = virt_to_page_or_null((char *)shadow + addr);
+		shadow_p = virt_to_page((char *)shadow + addr);
 		set_no_shadow_origin_page(shadow_p);
 		shadow_page_for(page) = shadow_p;
-		origin_p = virt_to_page_or_null((char *)origin + addr);
+		origin_p = virt_to_page((char *)origin + addr);
 		set_no_shadow_origin_page(origin_p);
 		origin_page_for(page) = origin_p;
 	}