MIPS: mm: Fix out-of-bounds write in maar_res_walk()

[Yadan Fan <ydfan@suse.com>]

maar_res_walk() uses wi->num_cfg as the index into the fixed-size
wi->cfg array, but checks whether the array is full only after it has
filled the selected entry. If walk_system_ram_range() reports more than
16 memory ranges, the overflow call writes one struct maar_config past
the end of the array before WARN_ON() prevents num_cfg from advancing.

Move the full-array check before taking the array slot. Keep the
existing behavior of warning and returning 0 when the scratch array has
no room left.

Fixes: a5718fe8f70f ("MIPS: mm: Drop boot_mem_map")
Signed-off-by: Yadan Fan <ydfan@suse.com>
---
 arch/mips/mm/init.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/mips/mm/init.c b/arch/mips/mm/init.c
index 55b25e85122a..0ba958b7ffa5 100644
--- a/arch/mips/mm/init.c
+++ b/arch/mips/mm/init.c
@@ -272,9 +272,15 @@ static int maar_res_walk(unsigned long start_pfn, unsigned long nr_pages,
 			 void *data)
 {
 	struct maar_walk_info *wi = data;
-	struct maar_config *cfg = &wi->cfg[wi->num_cfg];
+	struct maar_config *cfg;
 	unsigned int maar_align;
 
+	/* Ensure we don't overflow the cfg array */
+	if (WARN_ON(wi->num_cfg >= ARRAY_SIZE(wi->cfg)))
+		return 0;
+
+	cfg = &wi->cfg[wi->num_cfg];
+
 	/* MAAR registers hold physical addresses right shifted by 4 bits */
 	maar_align = BIT(MIPS_MAAR_ADDR_SHIFT + 4);
 
@@ -283,9 +289,7 @@ static int maar_res_walk(unsigned long start_pfn, unsigned long nr_pages,
 	cfg->upper = ALIGN_DOWN(PFN_PHYS(start_pfn + nr_pages), maar_align) - 1;
 	cfg->attrs = MIPS_MAAR_S;
 
-	/* Ensure we don't overflow the cfg array */
-	if (!WARN_ON(wi->num_cfg >= ARRAY_SIZE(wi->cfg)))
-		wi->num_cfg++;
+	wi->num_cfg++;
 
 	return 0;
 }
-- 
2.51.0