[PATCH v2] memblock: reject non-page-aligned reserve_mem regions

[Sang-Heon Jeon <ekffu200098@gmail.com>]

When map->start or map->size is not page aligned,
reserved_mem_preserve() does not preserve the last one or two pages of
the region from map->start to map->start + map->size.

After kexec the new kernel reserves only the preserved pages, so those
pages are released to the buddy allocator. But
reserve_mem_find_by_name() still returns the whole region including
them, so they can be allocated while the subsystem is still using them.

So reject a reserve_mem= size or alignment that is not a multiple of
PAGE_SIZE to keep both map->start and map->size page aligned.

Fixes: 8375b76517cb ("kho: replace kho_preserve_phys() with kho_preserve_pages()")
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
---
Changes from v1 [1]
- reject non-page-aligned size and align instead of fix page count when
  preserving reserve_mem regions

[1] https://lore.kernel.org/all/20260901165237.1025973-1-ekffu200098@gmail.com/
---
 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 mm/memblock.c                                   | 9 +++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 68647ff4bdd2..f969085fffb7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6648,6 +6648,9 @@ Kernel parameters
 
 			reserve_mem=12M:4096:oops ramoops.mem_name=oops
 
+			Both the size and the alignment must be multiples of
+			the page size.
+
 	reservetop=	[X86-32,EARLY]
 			Format: nn[KMG]
 			Reserves a hole at the top of the kernel virtual
diff --git a/mm/memblock.c b/mm/memblock.c
index 4302fb4ab85c..5e93b6da53e3 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2816,12 +2816,17 @@ static int __init reserve_mem(char *p)
 	if (*p != ':')
 		goto err_param;
 
+	if (!IS_ALIGNED(size, PAGE_SIZE) || !IS_ALIGNED(align, PAGE_SIZE)) {
+		pr_err("reserve_mem: size and align must be multiples of the page size\n");
+		return -EINVAL;
+	}
+
 	/*
 	 * memblock_phys_alloc() doesn't like a zero size align,
 	 * but it is OK for this command to have it.
 	 */
-	if (align < SMP_CACHE_BYTES)
-		align = SMP_CACHE_BYTES;
+	if (!align)
+		align = PAGE_SIZE;
 
 	name = p + 1;
 	len = strlen(name);

base-commit: dbffc67777cd752ef3f77818109e1d2ef13e2949
-- 
2.43.0