Re: [PATCH 1/2] mm/memblock: Add "reserve_mem" to reserved named memory at boot up

[Kees Cook <kees@kernel.org>]

On June 3, 2024 4:33:31 PM PDT, Steven Rostedt <rostedt@goodmis.org> wrote:
>From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
>
>In order to allow for requesting a memory region that can be used for
>things like pstore on multiple machines where the memory layout is not the
>same, add a new option to the kernel command line called "reserve_mem".
>
>The format is:  reserve_mem=nn:align:name
>
>Where it will find nn amount of memory at the given alignment of align.
>The name field is to allow another subsystem to retrieve where the memory
>was found. For example:
>
>  reserve_mem=12M:4096:oops ramoops.mem_name=oops

How does this interact with KASLR? It has chosen its physical location before this parsing happens, so I'd expect this to fail once in a while, unless the size/alignment is lucky enough that KASLR never uses that portion of the physical memory...

-Kees

>
>Where ramoops.mem_name will tell ramoops that memory was reserved for it
>via the reserve_mem option and it can find it by calling:
>
>  if (reserve_mem_find_by_name("oops", &start, &size)) {
>	// start holds the start address and size holds the size given
>
>Link: https://lore.kernel.org/all/ZjJVnZUX3NZiGW6q@kernel.org/
>
>Suggested-by: Mike Rapoport <rppt@kernel.org>
>Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
>---
> include/linux/mm.h |  2 +
> mm/memblock.c      | 97 ++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 99 insertions(+)
>
>diff --git a/include/linux/mm.h b/include/linux/mm.h
>index 9849dfda44d4..b4455cc02f2c 100644
>--- a/include/linux/mm.h
>+++ b/include/linux/mm.h
>@@ -4263,4 +4263,6 @@ static inline bool pfn_is_unaccepted_memory(unsigned long pfn)
> void vma_pgtable_walk_begin(struct vm_area_struct *vma);
> void vma_pgtable_walk_end(struct vm_area_struct *vma);
> 
>+int reserve_mem_find_by_name(const char *name, unsigned long *start, unsigned long *size);
>+
> #endif /* _LINUX_MM_H */
>diff --git a/mm/memblock.c b/mm/memblock.c
>index d09136e040d3..a8bf0ee9e2b4 100644
>--- a/mm/memblock.c
>+++ b/mm/memblock.c
>@@ -2244,6 +2244,103 @@ void __init memblock_free_all(void)
> 	totalram_pages_add(pages);
> }
> 
>+/* Keep a table to reserve named memory */
>+#define RESERVE_MEM_MAX_ENTRIES		8
>+#define RESERVE_MEM_NAME_SIZE		16
>+struct reserve_mem_table {
>+	char			name[RESERVE_MEM_NAME_SIZE];
>+	unsigned long		start;
>+	unsigned long		size;
>+};
>+static struct reserve_mem_table reserved_mem_table[RESERVE_MEM_MAX_ENTRIES];
>+static int reserved_mem_count;
>+
>+/* Add wildcard region with a lookup name */
>+static int __init reserved_mem_add(unsigned long start, unsigned long size,
>+				   const char *name)
>+{
>+	struct reserve_mem_table *map;
>+
>+	if (!name || !name[0] || strlen(name) >= RESERVE_MEM_NAME_SIZE)
>+		return -EINVAL;
>+
>+	if (reserved_mem_count >= RESERVE_MEM_MAX_ENTRIES)
>+		return -1;
>+
>+	map = &reserved_mem_table[reserved_mem_count++];
>+	map->start = start;
>+	map->size = size;
>+	strscpy(map->name, name);
>+	return 0;
>+}
>+
>+/**
>+ * reserve_mem_find_by_name - Find reserved memory region with a given name
>+ * @name: The name that is attached to a reserved memory region
>+ * @start: If found, holds the start address
>+ * @size: If found, holds the size of the address.
>+ *
>+ * Returns: 1 if found or 0 if not found.
>+ */
>+int reserve_mem_find_by_name(const char *name, unsigned long *start, unsigned long *size)
>+{
>+	struct reserve_mem_table *map;
>+	int i;
>+
>+	for (i = 0; i < reserved_mem_count; i++) {
>+		map = &reserved_mem_table[i];
>+		if (!map->size)
>+			continue;
>+		if (strcmp(name, map->name) == 0) {
>+			*start = map->start;
>+			*size = map->size;
>+			return 1;
>+		}
>+	}
>+	return 0;
>+}
>+
>+/*
>+ * Parse early_reserve_mem=nn:align:name
>+ */
>+static int __init reserve_mem(char *p)
>+{
>+	phys_addr_t start, size, align;
>+	char *oldp;
>+	int err;
>+
>+	if (!p)
>+		return -EINVAL;
>+
>+	oldp = p;
>+	size = memparse(p, &p);
>+	if (p == oldp)
>+		return -EINVAL;
>+
>+	if (*p != ':')
>+		return -EINVAL;
>+
>+	align = memparse(p+1, &p);
>+	if (*p != ':')
>+		return -EINVAL;
>+
>+	start = memblock_phys_alloc(size, align);
>+	if (!start)
>+		return -ENOMEM;
>+
>+	p++;
>+	err = reserved_mem_add(start, size, p);
>+	if (err) {
>+		memblock_phys_free(start, size);
>+		return err;
>+	}
>+
>+	p += strlen(p);
>+
>+	return *p == '\0' ? 0: -EINVAL;
>+}
>+__setup("reserve_mem=", reserve_mem);
>+
> #if defined(CONFIG_DEBUG_FS) && defined(CONFIG_ARCH_KEEP_MEMBLOCK)
> static const char * const flagname[] = {
> 	[ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",

-- 
Kees Cook