[v2,5/8] riscv: mm: Take memory hotplug read-lock during kernel page table dump

Commit Message

Björn Töpel May 14, 2024, 2:04 p.m. UTC

From: Björn Töpel <[e-mail]>

During memory hot remove, the ptdump functionality can end up touching
stale data. Avoid any potential crashes (or worse), by holding the
memory hotplug read-lock while traversing the page table.

This change is analogous to arm64's commit bf2b59f60ee1 ("arm64/mm:
Hold memory hotplug lock while walking for kernel page table dump").

Signed-off-by: Björn Töpel <[e-mail]>
---
 arch/riscv/mm/ptdump.c | 3 +++
 1 file changed, 3 insertions(+)

Comments

David Hildenbrand May 14, 2024, 8:39 p.m. UTC | #1

On 14.05.24 16:04, Björn Töpel wrote:
> From: Björn Töpel <[e-mail]>
> 
> During memory hot remove, the ptdump functionality can end up touching
> stale data. Avoid any potential crashes (or worse), by holding the
> memory hotplug read-lock while traversing the page table.
> 
> This change is analogous to arm64's commit bf2b59f60ee1 ("arm64/mm:
> Hold memory hotplug lock while walking for kernel page table dump").
> 
> Signed-off-by: Björn Töpel <[e-mail]>
> ---
>   arch/riscv/mm/ptdump.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/riscv/mm/ptdump.c b/arch/riscv/mm/ptdump.c
> index 1289cc6d3700..9d5f657a251b 100644
> --- a/arch/riscv/mm/ptdump.c
> +++ b/arch/riscv/mm/ptdump.c
> @@ -6,6 +6,7 @@
>   #include <linux/efi.h>
>   #include <linux/init.h>
>   #include <linux/debugfs.h>
> +#include <linux/memory_hotplug.h>
>   #include <linux/seq_file.h>
>   #include <linux/ptdump.h>
>   
> @@ -370,7 +371,9 @@ bool ptdump_check_wx(void)
>   
>   static int ptdump_show(struct seq_file *m, void *v)
>   {
> +	get_online_mems();
>   	ptdump_walk(m, m->private);
> +	put_online_mems();
>   
>   	return 0;
>   }

Reviewed-by: David Hildenbrand <[e-mail]>

Oscar Salvador May 14, 2024, 9:03 p.m. UTC | #2

On Tue, May 14, 2024 at 04:04:43PM +0200, Björn Töpel wrote:
> From: Björn Töpel <[e-mail]>
> 
> During memory hot remove, the ptdump functionality can end up touching
> stale data. Avoid any potential crashes (or worse), by holding the
> memory hotplug read-lock while traversing the page table.
> 
> This change is analogous to arm64's commit bf2b59f60ee1 ("arm64/mm:
> Hold memory hotplug lock while walking for kernel page table dump").
> 
> Signed-off-by: Björn Töpel <[e-mail]>

Reviewed-by: Oscar Salvador <[e-mail]>

funny enough, it seems arm64 and riscv are the only ones holding the
hotplug lock here.
I think we have the same problem on the other arches as well (at least
on x86_64 that I can see).

If we happen to finally need the lock in those, I would rather have a
centric function in the generic mm code with the locking and then
calling an arch specific ptdump_show function, so the lock is not
scattered. But that is another story.

Patch

diff --git a/arch/riscv/mm/ptdump.c b/arch/riscv/mm/ptdump.c
index 1289cc6d3700..9d5f657a251b 100644
--- a/arch/riscv/mm/ptdump.c
+++ b/arch/riscv/mm/ptdump.c
@@ -6,6 +6,7 @@ 
 #include <linux/efi.h>
 #include <linux/init.h>
 #include <linux/debugfs.h>
+#include <linux/memory_hotplug.h>
 #include <linux/seq_file.h>
 #include <linux/ptdump.h>
 
@@ -370,7 +371,9 @@  bool ptdump_check_wx(void)
 
 static int ptdump_show(struct seq_file *m, void *v)
 {
+	get_online_mems();
 	ptdump_walk(m, m->private);
+	put_online_mems();
 
 	return 0;
 }