[PATCH] x86/microcode/amd: Add __list_del_entry_valid() in front of __list_del() in free_cache()

[PATCH] x86/microcode/amd: Add __list_del_entry_valid() in front of __list_del() in free_cache()

[Jiasheng Jiang]

Directly use __list_del() to remove 'p->plist' might be unsafe,
as we can see from the __list_del_entry_valid() of 'lib/list_debug.c'
that p->plist->prev or p->plist->next may be the LIST_POISON,
or p->list is not a valid double list.
In that case, __list_del() will be corruption.
Therefore, we suggest that __list_del_entry_valid()
should be added in front of the __list_del() in free_cache.

Fixes: bad5fa6 ("x86, microcode: Move to a proper location")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
---
 arch/x86/kernel/cpu/microcode/amd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index 3d4a483..e589cff 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -626,6 +626,8 @@ static void free_cache(void)
 	struct ucode_patch *p, *tmp;
 
 	list_for_each_entry_safe(p, tmp, &microcode_cache, plist) {
+		if (!__list_del_entry_valid(&p->plist))
+			continue;
 		__list_del(p->plist.prev, p->plist.next);
 		kfree(p->data);
 		kfree(p);
-- 
2.7.4

Re: [PATCH] x86/microcode/amd: Add __list_del_entry_valid() in front of __list_del() in free_cache()

[Christoph Hellwig]

On Tue, Sep 28, 2021 at 09:04:44AM +0000, Jiasheng Jiang wrote:
> Directly use __list_del() to remove 'p->plist' might be unsafe,
> as we can see from the __list_del_entry_valid() of 'lib/list_debug.c'
> that p->plist->prev or p->plist->next may be the LIST_POISON,
> or p->list is not a valid double list.
> In that case, __list_del() will be corruption.
> Therefore, we suggest that __list_del_entry_valid()
> should be added in front of the __list_del() in free_cache.

No one should be using list_del directly anyway, so please fix that.