fork,memcg: alloc_thread_stack_node needs to set tsk->stack

BugLink: https://bugs.launchpad.net/bugs/1863019
[ Upstream commit 1bf4580e00a248a2c86269125390eb3648e1877c ]

Commit 5eed6f1dff87 ("fork,memcg: fix crash in free_thread_stack on
memcg charge fail") corrected two instances, but there was a third
instance of this bug.

Without setting tsk->stack, if memcg_charge_kernel_stack fails, it'll
execute free_thread_stack() on a dangling pointer.

Enterprise kernels are compiled with VMAP_STACK=y so this isn't
critical, but custom VMAP_STACK=n builds should have some performance
advantage, with the drawback of risking to fail fork because compaction
didn't succeed.  So as long as VMAP_STACK=n is a supported option it's
worth fixing it upstream.

Link: http://lkml.kernel.org/r/20190619011450.28048-1-aarcange@redhat.com
Fixes: 9b6f7e163cd0 ("mm: rework memcg kernel stack accounting")
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/kernel/fork.c b/kernel/fork.c
index 8358236909d724073cec2463b802fd7afe95e1bd..d76eb64182fe75595e58eee1e230354c951df81b 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -244,7 +244,11 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
        struct page *page = alloc_pages_node(node, THREADINFO_GFP,
                                             THREAD_SIZE_ORDER);
 
-       return page ? page_address(page) : NULL;
+       if (likely(page)) {
+               tsk->stack = page_address(page);
+               return tsk->stack;
+       }
+       return NULL;
 #endif
 }