linux-kernel - Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused after being freed

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Date:   Thu, 17 Feb 2022 12:50:56 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Michal Hocko <mhocko@...e.com>, shy828301@...il.com,
        rientjes@...gle.com, willy@...radead.org, hannes@...xchg.org,
        guro@...com, riel@...riel.com, minchan@...nel.org,
        kirill@...temov.name, aarcange@...hat.com, brauner@...nel.org,
        christian@...uner.io, hch@...radead.org, oleg@...hat.com,
        david@...hat.com, jannh@...gle.com, shakeelb@...gle.com,
        luto@...nel.org, christian.brauner@...ntu.com, fweimer@...hat.com,
        jengelh@...i.de, timmurray@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        syzbot+2ccf63a4bd07cf39cab0@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] mm: fix use-after-free bug when mm->mmap is reused
 after being freed

On Thu, 17 Feb 2022 11:51:13 -0800 Suren Baghdasaryan <surenb@...gle.com> wrote:

> This changelog is very detailed and I support switching to it. Andrew,
> please let me know if I should re-post the patch with this description
> or you will just amend the one in your tree.

I have made that change.


From: Suren Baghdasaryan <surenb@...gle.com>
Subject: mm: fix use-after-free bug when mm->mmap is reused after being freed

oom reaping (__oom_reap_task_mm) relies on a 2 way synchronization with
exit_mmap.  First it relies on the mmap_lock to exclude from unlock
path[1], page tables tear down (free_pgtables) and vma destruction. 
This alone is not sufficient because mm->mmap is never reset.  For
historical reasons[2] the lock is taken there is also MMF_OOM_SKIP set
for oom victims before.

The oom reaper only ever looks at oom victims so the whole scheme works
properly but process_mrelease can opearate on any task (with fatal
signals pending) which doesn't really imply oom victims.  That means
that the MMF_OOM_SKIP part of the synchronization doesn't work and it
can see a task after the whole address space has been demolished and
traverse an already released mm->mmap list.  This leads to use after
free as properly caught up by KASAN report.

Fix the issue by reseting mm->mmap so that MMF_OOM_SKIP synchronization
is not needed anymore.  The MMF_OOM_SKIP is not removed from exit_mmap
yet but it acts mostly as an optimization now.

[1] 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3")
[2] 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")

[mhocko@...e.com: changelog rewrite]
Link: https://lore.kernel.org/all/00000000000072ef2c05d7f81950@google.com/
Link: https://lkml.kernel.org/r/20220215201922.1908156-1-surenb@google.com
Fixes: 64591e8605d6 ("mm: protect free_pgtables with mmap_lock write lock in exit_mmap")
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Reported-by: syzbot+2ccf63a4bd07cf39cab0@...kaller.appspotmail.com
Suggested-by: Michal Hocko <mhocko@...e.com>
Reviewed-by: Rik van Riel <riel@...riel.com>
Reviewed-by: Yang Shi <shy828301@...il.com>
Acked-by: Michal Hocko <mhocko@...e.com>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Roman Gushchin <guro@...com>
Cc: Rik van Riel <riel@...riel.com>
Cc: Minchan Kim <minchan@...nel.org>
Cc: Kirill A. Shutemov <kirill@...temov.name>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: David Hildenbrand <david@...hat.com>
Cc: Jann Horn <jannh@...gle.com>
Cc: Shakeel Butt <shakeelb@...gle.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Christian Brauner <christian.brauner@...ntu.com>
Cc: Florian Weimer <fweimer@...hat.com>
Cc: Jan Engelhardt <jengelh@...i.de>
Cc: Tim Murray <timmurray@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 mm/mmap.c |    1 +
 1 file changed, 1 insertion(+)

--- a/mm/mmap.c~mm-fix-use-after-free-bug-when-mm-mmap-is-reused-after-being-freed
+++ a/mm/mmap.c
@@ -3186,6 +3186,7 @@ void exit_mmap(struct mm_struct *mm)
 		vma = remove_vma(vma);
 		cond_resched();
 	}
+	mm->mmap = NULL;
 	mmap_write_unlock(mm);
 	vm_unacct_memory(nr_accounted);
 }
_