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SubjectRe: [PATCH] fork: fix error handling in dup_task()
On Thu, 12 Jul 2012 20:04:53 +0900
Akinobu Mita <akinobu.mita@gmail.com> wrote:

> The function dup_task() may fail at the following function calls in
> the following order.
>
> 0) alloc_task_struct_node()
> 1) alloc_thread_info_node()
> 2) arch_dup_task_struct()
>
> Error by 0) is not a matter, it can just return. But error by 1)
> requires releasing task_struct allocated by 0) before it returns.
> Likewise, error by 2) requires releasing task_struct and thread_info
> allocated by 0) and 1).
>
> The existing error handling calls free_task_struct() and
> free_thread_info() which do not only release task_struct and
> thread_info, but also call architecture specific
> arch_release_task_struct() and arch_release_thread_info().
>
> The problem is that task_struct and thread_info are not fully
> initialized yet at this point, but arch_release_task_struct() and
> arch_release_thread_info() are called with them.
>
> For example, x86 defines its own arch_release_task_struct() that
> releases a task_xstate. If alloc_thread_info_node() fails in
> dup_task(), arch_release_task_struct() is called with task_struct
> which is just allocated and filled with garbage in this error handling.
>
> This actually happened with tools/testing/fault-injection/failcmd.sh
>
> # env FAILCMD_TYPE=fail_page_alloc \
> ./tools/testing/fault-injection/failcmd.sh --times=100 \
> --min-order=0 --ignore-gfp-wait=0 \
> -- make -C tools/testing/selftests/ run_tests
>
> In order to fix this issue, make free_{task_struct,thread_info}() not
> to call arch_release_{task_struct,thread_info}() and call
> arch_release_{task_struct,thread_info}() implicitly where needed.
>
> Default arch_release_task_struct() and arch_release_thread_info() are
> defined as empty by default. So this change only affects the
> architectures which implement their own arch_release_task_struct() or
> arch_release_thread_info() as listed below.

This conflicts with Salman's fix (below) which is in linux-next via
Ingo's tree.

It appears that we should drop Salman's patch altogether and use yours?


commit 164c33c6adee609b8b9062cce4c10f764d0dce13
Author: Salman Qazi <sqazi@google.com>
AuthorDate: Mon Jun 25 18:18:15 2012 -0700
Commit: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu Jul 5 20:57:32 2012 +0200

sched: Fix fork() error path to not crash

In dup_task_struct(), if arch_dup_task_struct() fails, the clean up
code fails to clean up correctly. That's because the clean up
code depends on unininitalized ti->task pointer. We fix this
by making sure that the task and thread_info know about each other
before we attempt to take the error path.

Signed-off-by: Salman Qazi <sqazi@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20120626011815.11323.5533.stgit@dungbeetle.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/fork.c b/kernel/fork.c
index ab5211b..f00e319 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -304,12 +304,17 @@ static struct task_struct *dup_task_struct(struct task_struct *orig)
}

err = arch_dup_task_struct(tsk, orig);
- if (err)
- goto out;

+ /*
+ * We defer looking at err, because we will need this setup
+ * for the clean up path to work correctly.
+ */
tsk->stack = ti;
-
setup_thread_stack(tsk, orig);
+
+ if (err)
+ goto out;
+
clear_user_return_notifier(tsk);
clear_tsk_need_resched(tsk);
stackend = end_of_stack(tsk);


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