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smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WJqDVD7J; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="WJqDVD7J" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1C6D51F00893; Mon, 1 Jun 2026 12:04:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780315486; bh=/IonyAXTK4z3N68EQaWEo5DEKU7B9ZtHA9Q/mLTloro=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=WJqDVD7Jm0TsknGaT29e7GQKi5+cL4Mmj+mlFLXfWi8hwdxVUAYSdscOiM+SHx5qV L440LJE+rpRT1xKV09jJnksJqvSsoCoVM/gQFiNBWemrfIe9sqMb70hHY4jlsJwiEW pnJPvDFxv2X3QUuB5Pic5JdbG1eY3Jacc8JPMm1x3+qsdrnPOmgnq1vhfx2AFNIr56 o/ncbK9tLTTVHT2jBqWSANG2j8rxZUDBfEIKqB4EEFcvfx2hLFSvhSPjy4X48zQSfc sz0vuQTcfSwKJdthZuRyMrUJtmKbwr14Q9cZ65nM/cK+qkZTfh+SKdGBS9oGxcHHEL qtutPZLnpSCSg== Message-ID: Date: Mon, 1 Jun 2026 14:04:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm_access: simplify the security checks To: Oleg Nesterov , Andrew Morton Cc: Christian Brauner , Jann Horn , Kees Cook , Lorenzo Stoakes , Michal Hocko , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/30/26 16:12, Oleg Nesterov wrote: I am not sure I spot the "simplification" here? Looks more like an optimization that makes the code slightly more complicated. > 1. Shift the fast-path "mm == current->mm" check from may_access_mm() > to mm_access(), and do it locklessly. > > task->mm is not stable but we do not care. We can race with exec, > but in this case we pin/return current->mm. This doesn't differ > from the case where the target execs after we drop exec_update_lock. > > All we need for correctness is READ_ONCE() to ensure the compiler > won't reload task->mm. This is not enough for KCSAN, but we already > have other lockless ->mm LOAD's. We should probably change exec_mmap/ > exit_mm to use WRITE_ONCE(). > > 2. With the change above, may_access_mm() doesn't need the "mm" argument, > so we do not need to call get_task_mm() beforehand. We can call it > only if may_access_mm() succeeds. > > Signed-off-by: Oleg Nesterov > --- > kernel/fork.c | 30 ++++++++++++++++-------------- > 1 file changed, 16 insertions(+), 14 deletions(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index b8b651abce8b..3239380ab93b 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1381,10 +1381,8 @@ struct mm_struct *get_task_mm(struct task_struct *task) > } > EXPORT_SYMBOL_GPL(get_task_mm); > > -static bool may_access_mm(struct mm_struct *mm, struct task_struct *task, unsigned int mode) > +static bool may_access_mm(struct task_struct *task, unsigned int mode) We now have a function named "may_access_mm" and not providing an mm struct pointer ... it now looks more like an extended, mm-independent ptrace check. So probably we should rename that function. > { > - if (mm == current->mm) > - return true; > if (ptrace_may_access(task, mode)) > return true; > if ((mode & PTRACE_MODE_READ) && perfmon_capable()) > @@ -1394,20 +1392,24 @@ static bool may_access_mm(struct mm_struct *mm, struct task_struct *task, unsign > > struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) > { > - struct mm_struct *mm; > - int err; > + struct mm_struct *mm = READ_ONCE(task->mm); > > - err = down_read_killable(&task->signal->exec_update_lock); > - if (err) > - return ERR_PTR(err); > + if (!mm || (task->flags & PF_KTHREAD)) > + return ERR_PTR(-ESRCH); > > - mm = get_task_mm(task); > - if (!mm) { > - mm = ERR_PTR(-ESRCH); > - } else if (!may_access_mm(mm, task, mode)) { > - mmput(mm); > - mm = ERR_PTR(-EACCES); > + if (mm == current->mm) { > + mmget(mm); > + return mm; > } > + > + if (down_read_killable(&task->signal->exec_update_lock)) > + return ERR_PTR(-EINTR); > + > + if (may_access_mm(task, mode)) > + mm = get_task_mm(task) ?: ERR_PTR(-ESRCH); > + else > + mm = ERR_PTR(-EACCES); > + > up_read(&task->signal->exec_update_lock); > > return mm; -- Cheers, David