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X-CSE-ConnectionGUID: iD2H5zBfRPKeVdxaX6iAMQ== X-CSE-MsgGUID: 1PVaRuTmQc2mPufGA7HSHA== X-IronPort-AV: E=McAfee;i="6800,10657,11777"; a="101544189" X-IronPort-AV: E=Sophos;i="6.23,217,1770624000"; d="scan'208";a="101544189" Received: from fmviesa004.fm.intel.com ([10.60.135.144]) by orvoesa101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 05 May 2026 09:39:07 -0700 X-CSE-ConnectionGUID: zM7lKNCuSY2PAn++OaK1yw== X-CSE-MsgGUID: EjEMenP6Ri62q/VYCWrCQw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.23,217,1770624000"; d="scan'208";a="237639686" Received: from sghuge-mobl2.amr.corp.intel.com (HELO [10.125.110.95]) ([10.125.110.95]) by fmviesa004-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 05 May 2026 09:39:06 -0700 Message-ID: <7a82a652-f861-4d72-8bd7-4e082af482f2@intel.com> Date: Tue, 5 May 2026 09:39:09 -0700 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 6/6] x86/mm: Avoid mmap lock for shadow stack pop fast path To: "Edgecombe, Rick P" , "linux-kernel@vger.kernel.org" , "dave.hansen@linux.intel.com" Cc: "Liam.Howlett@oracle.com" , "linux-mm@kvack.org" , "ljs@kernel.org" , "surenb@google.com" , "vbabka@kernel.org" , "shakeel.butt@linux.dev" , "akpm@linux-foundation.org" References: <20260429181954.F50224AE@davehans-spike.ostc.intel.com> <20260429182005.00BF70D8@davehans-spike.ostc.intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/4/26 16:15, Edgecombe, Rick P wrote: > > I guess the problem is the lock ordering. Not sure if there is any slow path > avoidance details that could make this splat a false positive. But how about > this simpler munmap() case: > > Shadow stack signal munmap() > ------------------- -------- > vma_start_read() (VM_SHADOW_STACK check) > mmap_write_lock() > mmap_read_lock() (user fault) <- deadlock > vma_start_write() <-deadlock It's a little more complicated than that in practice, but I think you're right. I'm not sure when this would happen in practice because the fault is actually on the VMA that's being held for read. So I think another writer would have had to sneak in there and zap the VMA. The funny thing is that the fault handler is really just trying to find the VMA. The thing causing the fault *has* the VMA. So it's as simple as just passing the VMA down into the fault handler, right? How hard could it be? ;) There are still games to play, but they all involve dropping locks and retrying, like: retry: vma = lock_vma_under_rcu() // muck with VMA pagefault_disable() // avoid deadlock ret = copy_from_user() pagefault_enable() vma_end_read(); if (!ret) return SUCCESS; mmap_read_lock() vma = vma_lookup() mmap_read_unlock() // avoid deadlock before touching userspace // check for valid VMA to avoid looping when there is no VMA if (!vma) return -ERRNO; // uh oh, slow path, something faulted get_user_pages()?? //or copy_from_user() without the VMA?? goto retry; This also needs some very careful thought, but something like this should work, where we avoid fault handling (and lock taking) in the actual #PF and do it in a context where the VMA lock is held: vma = lock_vma_under_rcu(); pagefault_disable() // avoid deadlock while (1) { ret = copy_from_user() if (!ret) break; handle_mm_fault(vma, address, FAULT_FLAG_VMA_LOCK...); }; pagefault_enable() vma_end_read(); That's effectively just short-circuiting the #PF code which does the: vma = lock_vma_under_rcu(mm, address); ... fault = handle_mm_fault(vma, address, ... FAULT_FLAG_VMA_LOCK) sequence _itself_.