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Thu, 11 Jul 2024 14:51:43 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFlzHXcQ41LSwTTFqc3QdjuN5WRgCohFU7c9FyUV8sGTTslvHz4sM17bH3ZcGwoezS+8JIZNg== X-Received: by 2002:a17:90a:f2cb:b0:2c9:9f4f:c787 with SMTP id 98e67ed59e1d1-2ca35d44b14mr7958966a91.35.1720734702661; Thu, 11 Jul 2024 14:51:42 -0700 (PDT) Received: from [172.20.2.228] ([4.28.11.157]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2cacd2d08a7sm71165a91.10.2024.07.11.14.51.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jul 2024 14:51:42 -0700 (PDT) Message-ID: Date: Thu, 11 Jul 2024 23:51:41 +0200 Precedence: bulk X-Mailing-List: linux-kernel-mentees@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: Fix mmap_assert_locked() in follow_pte() From: David Hildenbrand To: Pei Li , Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, skhan@linuxfoundation.org, syzkaller-bugs@googlegroups.com, linux-kernel-mentees@lists.linuxfoundation.org, syzbot+35a4414f6e247f515443@syzkaller.appspotmail.com, Peter Xu References: <20240710-bug12-v1-1-0e5440f9b8d3@gmail.com> <92a2dc30-6e48-44ea-9cde-693b911f200d@redhat.com> <3879ee72-84de-4d2a-93a8-c0b3dc3f0a4c@redhat.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11.07.24 23:45, David Hildenbrand wrote: > On 11.07.24 23:33, David Hildenbrand wrote: >> On 11.07.24 07:13, Pei Li wrote: >>> This patch fixes this warning by acquiring read lock before entering >>> untrack_pfn() while write lock is not held. >>> >>> syzbot has tested the proposed patch and the reproducer did not >>> trigger any issue. >>> >>> Reported-by: syzbot+35a4414f6e247f515443@syzkaller.appspotmail.com >>> Closes: https://syzkaller.appspot.com/bug?extid=35a4414f6e247f515443 >>> Tested-by: syzbot+35a4414f6e247f515443@syzkaller.appspotmail.com >>> Signed-off-by: Pei Li >>> --- >>> Syzbot reported the following warning in follow_pte(): >>> >>> WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 rwsem_assert_held include/linux/rwsem.h:195 [inline] >>> WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 mmap_assert_locked include/linux/mmap_lock.h:65 [inline] >>> WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 follow_pte+0x414/0x4c0 mm/memory.c:5980 >>> >>> This is because we are assuming that mm->mmap_lock should be held when >>> entering follow_pte(). This is added in commit c5541ba378e3 (mm: >>> follow_pte() improvements). >>> >>> However, in the following call stack, we are not acquring the lock: >>> follow_phys arch/x86/mm/pat/memtype.c:957 [inline] >>> get_pat_info+0xf2/0x510 arch/x86/mm/pat/memtype.c:991 >>> untrack_pfn+0xf7/0x4d0 arch/x86/mm/pat/memtype.c:1104 >>> unmap_single_vma+0x1bd/0x2b0 mm/memory.c:1819 >>> zap_page_range_single+0x326/0x560 mm/memory.c:1920 >> >> That implies that unmap_vmas() is called without the mmap lock in read >> mode, correct? >> >> Do we know how this happens? >> >> * exit_mmap() holds the mmap lock in read mode >> * unmap_region is documented to hold the mmap lock in read mode > > I think this is it (missed the call from zap_page_range_single()): > > follow_phys arch/x86/mm/pat/memtype.c:957 [inline] > get_pat_info+0xf2/0x510 arch/x86/mm/pat/memtype.c:991 > untrack_pfn+0xf7/0x4d0 arch/x86/mm/pat/memtype.c:1104 > unmap_single_vma+0x1bd/0x2b0 mm/memory.c:1819 > zap_page_range_single+0x326/0x560 mm/memory.c:1920 > unmap_mapping_range_vma mm/memory.c:3684 [inline] > unmap_mapping_range_tree mm/memory.c:3701 [inline] > unmap_mapping_pages mm/memory.c:3767 [inline] > unmap_mapping_range+0x1ee/0x280 mm/memory.c:3804 > truncate_pagecache+0x53/0x90 mm/truncate.c:731 > simple_setattr+0xf2/0x120 fs/libfs.c:886 > notify_change+0xec6/0x11f0 fs/attr.c:499 > do_truncate+0x15c/0x220 fs/open.c:65 > handle_truncate fs/namei.c:3308 [inline] > > I think Peter recently questioned whether untrack_pfn() should be even > called from the place, but I might misremember things. > > Fix should work (I suspect we are not violating some locking rules?), > PFNMAP should not happen there too often that we really care. ... thinking again, likely we reach this point with "!mm_wr_locked" and the mmap lock already held in read mode. So I suspect the fix won't work as is. -- Cheers, David / dhildenb