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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=htGvWvY7; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EF9E8606A327 for ; Tue, 20 Oct 2026 13:00:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=htGvWvY7 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 877C71F000CA; Tue, 20 Oct 2026 13:00:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792501209; bh=0SlxZ5UQXu81xsFhH9I1ceqg/5A7Zf9sbax9KJqUchU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=htGvWvY70hcOTpY92QXwy8gTYzLILBSSisBX8mhP1sRKfYjNS6z09IxQOvQBMZm7Z HaiPFoXiUlsj7EF2GdaS3DckfqOcJlBx5eAZmOR8drqfoFb1OWgj9srtF825m7IO+G 2Eaa29YIujwB+KexjanWX0jOM/ig/GHCHk1O6MUJy129SfiOabCY5J6Lon+eKe8DrT sBmXMH+3Uvvj+ga2A8kvkYsvuv9M9lWU91nP0W7O5WrUUcq2Tdle92kI5GZ3TTrE5n ZHCbpxm/cqjOqbaze53ai+CDC6++u7Q2WEtyGk8SYfmQdNa1RRwiF/MTjbhIAWLfTz 2KkdmVuPKPb7A== Message-ID: <3c80ab1a-d86d-4986-bfae-d513734a0d90@kernel.org> Date: Tue, 20 Oct 2026 15:00:07 +0200 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [BUG] mm: userfaultfd RWP drops a populated shmem PMD To: shaos , akpm@linux-foundation.org Cc: Mike Rapoport , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261020110759.960089-1-shawnq1@163.com> 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 10/20/26 13:07, shaos wrote: > Hello, > > UFFDIO_RWPROTECT_MODE_RWP can return success for a present shmem THP mapping while dropping the PMD mapping instead of leaving the populated page resident and write-protected. The next read refaults the page and completes without a UFFD_PAGEFAULT_FLAG_RWP event. > > The PoC verifies ShmemPmdMapped=2048 kB immediately before the ioctl. After the successful ioctl, smaps reports Rss=0 and ShmemPmdMapped=0; the read completes with the original byte and no event. Re-arming the refaulted PTE and a separate 4 KiB shmem control both produce the expected RWP event. The observed issue is loss of the mapping and protection tracking, not corruption of the file contents. > > The source path splits and clears the non-anonymous PMD for MM_CP_UFFD_RWP, but the retry does not repopulate a PTE table because pgtable_populate_needed() only recognizes MM_CP_UFFD_WP. The ioctl nevertheless returns success. > > HEAD commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907 > source version: Linux 7.3.0-rc7 > architecture: x86-64 > compiler: gcc 11.4.0, GNU ld 2.38 > QEMU: 6.2.0 > > console output: https://gist.githubusercontent.com/shaos26/3c336ea98e1e8fe09b5270c3a82ca268/raw/da13df3db0c05df09952f74bedb457f57c53b78b/console-output.txt > kernel config: https://gist.githubusercontent.com/shaos26/3c336ea98e1e8fe09b5270c3a82ca268/raw/baa5ee61ce748667104c480560ab5116a100e3fd/kernel-config.txt > reproducer: https://gist.githubusercontent.com/shaos26/3c336ea98e1e8fe09b5270c3a82ca268/raw/420975783e9d5dd88bc4a69190d8ef194faf5fdc/poc.c > > Build the kernel from the revision above with the attached config. Build the static PoC and initramfs from poc.c; the program is /init and creates the shmem mapping itself. A fresh-boot run used: > > timeout 300 qemu-system-x86_64 -m 2G -smp 2 \ > -kernel build/arch/x86/boot/bzImage -initrd initramfs.cpio.gz \ > -append "console=ttyS0 panic=1 oops=panic loglevel=7 kfence.sample_interval=0" \ > -nographic -no-reboot -monitor none > > Two fresh boots reported RESULT=RWP_SHMEM_PMD_BYPASS_CONFIRMED and exited cleanly. The affected code appears unchanged in upstream master f47d223627f8; I have only reproduced it on v7.3-rc7. No privilege escalation, memory corruption, or guest escape is claimed. > > Thanks, > > shaos > https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/ -- Cheers, David