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b=yzsqtF6jiYGBZlz4SdvbqNPJcJ/LvRiQ3K68q2rH7j7zCoAMQNZxZ4k79hD7wIro3 qd9HFDL4FYBAT8IrxeShgzpg8M6E1cmVZy88IY9KhUuvHYeX37sCPVrZwSD1zYbJ3I dNQ1gqUG95rncu/NCCOLwrGEYMztox5TSdNqdSA4= Date: Mon, 30 Mar 2026 13:40:21 -0700 From: Andrew Morton To: David Carlier Cc: Peter Xu , Mike Rapoport , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/userfaultfd: detect VMA replacement after copy retry in mfill_copy_folio_retry() Message-Id: <20260330134021.171441c4c236b03efebc9a77@linux-foundation.org> In-Reply-To: <20260330202909.136776-1-devnexen@gmail.com> References: <20260330202909.136776-1-devnexen@gmail.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 0EC531A000E X-Stat-Signature: gtew6jfepzyfecxuf9f7o353ejmgscq4 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1774903222-956402 X-HE-Meta: U2FsdGVkX1+Fg7oAfbCDLK5SsJLF34JjDleD1FX07ybmcAui22vud9dbo0ypqWzzJKPQROpyIBQZWL+hOVE0uFo5kCoy81u/QYhqoDsM5NUBU22+R1I3toxGP6sI83siluGPuw3yhFffi6/SnzgqQftgrv2yc4nfufdi/cBFhuVXCOjyo5HXYOTm4O5iG34+vHP8jRCk98/35gi8GDcs2jGbuwR4C47dq1p2/Ixrm8rVhI5zFUz5QzmbWAfzCm1HcX6ONQcY6uFsAOn97OPK/MKAK0KDXZ1ZuXyzKtMgE17lJWxV4+CBDc013+TnNaagJuSg2iBja+WgP887ilSx6Pdgbt8HqM65xTztmKXzLsQf56VEnj/zIO0njPNlj/JOtQf75IO/I6NKRQhfej+sizNqR0ufgBtUVu8AEQEiyK73LebSVk9ZVELkmK2aCbRmx5SpIhWjhPaeAxh7ETJkMweKXcPb9v9J9dM7CGDKykBAEot5JlnAWEaS9heP19hYdzxtNHh1EY/AWPSVXfgsGA1VwQ50wXtpUxcSJucBp8dguQEzRbYvCl5KJdrXh7ZVUNB1phg1khcgDLn2xJpAUkuciWqjkWQFp9IlOzty5qONLb+bsKf8d6M459S/1i+TPKz4Mt800tjlcos9E73p0cIlVtaJOtu0E8F+UkyP2Q0vUCVw+UGdRDZHVlaT/5g3kGEDa8TDQML3sMgUdtWbLgotERyw0Ypydvu3PDD70mTPp8Oxfy6AbwdS8ZfCAJgG062jonts5lFIs00sle7BPPewSXvc4eEMiKTZcxi8gT02wSWIqJ7HTgioGsKJ5hytgLODNsqse1/LdEHP7nyFzYZA9nRPrYNYlVB4/k5RP0oM8KVVpDx9ZOaNa2GGApvKJZJ1pHXgi4wWeBK7uwBtOvkDhuound1R/lWCHvAs2s3y7vGXnuWRHhoZihi41Vc8yFpJ2AWTzNyAyi/ewCt 5TzTx4h6 Gmt46GNZNgj+OD1S67jjUCEKv/49Vr4uvaiyMAhRrkDr81QF4kAGZWqYk+Tc+MSs7PbxUg7OYr3lBbml9FDfvg0uihxd+BjJNBy5+Y3jP0AFefFeq7pnG7Q8apTlKGdiP8Q3S9S7XitacEsReZqObbxgQUIII3uFHvM1P3oFvOoYzTJEFc0G0hYPTtHT9UBhZwFV0FCQlMNUCSnoY6qqr2tZuySAQasU6CUHsXgwIIdIt74ULUem+w/KmGkRCIu+cz4HCfCq/lLZmr+1zmDRpce+tHGGhxNqhBBi9h2zbtRL0uM8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 30 Mar 2026 21:29:09 +0100 David Carlier wrote: > In mfill_copy_folio_retry(), all locks are dropped to retry > copy_from_user() with page faults enabled. During this window, the VMA > can be replaced entirely (e.g. munmap + mmap + UFFDIO_REGISTER by > another thread), but the caller proceeds with a folio allocated from the > original VMA's backing store. > > Checking ops alone is insufficient: the replacement VMA could be the > same type (e.g. shmem -> shmem) with identical flags but a different > backing inode. Take a snapshot of the VMA's inode and flags before > dropping locks, and compare after re-acquiring them. If anything > changed, bail out with -EAGAIN. Thanks. What are the userspace-visible runtime effects of the bug? If they're serious we might be looking at a cc:stable and a Fixes: tag?