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Mon, 05 Oct 2026 02:20:02 +0000 X-Mizu-Trace-ID: 4cae8ba250b31cd8 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: akpm@linux-foundation.org Cc: neganhat@gmail.com, david@kernel.org, jgg@ziepe.ca, jhubbard@nvidia.com, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/gup: fix NULL pointer dereference in fixup_user_fault() Date: Mon, 5 Oct 2026 10:19:57 +0800 Message-ID: <20261005021957.5254-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20261004125601.c06953a6f0660df9859b0c2a@linux-foundation.org> References: <20261004125601.c06953a6f0660df9859b0c2a@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: g1zizx6943kmgqdugoc9cpx4iaic1kaj X-Rspamd-Queue-Id: 4598820005 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1791166805-277396 X-HE-Meta: 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 jrZ7Qwvg P2I41ym0s8pkC6LmDjD1kU6S/FlJG3szkmBVjP4QvqZAc31wr0+Dm8SQfew2jEnFknt/80cwConvt4Djy6VKiZzE9j1/qYy1qL723o42DYnrkWZkwkRggIHk8k3BH+J/9qLta7+Im88xHDDYQQ7k7fvPsbMGV5MMScJwJSTH3ynlE54o+v+FOgmMUXUrLaOSJ7zgNEuIVrhdZ8AaqYpt6GsbdsRnBa3709zLGuPe5mW7zAa8pti03QX4gXJZ6+USnzbj38TdrUOCEIBL378XKxXtvDGcxVdDkD4r2VBsDScK+wkzeFE9uVA4WNQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Oct 04, 2026 at 12:56:01PM -0700, Andrew Morton wrote: >On Sun, 4 Oct 2026 02:48:46 +0700 Nguyen Duy Nhat Anh wrote: > >> In fixup_user_fault(), the 'unlocked' parameter is checked for NULL >> early on, allowing callers to pass NULL if they do not track whether the >> mmap lock was dropped. >> >> However, if handle_mm_fault() returns VM_FAULT_COMPLETED, line 1597 >> dereferences 'unlocked' directly (*unlocked = true) without checking >> if it is NULL. Callers like s390's pci_mmio.c pass NULL for 'unlocked', >> leading to a kernel NULL pointer dereference when VM_FAULT_COMPLETED >> occurs. >> >> Fix this by checking if 'unlocked' is non-NULL before assigning to it. > >This code is too subtle so you aren't the first to attempt to "fix" it. > >The key hint is in the kerneldoc: > > * @unlocked: did we unlock the mmap_lock while retrying, maybe NULL if caller > * does not allow retry. If NULL, the caller must guarantee > * that fault_flags does not contain FAULT_FLAG_ALLOW_RETRY. > >Trace through the >FAULT_FLAG_ALLOW_RETRY/VM_FAULT_COMPLETED/VM_FAULT_RETRY logic >to confirm that the null deref is a cant-happen. IIUC, that's indeed a can't-happen :)