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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Li Xinhai , linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+f12658786a4153df5113@syzkaller.appspotmail.com, stable@vger.kernel.org Subject: Re: [PATCH] mm/mremap: reset unfaulted VMA page offset for MREMAP_DONTUNMAP Date: Thu, 27 Aug 2026 14:26:50 +0800 Message-ID: <20260827062652.305611-1-kunwu.chan@linux.dev> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260826192642.980c3aba921bf20cef425591@linux-foundation.org> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 AndrewTo: Andrew Morton Content-Transfer-Encoding: 8bit On Wed, 26 Aug 2026 19:26:42 -0700 Andrew Morton wrote: > On Tue, 25 Aug 2026 08:55:26 +0100 "Lorenzo Stoakes (ARM)" wrote: > > > Uniquely an mremap() invocation using the MREMAP_DONTUNMAP flag can reset > > a faulted VMA into an unfaulted one. > > > > It does so after the page tables have been moved to the copied VMA with > > MREMAP_DONTUNMAP leaving the old VMA in place which is naturally unfaulted > > as the page tables it had are no longer present. > > > > However, in doing so, it violates the invariant that the anonymous page > > offset of an unfaulted VMA is vma->vm_start >> PAGE_SHIFT. > > > > This is because a VMA may have been faulted in, mremap()'d (causing a delta > > between its page offset and vma->vm_start >> PAGE_SHIFT), and then > > mremap()'d again with MREMAP_DONTUNMAP resulting in the unfaulting. > > > > This condition is a violation of a fundamental assumption in mm, but now > > also triggers an assert in assert_sane_pgoff() which explicitly checks for > > this condition. > > > > Correct it by resetting the VMA's page offset at the point of completing > > the MREMAP_DONTUNMAP operation. > > Thanks. I'll park this in mm-new until mm.git is all merged up > (simplifying my life..) > > > Unrelatedly, Sashiko thinks we're messing up locked_vm accounting with > MREMAP_DONTUNMAP on a locked VMA. > https://sashiko.dev/#/patchset/20260825-fix-mremap-dontunmap-pgoff-v1-1-39a40b2c98b3@kernel.org > > > I had Sashiko write code to demonstrate this but am too lazy to test it > on a current kernel. If someone could oblige? Hi Andrew, I'll test the reproducer on a current kernel and check the mm->locked_vm accounting before and after MREMAP_DONTUNMAP. Thanks, Kunwu > > > #define _GNU_SOURCE > #include > #include > #include > #include > #include > #include > > /* Read VMLck (in kB) from /proc/self/status */ > static long get_vmlck_kb(void) { > FILE *f = fopen("/proc/self/status", "r"); > if (!f) { > perror("fopen /proc/self/status"); > return -1; > } > > char line[256]; > long vmlck = -1; > while (fgets(line, sizeof(line), f)) { > if (strncmp(line, "VMLck:", 6) == 0) { > sscanf(line + 6, "%ld", &vmlck); > break; > } > } > fclose(f); > return vmlck; > } > > int main(void) { > size_t size = 4096 * 10; // 40 kB > long initial_vmlck, post_mlock, post_mremap, post_munmap; > > initial_vmlck = get_vmlck_kb(); > printf("[1] Initial VMLck: %ld kB\n", initial_vmlck); > > /* 1. Allocate initial VMA */ > void *addr = mmap(NULL, size, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > if (addr == MAP_FAILED) { > perror("mmap initial"); > return 1; > } > > /* 2. Lock the VMA (increments mm->locked_vm) */ > if (mlock(addr, size) != 0) { > perror("mlock"); > return 1; > } > post_mlock = get_vmlck_kb(); > printf("[2] Post-mlock VMLck: %ld kB (+%ld kB)\n", > post_mlock, post_mlock - initial_vmlck); > > /* 3. mremap with MREMAP_DONTUNMAP > * move_vma() increments mm->locked_vm for the destination VMA, > * while dontunmap_complete() clears VMA_LOCKED_MASK on source VMA > * without decrementing mm->locked_vm. > */ > void *new_addr = mremap(addr, size, size, > MREMAP_MAYMOVE | MREMAP_DONTUNMAP, NULL); > if (new_addr == MAP_FAILED) { > perror("mremap MREMAP_DONTUNMAP"); > return 1; > } > post_mremap = get_vmlck_kb(); > printf("[3] Post-mremap VMLck: %ld kB (+%ld kB from initial)\n", > post_mremap, post_mremap - initial_vmlck); > > /* 4. Unmap source VMA > * Since VMA_LOCKED_BIT was cleared on source VMA, > * munmap fails to decrement mm->locked_vm for this region. > */ > munmap(addr, size); > post_munmap = get_vmlck_kb(); > printf("[4] Post-munmap source: %ld kB\n", post_munmap); > > /* 5. Clean up destination VMA */ > munmap(new_addr, size); > long final_vmlck = get_vmlck_kb(); > printf("[5] Final VMLck: %ld kB\n", final_vmlck); > > /* Evaluation */ > printf("\n--- Result ---\n"); > if (final_vmlck > initial_vmlck) { > printf("BUG DEMONSTRATED: Leaked %ld kB in mm->locked_vm counter.\n", > final_vmlck - initial_vmlck); > } else { > printf("NO LEAK: mm->locked_vm returned to initial state.\n"); > } > > return 0; > } > >