From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C3F70302768; Wed, 1 Oct 2025 20:52:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759351966; cv=none; b=UoPchMISurHqf2k+20xEeXF09s1Bj8bXFvcKXmqKNeFmjubwvL7fb4zOAQhu3WzZylsKfa7rh5ljA8gmaidM62ViM2XcCfs0YOnCpTB3M+odgAbcY0iRZRm95aY6A2pM1UHulXgTj9jH7H3hqQNItSjS923nuIgxK/rpMN8fd+c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759351966; c=relaxed/simple; bh=nuggKT4B+/cwdFthBZZHXFM/gZ5aXtLjPy9a5wkLf9A=; h=Date:To:From:Subject:Message-Id; b=HxOgmyecskc4vAd0yaWcaRztL6BHGboPvMD2qzlwcdaIx3LgUC1sw1d443smUVytYxoWBMEp31558nFylNVrjsH5LPC53fCD5DHm3HfDw32kCy3CwayQtWZQCKj70d5qKVhkY4qa1RIrf/j8hLrmRUsVMM2axd97p2IvcWPFjTc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=zyybq6Pt; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="zyybq6Pt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 30014C4CEF4; Wed, 1 Oct 2025 20:52:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1759351966; bh=nuggKT4B+/cwdFthBZZHXFM/gZ5aXtLjPy9a5wkLf9A=; h=Date:To:From:Subject:From; b=zyybq6Ptc4wn29wkV9AGwrjGxCxwTxZ/KcP6tPaKEWwjv71eE9lY1Vy2ogb+iGlZL PwTa7POjVHBUOBALY3qAnSRAw03NGXYmBBH+db2uemhCwlW4qqN1VxQJq9/I1e5MeC VGmopQ/KhGK5Zln+B85pG/70szfKcFaDqsJ81GAU= Date: Wed, 01 Oct 2025 13:52:45 -0700 To: mm-commits@vger.kernel.org,xu.xin16@zte.com.cn,stable@vger.kernel.org,sj@kernel.org,peterx@redhat.com,david@redhat.com,chengming.zhou@linux.dev,axelrasmussen@google.com,acsjakub@amazon.de,akpm@linux-foundation.org From: Andrew Morton Subject: + mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise.patch added to mm-hotfixes-unstable branch Message-Id: <20251001205246.30014C4CEF4@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: mm/ksm: fix flag-dropping behavior in ksm_madvise has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Jakub Acs Subject: mm/ksm: fix flag-dropping behavior in ksm_madvise Date: Wed, 1 Oct 2025 09:03:52 +0000 syzkaller discovered the following crash: (kernel BUG) [ 44.607039] ------------[ cut here ]------------ [ 44.607422] kernel BUG at mm/userfaultfd.c:2067! [ 44.608148] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI [ 44.608814] CPU: 1 UID: 0 PID: 2475 Comm: reproducer Not tainted 6.16.0-rc6 #1 PREEMPT(none) [ 44.609635] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 [ 44.610695] RIP: 0010:userfaultfd_release_all+0x3a8/0x460 [ 44.617726] Call Trace: [ 44.617926] [ 44.619284] userfaultfd_release+0xef/0x1b0 [ 44.620976] __fput+0x3f9/0xb60 [ 44.621240] fput_close_sync+0x110/0x210 [ 44.622222] __x64_sys_close+0x8f/0x120 [ 44.622530] do_syscall_64+0x5b/0x2f0 [ 44.622840] entry_SYSCALL_64_after_hwframe+0x76/0x7e [ 44.623244] RIP: 0033:0x7f365bb3f227 Kernel panics because it detects UFFD inconsistency during userfaultfd_release_all(). Specifically, a VMA which has a valid pointer to vma->vm_userfaultfd_ctx, but no UFFD flags in vma->vm_flags. The inconsistency is caused in ksm_madvise(): when user calls madvise() with MADV_UNMEARGEABLE on a VMA that is registered for UFFD in MINOR mode, it accidentally clears all flags stored in the upper 32 bits of vma->vm_flags. Assuming x86_64 kernel build, unsigned long is 64-bit and unsigned int and int are 32-bit wide. This setup causes the following mishap during the &= ~VM_MERGEABLE assignment. VM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000. After ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then promoted to unsigned long before the & operation. This promotion fills upper 32 bits with leading 0s, as we're doing unsigned conversion (and even for a signed conversion, this wouldn't help as the leading bit is 0). & operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff instead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears the upper 32-bits of its value. Fix it by changing `VM_MERGEABLE` constant to unsigned long, using the BIT() macro. Note: other VM_* flags are not affected: This only happens to the VM_MERGEABLE flag, as the other VM_* flags are all constants of type int and after ~ operation, they end up with leading 1 and are thus converted to unsigned long with leading 1s. Note 2: After commit 31defc3b01d9 ("userfaultfd: remove (VM_)BUG_ON()s"), this is no longer a kernel BUG, but a WARNING at the same place: [ 45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067 but the root-cause (flag-drop) remains the same. Link: https://lkml.kernel.org/r/20251001090353.57523-2-acsjakub@amazon.de Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode") Signed-off-by: Jakub Acs Acked-by: David Hildenbrand Acked-by: SeongJae Park Cc: Xu Xin Cc: Chengming Zhou Cc: Peter Xu Cc: Axel Rasmussen Cc: Signed-off-by: Andrew Morton --- include/linux/mm.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/include/linux/mm.h~mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise +++ a/include/linux/mm.h @@ -296,7 +296,7 @@ extern unsigned int kobjsize(const void #define VM_MIXEDMAP 0x10000000 /* Can contain "struct page" and pure PFN pages */ #define VM_HUGEPAGE 0x20000000 /* MADV_HUGEPAGE marked this vma */ #define VM_NOHUGEPAGE 0x40000000 /* MADV_NOHUGEPAGE marked this vma */ -#define VM_MERGEABLE 0x80000000 /* KSM may merge identical pages */ +#define VM_MERGEABLE BIT(31) /* KSM may merge identical pages */ #ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS #define VM_HIGH_ARCH_BIT_0 32 /* bit only usable on 64-bit architectures */ _ Patches currently in -mm which might be from acsjakub@amazon.de are mm-ksm-fix-flag-dropping-behavior-in-ksm_madvise.patch