From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 A8F632EEE80; Wed, 15 Jul 2026 15:49:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784130581; cv=none; b=a94mc97NIRJkUuaLw3eymK9f063snZrkaDp74WodFSFY+mxTVNRX/liUAYuf166OWFnGoEFTT8Z6+rwRts0OUPySuhcOkeiKEB8sjh/pd9zqVdvz9a5YocezlJR7dIWKvHehhcQawYaNM0/xaC4pSCF75q4Slx7fdjQmg/KZjeY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784130581; c=relaxed/simple; bh=REy7UL312QrEDzK1bW0DMkqqh4m8Ol8H1Ad9I4djheA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=CunHmnjtA1++LAnbMLVTpfHHMlpwi0ud0aO6pBbViRiD5j2urjProa0TjfN6wDJfX1IcNBRJCuEEFz9HDHyktZ7kS0qDyoRnR/FBQsSHPkNCruS8JJAjopGfoDXgtXfKOr20mJ5HYfBLce8tihmbx26HDtD68miR93WqVXqosNo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JfStKliW; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JfStKliW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 27E021F000E9; Wed, 15 Jul 2026 15:49:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784130580; bh=dppU3jext8Cfhc/a4fcob/OWEauwlB4pCHse6dF5FfE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=JfStKliWix9MQgnZ5RZg9SJ72c2MTpasGugfWwP7+RJOvdJ96GK2cdQR/hyjOHt3N PoICC3f/12tQsSjwwmrk4qdrpdx1oTc2w0cCF1zgOQtDB3SYTldCH7cLg9v+LTWwhz ae8/7OsggPgfBKlm6DusfuBPm0vDTJvOIhJKlGSlqVZGP3XK5SlSJyzr1dntetduZK QL307ELqTSxqUUyNDPoqjM2I4XRG1/wxlv+2MHaJ4ORE4CwT1irifJgiPl1kCjBUZy qoHp+UfSipV3mdjpYbAVCBIMkI5oFsyDTv1YESKX7JKF2Logz1eO7y05C7pC1uVQ6a D7LztoDurYkGA== Date: Wed, 15 Jul 2026 18:49:30 +0300 From: Mike Rapoport To: Will Deacon Cc: Lorenzo Stoakes , Andrew Morton , Suren Baghdasaryan , "Liam R. Howlett" , Vlastimil Babka , Shakeel Butt , David Hildenbrand , Michal Hocko , Uladzislau Rezki , Toshi Kani , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , Kiryl Shutsemau , Catalin Marinas , Dev Jain , Ryan Roberts , David Carlier , linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org, abarnas@google.com Subject: Re: [PATCH mm-hotfixes v3 2/4] x86/mm/pat: acquire mmap lock on page table free to avoid ptdump UAF Message-ID: References: <20260714-series-vmap-race-fix-v3-0-b812eccfa0f9@kernel.org> <20260714-series-vmap-race-fix-v3-2-b812eccfa0f9@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Hi Will, On Wed, Jul 15, 2026 at 04:24:59PM +0100, Will Deacon wrote: > Hi Lorenzo, > > I'm certainly no x86 expert (quite the opposite!), but I was looking at > this with Adrian and got myself confused. See below. > > On Tue, Jul 14, 2026 at 06:24:24PM +0100, Lorenzo Stoakes wrote: > > x86 implements page attribute modification using its Change Page > > Attributes (CPA) mechanism. > > > > This tracks properties of ranges such as cache mode through x86 page > > attributes, and as part of that logic manipulates kernel page tables. > > > > Since commit 41d88484c71c ("x86/mm/pat: restore large ROX pages after > > fragmentation") ranges of kernel page table entries can be collapsed into > > huge page table entries as part of this logic. > > > > As part of this collapse, it frees the page tables which the collapsed > > entries previously pointed to, and it does so without any relevant locks > > being held to preclude concurrent kernel page table walkers. > > > > The only way this code can be reached is if CPA_COLLAPSE is specified, and > > this is only set in set_memory_rox() via: > > > > set_memory_rox() > > -> change_page_attr_set_clr() > > -> cpa_flush() > > -> cpa_collapse_large_pages() > > > > Notable users of this are execmem and bpf when manipulating executable > > mappings. > > > > However, this is problematic for ptdump as it walks ranges it does not own > > and thus runs the risk of a use-after-free on page tables freed underneath > > it. > > > > Resolve the issue by acquiring the mmap read lock on init_mm which prevents > > a concurrent ptdump as it acquires the write lock. > > > > It is safe to acquire a sleeping lock as all the callers invoke > > set_memory_rox() from process context and in any case, > > change_page_attr_set_clr() calls vm_unmap_alias() which ultimately takes a > > mutex, disallowing atomic context here. > > > > Fixes: 41d88484c71c ("x86/mm/pat: restore large ROX pages after fragmentation") > > Cc: stable@vger.kernel.org > > Reviewed-by: Mike Rapoport (Microsoft) > > Reviewed-by: Kiryl Shutsemau (Meta) > > Signed-off-by: Lorenzo Stoakes > > --- > > arch/x86/mm/pat/set_memory.c | 14 +++++++++++--- > > 1 file changed, 11 insertions(+), 3 deletions(-) > > > > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c > > index d023a40a1e03..4c4b8244502f 100644 > > --- a/arch/x86/mm/pat/set_memory.c > > +++ b/arch/x86/mm/pat/set_memory.c > > @@ -22,6 +22,7 @@ > > #include > > #include > > #include > > +#include > > > > #include > > #include > > @@ -436,9 +437,16 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa) > > > > flush_tlb_all(); > > > > - list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) { > > - list_del(&ptdesc->pt_list); > > - pagetable_free(ptdesc); > > + /* > > + * ptdump might read these page tables, so avoid a use-after-free by > > + * acquiring the mmap read lock on init_mm (ptdump acquires the mmap > > + * write lock). > > + */ > > + scoped_guard(mmap_read_lock, &init_mm) { > > + list_for_each_entry_safe(ptdesc, tmp, &pgtables, pt_list) { > > + list_del(&ptdesc->pt_list); > > + pagetable_free(ptdesc); > > + } > > As I understand it, the argument for taking the read lock is that we're > operating on a region that we "wholly own" and therefore we can happily > run concurrently with CPUs walking distinct parts of the page-table. > However, from what I can tell, the CPA collapse logic will operate on > regions outside of the address range being manipulated by its caller > because it rounds up to the PMD size. > > As a made-up example, imagine I have a 2MiB aligned region where the > first 1MiB is read-only and the second 1MiB is in the default r/w state. > If one CPU calls set_memory_rw() on the first 1MiB while another CPU is > walking the second 1MiB (via some other API that doesn't take cpa_lock), > it looks to me like the first CPU can collapse the page-table and free > the unused pages under the feet of the other CPU. What prevents that > from happening? Nothing, and there's a patch to fix that that synchronizes cpa_collapse_large_pages() using cpa_lock: https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org This still won't be enough to sync with ptdump though. > If all concurrent walkers have interrupts disabled, I guess the TLB > invalidation logic would do it, but it would be good to call this out in > the commit message because it's not clear to me why the read_lock is > sufficient for the collapsing case. > > Cheers, > > Will -- Sincerely yours, Mike.