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 07E5A3859F4; Fri, 17 Jul 2026 11:45:19 +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=1784288724; cv=none; b=YOSUg2r/FVaKjWCadkIInc+ptscFJLsuRksosOs2eX1H1UGgDcvpFjK1zohyKOq/cxb6dAPiLrCrBXAber733OJINAj8GwdghC2XVQVIcwB0pRnw/WRBga5esccRvRT1HmeaXgOBBEERQcSfzr5REn7VMqSStP4J6w4vvFWjE7Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784288724; c=relaxed/simple; bh=Bo8k2rDEjaCscH2PzEJCclAdZ7/kWwdotP2KsgA8nmk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TyIrGh0lKmjtzzbmdd/h13HmA0iB/YH+ZIaFuKvZ+rErGcRfcDQNEwptRPAWZwGl0wKq8webKv0f9TrbJQr2sqhmi4BhojRKjfCvPRjnGGbdku7lKBBRof3tgg6PDhwIGVlmWAC4CQDbNe3QrsMp7msIv+NwBF1aeZ5XkjXJV7U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WRNVSRpt; 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="WRNVSRpt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D93521F000E9; Fri, 17 Jul 2026 11:45:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784288719; bh=H0iH+iLx4WpWMlR/UNqi4frhPXGJpdzII4B/mC2Dvak=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=WRNVSRptWK6XYa4Qx4ycFwy+YSZ0sh256CyMtcxwLqRGQnZnv20dPjhLgDnYR5hO+ wYBKKKgRCGTBm7davcd2HvN/ei6UTurGSSe0PiUsyoeJCpKX0q/1NdQNS2bbbtU0Pi 7zs92fuSaLV/9lgBGyXrGWWymB42knidq/161vuAenO+Rv1iS4lh+BKmGuBxar03BR pVz9eNecpKtEVX03Hz/BRdsr0oog3dfvAPuKjXXSEZPD/miCvmhOK4u9mylALq73W7 GSg4C7H7YIv6cJbF5hkYfJ4g9DFLFUhHZov1hso1Q3pyOLXxBpTj+7z0UenW1/pORw yd0PhSue8nB0A== Date: Fri, 17 Jul 2026 12:45:00 +0100 From: "Lorenzo Stoakes (ARM)" To: Will Deacon Cc: Andrew Morton , Suren Baghdasaryan , "Liam R. Howlett" , Vlastimil Babka , Shakeel Butt , David Hildenbrand , Mike Rapoport , 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, "Denis V. Lunev" , stable@vger.kernel.org Subject: Re: [PATCH mm-hotfixes v4 2/4] x86/mm/pat: acquire init_mm write lock to avoid UAF Message-ID: References: <20260716-series-vmap-race-fix-v4-0-8c108c4317df@kernel.org> <20260716-series-vmap-race-fix-v4-2-8c108c4317df@kernel.org> Precedence: bulk X-Mailing-List: bpf@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: On Fri, Jul 17, 2026 at 12:30:38PM +0100, Will Deacon wrote: > On Thu, Jul 16, 2026 at 10:31:13PM +0100, Lorenzo Stoakes (ARM) 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. > > > > In addition, concurrent CPA collapse operations are possible which can also > > cause races. > > > > Resolve the issue by acquiring the mmap write lock on init_mm across the > > whole operation. > > Thanks, Lorenzo. > > This ain't worth much here, but all the same: > > Reviewed-by: Will Deacon Always appreciated :>) thanks! > > with one nit below: > > > diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c > > index d023a40a1e03..d1e63f7d267f 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 > > @@ -410,7 +411,7 @@ static void __cpa_flush_tlb(void *data) > > > > static int collapse_large_pages(unsigned long addr, struct list_head *pgtables); > > > > -static void cpa_collapse_large_pages(struct cpa_data *cpa) > > +static void __cpa_collapse_large_pages(struct cpa_data *cpa) > > { > > unsigned long start, addr, end; > > struct ptdesc *ptdesc, *tmp; > > @@ -442,6 +443,18 @@ static void cpa_collapse_large_pages(struct cpa_data *cpa) > > } > > } > > > > +static void cpa_collapse_large_pages(struct cpa_data *cpa) > > +{ > > + /* > > + * Take the mmap write lock on init_mm to: > > + * - Avoid a use-after-free if raced by ptdump (which takes its own > > + * write lock on init_mm). > > + * - Serialise concurrent CPA walkers. > > The CPA lock should handle those, so I think it's the non-CPA walkers Ah well no actually, see Denis's patch at [2] - we actually have a race here that this doesn't quite solve. But. See my reply at [3] - if people are agreeable to that I'll respin with an extra patch to fix that which this change now permits :)) > that are problematic. Looks like Sashiko reckons it's found some that > don't take the mmap_lock at all [1], but this is still a big step in the > right direction, fixes the ptdump case and we're not trying to boil the > ocean. > > Will > > [1] https://lore.kernel.org/all/20260716215307.14A6E1F000E9@smtp.kernel.org/#t In the long run I'm looking into RCU-safe page table freeing more generally, but obviously here we're trying to get things to a state of vague sanity (and probably we're not getting _all_ the races anyway) first. Cheers, Lorenzo [2]:https://lore.kernel.org/all/20260626163213.2284080-1-den@openvz.org/ [3]:https://lore.kernel.org/all/20260716-series-vmap-race-fix-v4-0-8c108c4317df@kernel.org/