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 831583B47CD; Mon, 7 Sep 2026 16:56:59 +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=1788800221; cv=none; b=OLihD2X0NLIwR+EaJy3cEUuG0GqJSDRG09dD8ohTninaXQiyVQY4ohYP7swSGU3Ew0CcWgEUdM48VduCvK6dr4hk7TaXsqh2unwdw8c8q2ZQjCmBngvLP7hTYghYuW+b0cg6JJ33RIrnrWplGm4HJQuvS+B5w8Xxi3BWS3MFH4c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788800221; c=relaxed/simple; bh=aTuazGd35Bcu9csBhTUgz4cz9heaPWV36zCjuHxVwA0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=pdEYwu7gLcvMCt2kva/RCjM5MChEnx+Xampw9l4Xm3rcoeM718KBZ6WLUcxB6A0XJ8N/GQWMRkzDnyg+dfXpjvv3LvH3OwttTJFZTN5kE8UALtjTxSWkGwRVbVUdIwKMzLxSPrzGm7TrO0Joh9OJyU+Ps1tpD4O+GG9nvtI2PSM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=U2eMkJBn; 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="U2eMkJBn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1A1461F00A3A; Mon, 7 Sep 2026 16:56:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788800219; bh=7sMiZud/gecw2GU2/OU3WSFMgRwBwDYM1b6ol/3L2AQ=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=U2eMkJBnxejCrjOQYxNf49B/y7ghwS63xl8kEFfSHPWHnEslOhZPUH9PrfOaFb/oO vhBO4qgPGcA4rgq/5XcEfm7JlWYqx1IgFGQwjh1wWy1/O6JVpIHy2U4n8+BXHPsDNx 1VLmZw2b6mikVBQRxjIYCzdsH5OuRk7gr2NrmNJMcOj+DSky/Ir00/zP+/t1IlvVKT b2ju0GSQNWjg7+im/JUVMHfkNJdv0WspOrigCfBEvB4oyND2m/Ye03Ju/EQ9/Hr6j1 HFdX4QrTcpf91Aj+wxuQwfBphRIEyrP22dI461P1S/nE4ua3v7dNt1qETwDaMYrngQ Wxon93uiVUv/A== Message-ID: Date: Mon, 7 Sep 2026 18:56:51 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/6] mm/vma: only permit MAP_PRIVATE /dev/zero to be mapped anonymous To: "Lorenzo Stoakes (ARM)" , Arnd Bergmann , Greg Kroah-Hartman , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Hugh Dickins , Baolin Wang , "Matthew Wilcox (Oracle)" , Jan Kara Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260902-map-private-dev-zero-v1-0-a578c730cec7@kernel.org> <20260902-map-private-dev-zero-v1-3-a578c730cec7@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260902-map-private-dev-zero-v1-3-a578c730cec7@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/2/26 20:00, Lorenzo Stoakes (ARM) wrote: > In order to use mmap_prepare() with MAP_PRIVATE mappings of /dev/zero > without the success_hook hack we explicitly permitted mmap_prepare handlers > to set NULL vm_ops. > > However this is dangerous and we really only want to allow this for > MAP_PRIVATE-mapped /dev/zero. > > Therefore use the newly introduced file_is_dev_zero() to uniquely identify > MAP_PRIVATE-/dev/zero mappings and only permit this behaviour for them. > > Then, remove all ability for mmap_prepare or mmap hooks to set a VMA > anonymous and update mmap_zero_prepare() to leave it to the core mmap code > to do so. > > Note that this disallows nested MAP_PRIVATE-mappings of /dev/zero > regions. Doing this would be broken in any case. > > We therefore do not need to update the mmap_prepare() compatibility layer > to reflect these changes, as the mmap hook check suffices to disallow this > behaviour. > > Now we're setting vma->vm_ops to NULL for an mmap_prepare-initialised > MAP_PRIVATE-/dev/zero mapping, we have to avoid a subtle issue when > updating user-defined fields via set_vma_user_defined_fields(). > > The default for vma->vm_ops for all mmap_prepare-initialised mappings is > vma_dummy_vm_ops, so map->vm_ops will be set to this and setting > vma->vm_ops to this will render the VMA mistakenly non-anon. > > In general, we should never be setting user-defined fields for an anonymous > VMA, so explicitly check for this to avoid doing so for the one case where > a mapping can be both mmap_prepare and anonymous. > > In the case of legacy ->mmap hooks some drivers may set vma->vm_ops NULL > believing this is the equivalent of setting no VMA operations. Therefore > update mmap_file() to correct this by setting dummy VMA operations if this > occurs. > > An example of this is drm_gem_shmem_mmap() which deliberately clears > vma->vm_ops before handing the VMA to dma-buf. Cases such as this will be > updated when they are converted to mmap_prepare. > > Also, in order to avoid a single commit bisection hazard, add a temporary > workaround to set the VMA anonymous only after vma->vm_file is assigned in > __mmap_new_file_vma(). > > This is because vma_set_range() calls vma_set_pgoff() and > assert_sane_pgoff() in turn, prior to the vma->vm_file being assigned. If > we set the VMA anonymous early then this assert will fail. > > This is removed in the subsequent commit. > > Signed-off-by: Lorenzo Stoakes (ARM) > --- > mm/char-mem.c | 6 +----- > mm/internal.h | 17 ++++++++++------- > mm/vma.c | 33 +++++++++++++++++++++++++-------- > 3 files changed, 36 insertions(+), 20 deletions(-) > > diff --git a/mm/char-mem.c b/mm/char-mem.c > index e53e89e6ddd8..c0b5fb019223 100644 > --- a/mm/char-mem.c > +++ b/mm/char-mem.c > @@ -508,11 +508,7 @@ static int mmap_zero_prepare(struct vm_area_desc *desc) > if (vma_desc_test(desc, VMA_SHARED_BIT)) > return shmem_zero_setup_desc(desc); > > - /* > - * This is a highly unique situation where we mark a MAP_PRIVATE mapping > - * of /dev/zero anonymous, despite it not being. > - */ > - vma_desc_set_anonymous(desc); > + /* MAP_PRIVATE semantics are taken care of for us by core mm. */ > return 0; > } > > diff --git a/mm/internal.h b/mm/internal.h > index 5d474e5f7709..da14c56fb24e 100644 > --- a/mm/internal.h > +++ b/mm/internal.h > @@ -226,15 +226,18 @@ static inline int mmap_file(struct file *file, struct vm_area_struct *vma) > { > int err = vfs_mmap(file, vma); > > - if (likely(!err)) > - return 0; > - > /* > - * OK, we tried to call the file hook for mmap(), but an error > - * arose. The mapping is in an inconsistent state and we must not invoke > - * any further hooks on it. > + * Either we tried to call the file hook for mmap() and an error arose > + * or a driver set vma->vm_ops = NULL intending there to be no VMA > + * operations. > + * > + * In the former case the VMA is in an inconsistent state and we mustn't > + * invoke any further hooks on it, in the latter case the hook actually > + * wanted no further hooks to be invoked, so fix both by setting dummy > + * VMA ops. > */ > - vma->vm_ops = &vma_dummy_vm_ops; > + if (unlikely(err || !vma->vm_ops)) > + vma->vm_ops = &vma_dummy_vm_ops; > > return err; > } > diff --git a/mm/vma.c b/mm/vma.c > index 35e7a64855fa..4b8d430d9619 100644 > --- a/mm/vma.c > +++ b/mm/vma.c > @@ -2621,6 +2621,19 @@ static int __mmap_new_file_vma(struct mmap_state *map, > return 0; > } > > +static bool map_is_private(const struct mmap_state *map) > +{ > + return !vma_flags_test(&map->vma_flags, VMA_SHARED_BIT); > +} > + > +static bool map_is_anon(const struct mmap_state *map) I was wondering whether we should call this "map_is_private_anon", due to MAP_ANON|MAP_SHARED. But looking at __mmap_new_vma(), the existing "is_anon" is also limited to MAP_ANON|MAP_PRIVATE. > +{ > + if (!map_is_private(map)) > + return false; > + > + return !map->file || file_is_dev_zero(map->file); > +} > + > /* > * __mmap_new_vma() - Allocate a new VMA for the region, as merging was not > * possible. > @@ -2634,8 +2647,7 @@ static int __mmap_new_file_vma(struct mmap_state *map, > static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap, > struct mmap_action *action) > { > - const bool is_anon = !map->file && > - !vma_flags_test(&map->vma_flags, VMA_SHARED_BIT); > + const bool is_anon = map_is_anon(map); > struct vma_iterator *vmi = map->vmi; > int error = 0; > struct vm_area_struct *vma; > @@ -2651,7 +2663,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap, > > vma_iter_config(vmi, map->addr, map->end); > > - if (is_anon) > + if (is_anon && !map->file) > vma_set_anonymous(vma); > > vma_set_range(vma, map->addr, map->end, map->pgoff, map->anon_pgoff); > @@ -2669,6 +2681,10 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap, > else if (!is_anon) > error = shmem_zero_setup(vma); > > + /* Temporary MAP_PRIVATE-/dev/zero workaround. */ > + if (is_anon && map->file) > + vma_set_anonymous(vma); > + > if (error) > goto free_iter_vma; > > @@ -2777,6 +2793,10 @@ static int call_mmap_prepare(struct mmap_state *map, > if (err) > return err; > > + /* Hooks cannot mark themselves anonymous. */ I guess this comment will be stale soon (after #4 where you drop the set_anonymous part). Should it be "vm_ops are strictly required with mmap_prepare" or sth like that? > + if (!desc->vm_ops) > + return -EINVAL; > + > err = call_action_prepare(map, desc); > if (err) > return err; > @@ -2799,10 +2819,7 @@ static int call_mmap_prepare(struct mmap_state *map, > static void set_vma_user_defined_fields(struct vm_area_struct *vma, > struct mmap_state *map) > { > - if (map->vm_ops) > - vma->vm_ops = map->vm_ops; > - else /* Only /dev/zero should do this. */ > - vma_set_anonymous(vma); > + vma->vm_ops = map->vm_ops; > vma->vm_private_data = map->vm_private_data; > } > > @@ -2882,7 +2899,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr, > allocated_new = true; > } > > - if (have_mmap_prepare) > + if (have_mmap_prepare && !map_is_anon(&map)) > set_vma_user_defined_fields(vma, &map); Ah, we have mmap_zero_prepare() for handling the shmem_zero_setup_desc(). I was just about to ask whether we can just get rid of this here. But, hold on, do we now even need that? Could core-mm now take care of that as well, and we could just remove mmap_zero_prepare() entirely? That is, we'd make shmem_zero_setup() in __mmap_new_vma() take care of this? Then we might not even need shmem_zero_setup_desc() anymore. Maybe harder than it sounds at first. -- Cheers, David