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 41B2852ED38; Tue, 8 Sep 2026 11:24:57 +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=1788866704; cv=none; b=giacjSkS57cKvdYor27Pslnuvs4cgx/1liGBOn/J33Z6TsaovkkiEDluqlEke2tdcF63dCs2L7sDRhH2OtnAieKhJRWioaZiXcxNn7OmuiyLU+gGwnaAs6J0zYDu4k0L5nHNI/j5COAuPyEQH2flObKgJQ3KmoFmYE1Xm/jCArQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788866704; c=relaxed/simple; bh=K4kKSpQaR4XCTcqNOsKhlctxc2n/RJkZ2c1LQ0Kbg9I=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=iqDeZdxthTZ+ByvVV+hQIqVs3tJShOt0+OrAl5TLhWgfrlUgcv2JToYyEnL4hnoLRqTz+W8x6fHKrxZc4jpSrNgteJ/PsA3+eSHnNyET9OZXeo4IM85GhUyrDG9aI2Z96HrrBSIwuQfhIfsqir7QpcZKtkIASkJhJTz3zAEiVUM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RA6OXMH7; 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="RA6OXMH7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 98E231F00A3F; Tue, 8 Sep 2026 11:24:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788866694; bh=Sr9jsabfi8EcucrWWLeAxRfyM9FSn5Wtyl4+cNbIDm8=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=RA6OXMH77Zgvaug81fCjs6q8EFJrkbGmeb5ZTjWBjxCYmZgZ8iTsBWQli9NNSLlxI VA9VdFEgvov7JLmT1QlJk6DibaCmUI1gcg6hlnXFqhxznqkJ24BwmuOQ0rVhFXxXnf f510ZISKCnXtVnIpHmiv9lPnAaCpG5I9H+RxQSMb7Dwu949ia2SkyAH/qxp3hbWNy/ yRi966BwRmoku4PUIcg+v8K+NkJ1zLsDDdBnM/RFgWbMPcLsXOET7kSO7/1Tlz5GbN 9wt8xwfZvE+Y2kRVil6nc5rE4jqeJhy/Ul45Z6GvJwBngeBQeXWyuNoj4SHsvHCsLi bUAhZ8HJegA5w== From: "Lorenzo Stoakes (ARM)" Date: Tue, 08 Sep 2026 12:23:40 +0100 Subject: [PATCH v2 3/6] mm/vma: only permit MAP_PRIVATE /dev/zero to be mapped anonymous Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260908-map-private-dev-zero-v2-3-acc7b5625305@kernel.org> References: <20260908-map-private-dev-zero-v2-0-acc7b5625305@kernel.org> In-Reply-To: <20260908-map-private-dev-zero-v2-0-acc7b5625305@kernel.org> To: Arnd Bergmann , Greg Kroah-Hartman , Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , David Hildenbrand , 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, "Lorenzo Stoakes (ARM)" X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=6758; i=ljs@kernel.org; h=from:subject:message-id; bh=K4kKSpQaR4XCTcqNOsKhlctxc2n/RJkZ2c1LQ0Kbg9I=; b=owGbwMvMwCV2fu7ZrsZH9SKMp9WSGLLmfyhce1aq6l3qAWex1cw/52hrmF+YdSzvbuuxBJ62A plnb8NEOkpZGMS4GGTFFFmefxHfHyQSNq/zgr8bzBxWJpAhDFycAjCRP9sZ/kqxlcwXi/dMeuN4 6sIhzZs8PNlG/V4bxdusvSPMt6ot/8/IsKXiQvjFou2VZZd/vugSuqSqEtg+d0K0fmTuiYUsgo+ NmQA= X-Developer-Key: i=ljs@kernel.org; a=openpgp; fpr=E7F417BF5214569E89D04F46CF9DCD8A81E27F14 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) +{ + 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. */ + 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); __mmap_complete(&map, vma); -- 2.55.0