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 59AC71427A for ; Tue, 30 Dec 2025 02:54:47 +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=1767063287; cv=none; b=taNUvAglSuJKNH48Wht+7qdLFX8DqhOiOsEKnwWlFs+7RMeOn4u6gZ+zw1kjGyG6IvDE8Db9gim/VK3V8CJ3XNteiIS79+TLU8rWw2whTsDwV7L++JARSiP87l2jVj68F+ySRoGwSE/MHrhTBnVMu1gDby7jkqOjnxp/0NSLjKY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767063287; c=relaxed/simple; bh=GeQc9UTFkim6ObLNMEA+TRdhwkjmuw0xg5uFoQFfsO4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=F3PmipaQv/Pc1dnjbb+Ppw/EbZBg7zxUA8UQlpDMJG9Ie2Rp2WYW7oxMBnM/O/bLmSx0VfdWnpsgB2N27gnbw3F5VcNIfXS69SgWmTbxXyhTgCjLUVvqCSgXJaKZaYBfQQuIxYMi5+uAFsJHPTswn5Ss0HoH+hk+DrPpGlK8mdE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ny3Wqblb; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ny3Wqblb" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EF7BFC4CEF7; Tue, 30 Dec 2025 02:54:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767063286; bh=GeQc9UTFkim6ObLNMEA+TRdhwkjmuw0xg5uFoQFfsO4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ny3Wqblb4K+31z2qoERG4Bz8O8Jzdgp8z+ua9gv7M8Sc3lInIvnLqVdReOEotFEtA dE5LoWt7HxSFhL43o/K85VdoOPNxtDUnGJspxVjZ9tH9Gcm298qiF54yQnhD/FAfb2 FafiZA/cKT4X4hKrad/IqCMPAph+DwiEZq/xoqwDMuhB1r/bYnpCiaANeGsVlHmNJa sk8HXA6YCaL7VsTAuKfAHQ1mqkT49lC3l8vUBUkIxu0xccMcYGS2vitkS47wjk9ZK/ Y6eyRvg0IelurBmzmIAXRXBrtuKgENav/mF55Pxj9r2R5GtmbGKoa9TPYmXLWdibwi 37aJW2i686YLw== From: Sasha Levin To: stable@vger.kernel.org Cc: xu xin , Stefan Roesch , David Hildenbrand , Jinjiang Tu , Wang Yaxin , Yang Yang , Andrew Morton , Sasha Levin Subject: [PATCH 6.6.y] mm/ksm: fix exec/fork inheritance support for prctl Date: Mon, 29 Dec 2025 21:54:43 -0500 Message-ID: <20251230025443.1980197-1-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <2025122925-designed-overture-2e7d@gregkh> References: <2025122925-designed-overture-2e7d@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: xu xin [ Upstream commit 590c03ca6a3fbb114396673314e2aa483839608b ] Patch series "ksm: fix exec/fork inheritance", v2. This series fixes exec/fork inheritance. See the detailed description of the issue below. This patch (of 2): Background ========== commit d7597f59d1d33 ("mm: add new api to enable ksm per process") introduced MMF_VM_MERGE_ANY for mm->flags, and allowed user to set it by prctl() so that the process's VMAs are forcibly scanned by ksmd. Subsequently, the 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") supported inheriting the MMF_VM_MERGE_ANY flag when a task calls execve(). Finally, commit 3a9e567ca45fb ("mm/ksm: fix ksm exec support for prctl") fixed the issue that ksmd doesn't scan the mm_struct with MMF_VM_MERGE_ANY by adding the mm_slot to ksm_mm_head in __bprm_mm_init(). Problem ======= In some extreme scenarios, however, this inheritance of MMF_VM_MERGE_ANY during exec/fork can fail. For example, when the scanning frequency of ksmd is tuned extremely high, a process carrying MMF_VM_MERGE_ANY may still fail to pass it to the newly exec'd process. This happens because ksm_execve() is executed too early in the do_execve flow (prematurely adding the new mm_struct to the ksm_mm_slot list). As a result, before do_execve completes, ksmd may have already performed a scan and found that this new mm_struct has no VM_MERGEABLE VMAs, thus clearing its MMF_VM_MERGE_ANY flag. Consequently, when the new program executes, the flag MMF_VM_MERGE_ANY inheritance missed. Root reason =========== commit d7597f59d1d33 ("mm: add new api to enable ksm per process") clear the flag MMF_VM_MERGE_ANY when ksmd found no VM_MERGEABLE VMAs. Solution ======== Firstly, Don't clear MMF_VM_MERGE_ANY when ksmd found no VM_MERGEABLE VMAs, because perhaps their mm_struct has just been added to ksm_mm_slot list, and its process has not yet officially started running or has not yet performed mmap/brk to allocate anonymous VMAS. Secondly, recheck MMF_VM_MERGEABLE again if a process takes MMF_VM_MERGE_ANY, and create a mm_slot and join it into ksm_scan_list again. Link: https://lkml.kernel.org/r/20251007182504440BJgK8VXRHh8TD7IGSUIY4@zte.com.cn Link: https://lkml.kernel.org/r/20251007182821572h_SoFqYZXEP1mvWI4n9VL@zte.com.cn Fixes: 3c6f33b7273a ("mm/ksm: support fork/exec for prctl") Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process") Signed-off-by: xu xin Cc: Stefan Roesch Cc: David Hildenbrand Cc: Jinjiang Tu Cc: Wang Yaxin Cc: Yang Yang Cc: Signed-off-by: Andrew Morton [ changed mm_flags_test() and mm_flags_clear() calls to test_bit() and clear_bit() ] Signed-off-by: Sasha Levin --- mm/ksm.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index 2e4cd681622d..96a0feb19c09 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2451,8 +2451,14 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page) spin_unlock(&ksm_mmlist_lock); mm_slot_free(mm_slot_cache, mm_slot); + /* + * Only clear MMF_VM_MERGEABLE. We must not clear + * MMF_VM_MERGE_ANY, because for those MMF_VM_MERGE_ANY process, + * perhaps their mm_struct has just been added to ksm_mm_slot + * list, and its process has not yet officially started running + * or has not yet performed mmap/brk to allocate anonymous VMAS. + */ clear_bit(MMF_VM_MERGEABLE, &mm->flags); - clear_bit(MMF_VM_MERGE_ANY, &mm->flags); mmap_read_unlock(mm); mmdrop(mm); } else { @@ -2567,8 +2573,16 @@ void ksm_add_vma(struct vm_area_struct *vma) { struct mm_struct *mm = vma->vm_mm; - if (test_bit(MMF_VM_MERGE_ANY, &mm->flags)) + if (test_bit(MMF_VM_MERGE_ANY, &mm->flags)) { __ksm_add_vma(vma); + /* + * Generally, the flags here always include MMF_VM_MERGEABLE. + * However, in rare cases, this flag may be cleared by ksmd who + * scans a cycle without finding any mergeable vma. + */ + if (unlikely(!test_bit(MMF_VM_MERGEABLE, &mm->flags))) + __ksm_enter(mm); + } } static void ksm_add_vmas(struct mm_struct *mm) -- 2.51.0