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 D257D1F19A; Thu, 2 Oct 2025 15:30:40 +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=1759419040; cv=none; b=k6WYO4cdGCn8zTetSWQzEphuZ883ac1BHYB3hUGt/EAeUxt9wQwwB6d45mKP6+dKwrqBju0W5NDWDfF/ZNF4EL6dEUhdq8YtoOo6ZiKGfipJYlkR5tCH/F5NVsj4cdtzVZ7ZoH4qmty9hbUMBPG6yWpb4WE20gj70LqrOzb7jLg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759419040; c=relaxed/simple; bh=jXFvqADOXhoMBxDmKNZjCgPHb2ho0Ydepvi1UXzuC+M=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=MKtLk07qEdC+lPowEUxDSA5VDJj2moBRHhlPyuk89ZsbMAch+D1CuTIr9HzkRhQoB2fncEdA2h0cOeLg1PRk1EHYal10dO7PbJvmmlWR9uG9rm5vpHyS+X9voJArNX5y5dc/wOlX2SqXeJF/prsL4kduk8IP7fH1C6v1arG9EMk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=alJXUrmf; 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="alJXUrmf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 03D4CC4CEF9; Thu, 2 Oct 2025 15:30:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1759419040; bh=jXFvqADOXhoMBxDmKNZjCgPHb2ho0Ydepvi1UXzuC+M=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=alJXUrmf95I8heIuO7zt8RADFO/5+6S4FJaPix5nefsjIzRshZ3AQFgx80NLV9DXk 9nR4RteOdoIjKmTnhzGCRQDtVeh/OrncPDzhMe3V3cs1w2HAn2VO3hsnyjw1+8HiRK YzthvKQMWnpa7OFHyQ517kdb+b9fesHoqryoBiV6LAdbZsJ0QTN9rjjbcKt0OcW/2c yEooYMqxGIymQd2uBfcwTZsM2x9T5lssRTpckdjjPIZqZ96vy1P3sopuaQkBuJPnAs oGjW8yRQLvbQih2JxDjd6c3LTNQbGfB4tFACY/4xXaCH6E/02Sm2vPpb6LyA5P3866 dkmezmJzm6uEg== From: Sasha Levin To: patches@lists.linux.dev, stable@vger.kernel.org Cc: Simon Schuster , Andreas Oetken , Dinh Nguyen , Sasha Levin Subject: [PATCH AUTOSEL 6.17-5.15] nios2: ensure that memblock.current_limit is set when setting pfn limits Date: Thu, 2 Oct 2025 11:29:58 -0400 Message-ID: <20251002153025.2209281-11-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251002153025.2209281-1-sashal@kernel.org> References: <20251002153025.2209281-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 6.17 Content-Transfer-Encoding: 8bit From: Simon Schuster [ Upstream commit a20b83cf45be2057f3d073506779e52c7fa17f94 ] On nios2, with CONFIG_FLATMEM set, the kernel relies on memblock_get_current_limit() to determine the limits of mem_map, in particular for max_low_pfn. Unfortunately, memblock.current_limit is only default initialized to MEMBLOCK_ALLOC_ANYWHERE at this point of the bootup, potentially leading to situations where max_low_pfn can erroneously exceed the value of max_pfn and, thus, the valid range of available DRAM. This can in turn cause kernel-level paging failures, e.g.: [ 76.900000] Unable to handle kernel paging request at virtual address 20303000 [ 76.900000] ea = c0080890, ra = c000462c, cause = 14 [ 76.900000] Kernel panic - not syncing: Oops [ 76.900000] ---[ end Kernel panic - not syncing: Oops ]--- This patch fixes this by pre-calculating memblock.current_limit based on the upper limits of the available memory ranges via adjust_lowmem_bounds, a simplified version of the equivalent implementation within the arm architecture. Signed-off-by: Simon Schuster Signed-off-by: Andreas Oetken Signed-off-by: Dinh Nguyen Signed-off-by: Sasha Levin --- LLM Generated explanations, may be completely bogus: Based on my comprehensive analysis, here is my determination: ## Backport Recommendation: **YES** ### Analysis **Bug Description:** The commit fixes a critical memory initialization bug in nios2 architecture with CONFIG_FLATMEM. The `find_limits()` function (introduced in 2021 via commit 7f7bc20bc41a4) uses `memblock_get_current_limit()` to calculate `max_low_pfn` at arch/nios2/kernel/setup.c:140: ```c *max_low_pfn = PFN_DOWN(memblock_get_current_limit()); ``` However, `memblock.current_limit` is only default-initialized to `MEMBLOCK_ALLOC_ANYWHERE` (defined as `~(phys_addr_t)0` in include/linux/memblock.h:408, essentially ULONG_MAX). This causes `max_low_pfn` to be set to an enormous value that exceeds `max_pfn`, violating the critical invariant `max_low_pfn <= max_pfn`. **Impact:** When the kernel attempts memory operations using the incorrect `max_low_pfn`, it accesses memory beyond actual DRAM, resulting in kernel paging failures: ``` Unable to handle kernel paging request at virtual address 20303000 Kernel panic - not syncing: Oops ``` **The Fix:** The commit adds `adjust_lowmem_bounds()` (arch/nios2/kernel/setup.c:145-157) which: 1. Iterates through all memory ranges using `for_each_mem_range()` 2. Finds the highest `block_end` address 3. Calls `memblock_set_current_limit(memblock_limit)` to set the actual memory limit This function is called at line 174 before `find_limits()`, ensuring correct initialization. The implementation follows the proven pattern from ARM architecture (arch/arm/mm/mmu.c:1185). **Backporting Justification:** 1. **Critical bug with clear user impact**: Causes kernel panics on affected systems 2. **Small and well-contained**: Only 15 lines of code added 3. **Minimal regression risk**: - Architecture-specific (nios2 only) - zero risk to other systems - Pattern proven in ARM for years (since commit 985626564eedc from 2017) - Only affects boot-time initialization 4. **Follows stable tree rules**: Important bugfix, no new features, no architectural changes 5. **Similar issues in other architectures**: MIPS had a similar `max_low_pfn` bug fixed in commit 0f5cc249ff735 (2023) 6. **Long-lived bug**: Latent since 2021, affects all kernels with commit 7f7bc20bc41a4 **Historical Context:** - Bug introduced: 2021-02-19 (commit 7f7bc20bc41a4) - Bug fixed: 2025-08-21 (commit a20b83cf45be2) - Same author (Andreas Oetken) involved in both commits **Target Stable Trees:** Should be backported to all stable trees containing commit 7f7bc20bc41a4 (February 2021 onwards). arch/nios2/kernel/setup.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/arch/nios2/kernel/setup.c b/arch/nios2/kernel/setup.c index 2a40150142c36..f43f01c4ab934 100644 --- a/arch/nios2/kernel/setup.c +++ b/arch/nios2/kernel/setup.c @@ -142,6 +142,20 @@ static void __init find_limits(unsigned long *min, unsigned long *max_low, *max_high = PFN_DOWN(memblock_end_of_DRAM()); } +static void __init adjust_lowmem_bounds(void) +{ + phys_addr_t block_start, block_end; + u64 i; + phys_addr_t memblock_limit = 0; + + for_each_mem_range(i, &block_start, &block_end) { + if (block_end > memblock_limit) + memblock_limit = block_end; + } + + memblock_set_current_limit(memblock_limit); +} + void __init setup_arch(char **cmdline_p) { console_verbose(); @@ -157,6 +171,7 @@ void __init setup_arch(char **cmdline_p) /* Keep a copy of command line */ *cmdline_p = boot_command_line; + adjust_lowmem_bounds(); find_limits(&min_low_pfn, &max_low_pfn, &max_pfn); memblock_reserve(__pa_symbol(_stext), _end - _stext); -- 2.51.0