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[BUG] Recently, we encountered a KASAN warning as follows: BUG: KASAN: slab-out-of-bounds in ccw_putcs+0x8bd/0xa80 Read of size 1 at addr ff11000110067100 by task bash/1209 CPU: 10 UID: 0 PID: 1209 Comm: bash Not tainted 7.2.0-rc3 #69 PREEMPT(full) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 Call Trace: <TASK> ... kasan_report+0xf0/0x120 ? ccw_putcs+0x8bd/0xa80 ccw_putcs+0x8bd/0xa80 ? __pfx_ccw_putcs+0x10/0x10 fbcon_putcs+0x338/0x410 ? __pfx_ccw_putcs+0x10/0x10 do_update_region+0x21d/0x450 invert_screen+0x29d/0x5e0 ? __kmalloc_noprof+0x493/0x640 ? vc_do_resize+0x17c/0xe50 clear_selection+0x4c/0x60 vc_do_resize+0xaee/0xe50 fbcon_modechanged+0x2bd/0x640 rotate_all_store+0x298/0x380 ... reproduce: 1) issue two ioctls: first a KDFONTOP ioctl with op.op = KD_FONT_OP_SET, op.width = 1 and op.height = 1, then a TIOCL_SETSEL ioctl 2) echo 2 > /sys/devices/virtual/graphics/fbcon/rotate_all 3) issue two ioctls: first a KDFONTOP ioctl with op.op = KD_FONT_OP_SET, op.width = 8 and op.height = 1, then a TIOCL_SETSEL ioctl 4) echo 3 > /sys/devices/virtual/graphics/fbcon/rotate_all [CAUSE] The root cause is that fbcon_modechanged() first sets the current rotate's corresponding ops. Subsequently, during vc_resize(), it may trigger clear_selection(), and in fbcon_putcs->ccw_putcs[rotate=3], this can result in an out-of-bounds access to "src". This happens because par->rotated.buf is reallocated in fbcon_rotate_font(): 1) When rotate=2, its size is (width + 7) / 8 * height 2) When rotate=3, its size is (height + 7) / 8 * width And the call to fbcon_rotate_font() occurs after clear_selection(). In other words, the fontbuffer is allocated using the size calculated from the previous rotation 2, but before reallocating it with the new size, con_putcs is already using the new rotation 3: rotate_all_store fbcon_rotate_all fbcon_set_all_vcs fbcon_modechanged set_blitting_type ... par->bitops = &ccw_fbcon_bitops vc_resize ... clear_selection highlight ... do_update_region fbcon_putcs ... image.dy = vyres - ((xx + count) * vc->vc_font.width) [1] // overflow! ccw_putcs_aligned // old buf size is still being used during the read! src = par->rotated.buf + (scr_readw(s--) & charmask) * cellsize fb_pad_aligned_buffer----[src KASAN!!!] [2] info->fbops->fb_imageblit(info, image) sys_imageblit fb_imageblit fb_address_forward // offset: image->dy * bits_per_line + image->dx * bpp unsigned int bits = (unsigned int)adr->bits + offset adr->address += (bits & ~(BITS_PER_LONG - 1u)) / BITS_PER_BYTE [3] fb_bitmap_imageblit ... fb_read_offset // page fault! [4] update_screen redraw_screen ... ccw_cursor soft_cursor memcpy(src, image->data, dsize)----[src KASAN again!!!] [5] fbcon_switch fbcon_rotate_font font_data_rotate dst = kmalloc_array(charcount, d_cellsize, GFP_KERNEL) // the new size is allocated only here! par->rotated.buf = buf [6] [FIX] A fairly obvious approach is to follow fbcon_switch(): in fbcon_modechanged(), call rotate_font() before vc_resize() so that a correctly sized buffer is allocated in time, as done in [6]. This fix is necessary, but it is not sufficient on its own. In [1] it causes an image.dy overflow (ccw_putcs: vyres = 768, image.dy = 4294967040), because vc_cols has not been updated in time at this point (it is likewise only updated after clear_selection()). This allows (xx + count) * width to exceed vyres, causing image.dy to overflow. Subsequently, address in [3] is incremented by an even larger amount, which triggers a page fault at [4]. Therefore, a second fix is required in combination with the first: move clear_selection() earlier, before set_blitting_type() in fbcon_set_all_vcs(), to prevent the out-of-bounds access. fbcon_rotate() has a similar problem, so add the same clear there. Since vc_is_sel() is not exported, the fbdev side is currently forced to call clear_selection() unconditionally, causing the global selection to be cleared prematurely. And this will not cause any other significant impact. Signed-off-by: Zizhi Wo <wozizhi@huawei.com> Signed-off-by: Helge Deller <deller@gmx.de>
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros * AI Coding Assistant - LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst