From d15d51fb26e830af58f3f21964f1c09c239077ea Mon Sep 17 00:00:00 2001 From: Melbin K Mathew Date: Thu, 2 Jul 2026 01:42:46 +0200 Subject: [PATCH 1/6] fbdev: bound mode sysfs output to the sysfs buffer mode_string() uses snprintf() which can return a value larger than the remaining buffer space. show_modes() accumulates the return value into i without checking whether i has reached PAGE_SIZE, causing the offset to advance past the sysfs buffer if the modelist is long enough. Add a size parameter to mode_string() and use scnprintf() to return only the bytes actually written. Add an early return when offset already exceeds the buffer. In show_modes(), stop accumulating once the buffer is full. Cc: stable@vger.kernel.org # v7.1+ Signed-off-by: Melbin K Mathew Signed-off-by: Helge Deller --- drivers/video/fbdev/core/fbsysfs.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c index ea196603c7a8..af21dc5052df 100644 --- a/drivers/video/fbdev/core/fbsysfs.c +++ b/drivers/video/fbdev/core/fbsysfs.c @@ -27,12 +27,15 @@ static int activate(struct fb_info *fb_info, struct fb_var_screeninfo *var) return 0; } -static int mode_string(char *buf, unsigned int offset, +static int mode_string(char *buf, size_t size, unsigned int offset, const struct fb_videomode *mode) { char m = 'U'; char v = 'p'; + if (offset >= size) + return 0; + if (mode->flag & FB_MODE_IS_DETAILED) m = 'D'; if (mode->flag & FB_MODE_IS_VESA) @@ -45,7 +48,7 @@ static int mode_string(char *buf, unsigned int offset, if (mode->vmode & FB_VMODE_DOUBLE) v = 'd'; - return snprintf(&buf[offset], PAGE_SIZE - offset, "%c:%dx%d%c-%d\n", + return scnprintf(&buf[offset], size - offset, "%c:%dx%d%c-%d\n", m, mode->xres, mode->yres, v, mode->refresh); } @@ -64,7 +67,7 @@ static ssize_t store_mode(struct device *device, struct device_attribute *attr, list_for_each_entry(modelist, &fb_info->modelist, list) { mode = &modelist->mode; - i = mode_string(mstr, 0, mode); + i = mode_string(mstr, sizeof(mstr), 0, mode); if (strncmp(mstr, buf, max(count, i)) == 0) { var = fb_info->var; @@ -86,7 +89,7 @@ static ssize_t show_mode(struct device *device, struct device_attribute *attr, if (!fb_info->mode) return 0; - return mode_string(buf, 0, fb_info->mode); + return mode_string(buf, PAGE_SIZE, 0, fb_info->mode); } static ssize_t store_modes(struct device *device, @@ -136,7 +139,9 @@ static ssize_t show_modes(struct device *device, struct device_attribute *attr, i = 0; list_for_each_entry(modelist, &fb_info->modelist, list) { mode = &modelist->mode; - i += mode_string(buf, i, mode); + i += mode_string(buf, PAGE_SIZE, i, mode); + if (i >= PAGE_SIZE - 1) + break; } return i; } From 95e647d2a5304a8fd11f1ba3c8502de700650131 Mon Sep 17 00:00:00 2001 From: Melbin K Mathew Date: Thu, 2 Jul 2026 01:42:47 +0200 Subject: [PATCH 2/6] fbdev: clear fb_info->mode before deleting a videomode fb_set_var() can delete a mode from info->modelist when userspace passes FB_ACTIVATE_INV_MODE through FBIOPUT_VSCREENINFO. The code checks that the mode being deleted is not the current info->var and that fbcon is not using it, but it does not check fb_info->mode. fb_info->mode may still point into the modelist entry being deleted. If the entry is freed, later mode sysfs reads through show_mode() can dereference a stale pointer. Clear fb_info->mode before calling fb_delete_videomode() when it matches the mode being removed. Cc: stable@vger.kernel.org # v7.1+ Signed-off-by: Melbin K Mathew Signed-off-by: Helge Deller --- drivers/video/fbdev/core/fbmem.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c index 2f1c56e5a7a2..c8aa163b0ecf 100644 --- a/drivers/video/fbdev/core/fbmem.c +++ b/drivers/video/fbdev/core/fbmem.c @@ -246,8 +246,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var) ret = fb_mode_is_equal(&mode1, &mode2); if (!ret) { ret = fbcon_mode_deleted(info, &mode1); - if (!ret) + if (!ret) { + if (info->mode && fb_mode_is_equal(info->mode, &mode1)) + info->mode = NULL; fb_delete_videomode(&mode1, &info->modelist); + } } return ret ? -EINVAL : 0; From 061db6b7a910b8378f3b2df64f8c0a3ddc6e85f2 Mon Sep 17 00:00:00 2001 From: Melbin K Mathew Date: Thu, 2 Jul 2026 01:42:48 +0200 Subject: [PATCH 3/6] fbdev: serialize mode sysfs access with lock_fb_info() show_mode(), show_modes(), and store_mode() access fb_info->modelist and fb_info->mode without holding lock_fb_info(). store_modes() takes lock_fb_info() while replacing the modelist and freeing the old one. A concurrent reader or writer can load a pointer to an old modelist entry before store_modes() frees it, then dereference freed memory or store a stale freed pointer in fb_info->mode. Take lock_fb_info() in show_mode(), show_modes(), and store_mode() to serialize with store_modes(). In show_mode(), copy the mode to the stack and format after dropping the lock. In store_mode(), split activate() into a _locked variant to avoid double-locking, and hold the locks for the modelist walk, mode conversion, activation, and fb_info->mode assignment together. Cc: stable@vger.kernel.org # v7.1+ Signed-off-by: Melbin K Mathew Signed-off-by: Helge Deller --- drivers/video/fbdev/core/fbsysfs.c | 46 ++++++++++++++++++++++++------ 1 file changed, 38 insertions(+), 8 deletions(-) diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c index af21dc5052df..d3d60c555bb8 100644 --- a/drivers/video/fbdev/core/fbsysfs.c +++ b/drivers/video/fbdev/core/fbsysfs.c @@ -12,19 +12,24 @@ #include "fb_internal.h" #include "fbcon.h" +static int activate_locked(struct fb_info *fb_info, + struct fb_var_screeninfo *var) +{ + var->activate |= FB_ACTIVATE_FORCE; + return fb_set_var_from_user(fb_info, var); +} + static int activate(struct fb_info *fb_info, struct fb_var_screeninfo *var) { int err; - var->activate |= FB_ACTIVATE_FORCE; console_lock(); lock_fb_info(fb_info); - err = fb_set_var_from_user(fb_info, var); + err = activate_locked(fb_info, var); unlock_fb_info(fb_info); console_unlock(); - if (err) - return err; - return 0; + + return err; } static int mode_string(char *buf, size_t size, unsigned int offset, @@ -65,6 +70,9 @@ static ssize_t store_mode(struct device *device, struct device_attribute *attr, memset(&var, 0, sizeof(var)); + console_lock(); + lock_fb_info(fb_info); + list_for_each_entry(modelist, &fb_info->modelist, list) { mode = &modelist->mode; i = mode_string(mstr, sizeof(mstr), 0, mode); @@ -72,12 +80,22 @@ static ssize_t store_mode(struct device *device, struct device_attribute *attr, var = fb_info->var; fb_videomode_to_var(&var, mode); - if ((err = activate(fb_info, &var))) + err = activate_locked(fb_info, &var); + if (err) { + unlock_fb_info(fb_info); + console_unlock(); return err; + } fb_info->mode = mode; + unlock_fb_info(fb_info); + console_unlock(); return count; } } + + unlock_fb_info(fb_info); + console_unlock(); + return -EINVAL; } @@ -85,11 +103,20 @@ static ssize_t show_mode(struct device *device, struct device_attribute *attr, char *buf) { struct fb_info *fb_info = dev_get_drvdata(device); + struct fb_videomode mode; + bool have_mode = false; - if (!fb_info->mode) + lock_fb_info(fb_info); + if (fb_info->mode) { + mode = *fb_info->mode; + have_mode = true; + } + unlock_fb_info(fb_info); + + if (!have_mode) return 0; - return mode_string(buf, PAGE_SIZE, 0, fb_info->mode); + return mode_string(buf, PAGE_SIZE, 0, &mode); } static ssize_t store_modes(struct device *device, @@ -137,12 +164,15 @@ static ssize_t show_modes(struct device *device, struct device_attribute *attr, const struct fb_videomode *mode; i = 0; + lock_fb_info(fb_info); list_for_each_entry(modelist, &fb_info->modelist, list) { mode = &modelist->mode; i += mode_string(buf, PAGE_SIZE, i, mode); if (i >= PAGE_SIZE - 1) break; } + unlock_fb_info(fb_info); + return i; } From 81cc73be40c6f028f1ee3f438ace46afe666dbae Mon Sep 17 00:00:00 2001 From: Mingyu Wang <25181214217@stu.xidian.edu.cn> Date: Tue, 21 Jul 2026 16:19:42 +0800 Subject: [PATCH 4/6] fbdev: core: Fix pointer desynchronization in fb_io_read() In fb_io_read(), if copy_to_user() performs a partial copy (e.g., due to a faulty user buffer), the loop adjusts the chunk size 'c' and updates the remaining 'count'. However, the hardware 'src' pointer has already been eagerly advanced by the original chunk size. If the loop is allowed to continue, the read will resume from an incorrect, over-advanced offset. Since the remaining 'count' was only decremented by the successful bytes, this desynchronization causes the next iterations to execute more hardware reads than originally bounded, eventually leading to out-of-bounds I/O reads. Fix this by breaking out of the loop immediately upon a partial copy_to_user(). A partial copy indicates a faulty user buffer, making subsequent read attempts futile. Breaking out ensures we return the number of successfully read bytes without risking out-of-bounds hardware accesses in subsequent mismatched iterations. Fixes: 6121cd9ef911 ("fbdev: Move I/O read and write code into helper functions") Cc: stable@vger.kernel.org Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn> Signed-off-by: Helge Deller --- drivers/video/fbdev/core/fb_io_fops.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/drivers/video/fbdev/core/fb_io_fops.c b/drivers/video/fbdev/core/fb_io_fops.c index 6ab60fcd0050..0798e88799eb 100644 --- a/drivers/video/fbdev/core/fb_io_fops.c +++ b/drivers/video/fbdev/core/fb_io_fops.c @@ -61,6 +61,14 @@ ssize_t fb_io_read(struct fb_info *info, char __user *buf, size_t count, loff_t buf += c; cnt += c; count -= c; + + /* + * If there was a partial copy, the user buffer is faulty. + * Break out to avoid over-advancing the src pointer and + * reading out of bounds in the next iteration. + */ + if (trailing) + break; } kfree(buffer); From ef7656e85f1a4400999625cd398b655517368e7e Mon Sep 17 00:00:00 2001 From: Zizhi Wo Date: Wed, 29 Jul 2026 10:12:04 +0800 Subject: [PATCH 5/6] fbdev: Fix out-of-bounds access when rotating console after font resize [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: ... 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 Signed-off-by: Helge Deller --- drivers/video/fbdev/core/fbcon.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c index 9f5c4c101581..23b3c536d53d 100644 --- a/drivers/video/fbdev/core/fbcon.c +++ b/drivers/video/fbdev/core/fbcon.c @@ -2641,9 +2641,31 @@ static void fbcon_modechanged(struct fb_info *info) fbcon_info_from_console(par->currcon) != info) return; + /* + * Clear the selection before switching bitops. Without this, the + * clear_selection() inside vc_resize() below repaints the highlighted + * cells through the new bitops while the console geometry(vc_rows/vc_cols) + * has not been updated to match, so the repaint is computed from a + * half-switched geometry and overflows the framebuffer address. + * Pre-clearing makes that repaint a no-op. + */ + clear_selection(); + p = &fb_display[vc->vc_num]; set_blitting_type(vc, info); + /* + * Rebuild par->rotated.buf for the new rotation now that bitops have + * switched. The new putcs/cursor ops read this buffer; if it is still + * sized for the old rotation, fbcon_putcs() and the cursor path reached + * via update_screen() below overflow it. Mirrors fbcon_switch(); fall + * back to unrotated rendering on allocation failure. + */ + if (par->bitops->rotate_font && par->bitops->rotate_font(info, vc)) { + par->rotate = FB_ROTATE_UR; + set_blitting_type(vc, info); + } + if (con_is_visible(vc)) { var_to_display(p, &info->var, info); cols = FBCON_SWAP(par->rotate, info->var.xres, info->var.yres); @@ -2675,6 +2697,9 @@ static void fbcon_set_all_vcs(struct fb_info *info) if (!par || par->currcon < 0) return; + /* See the comment in fbcon_modechanged(). */ + clear_selection(); + for (i = first_fb_vc; i <= last_fb_vc; i++) { vc = vc_cons[i].d; if (!vc || vc->vc_mode != KD_TEXT || From e033cbf3975a8465f879ebd5989dc35b04423a4d Mon Sep 17 00:00:00 2001 From: Rik van Riel Date: Fri, 7 Aug 2026 22:19:56 -0400 Subject: [PATCH 6/6] fbdev: bitblit: bound-check glyph index in bit_cursor() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit bit_cursor() fetches the glyph under the cursor with c = scr_readw(vc_pos); src = vc_font.data + ((c & charmask) * w * height); where charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer value comes directly from scr_readw() and may be larger than the current font's glyph count. Syzkaller triggers this via vcs_write(). The Call Trace shows vcs_write() in vc_screen.c writing an arbitrary 16-bit value with writev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via vcs_scr_writew() without checking charcount. The stored value is later read in bit_cursor() in bitblit.c. When the font is changed from a font with 512 glyphs to a font with 256 glyphs, the screen buffer can retain characters with the high bit set from the previous mode, which could also produce the same out-of-bounds access. BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70 Read of size 16 at addr ffff800086c57970 Call Trace: soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70 bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365 fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427 hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883 update_region+0x100/0x18c drivers/tty/vt/vt.c:669 vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685 bit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph index to vc_font.charcount. Apply the same clamp in bit_cursor() after extracting the attribute and masking, before indexing fontdata. The fix completes the bounds checking started in commit 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in bit_putcs*"), which missed the cursor path. This change should be safe because the clamp reuses the existing contract from fbcon: charcount is maintained under console_lock in con_font_set() and fbcon_font_set(), and hi_font_mask is cleared when switching from 512 to 256 glyphs. When stale screen data with high bits remains after a font switch, or when vcs_write() stores an arbitrary value, clamping the index to 0 prevents the out-of-bounds read without changing cursor semantics — the same fallback bit_putcs uses. Reported-by: syzbot+61b1db46218109869c14@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=61b1db46218109869c14 Link: https://lore.kernel.org/all/6a75205c.01d0871a.3a0d52.0032.GAE@google.com/ Fixes: 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in bit_putcs*") Cc: stable@vger.kernel.org Assisted-by: Hermes:muse-spark-1.2 syzkaller Signed-off-by: Rik van Riel Signed-off-by: Helge Deller --- drivers/video/fbdev/core/bitblit.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c index 65681dcc5930..39f44258d793 100644 --- a/drivers/video/fbdev/core/bitblit.c +++ b/drivers/video/fbdev/core/bitblit.c @@ -273,9 +273,14 @@ static void bit_cursor(struct vc_data *vc, struct fb_info *info, bool enable, if (!vc->vc_font.data) return; - c = scr_readw((u16 *) vc->vc_pos); + c = scr_readw((u16 *) vc->vc_pos); attribute = get_attribute(info, c); - src = vc->vc_font.data + ((c & charmask) * (w * vc->vc_font.height)); + c &= charmask; + + /* Clamp to font size, same as bit_putcs_aligned() */ + if (c >= vc->vc_font.charcount) + c = 0; + src = vc->vc_font.data + (c * (w * vc->vc_font.height)); if (par->cursor_state.image.data != (const char *)src || par->cursor_reset) {