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The issue was found by our own race fuzzer. We have not analyzed the root cause, so we do not have a proposed fix to offer. To reproduce the race reliably, we applied the delay patch below to the kernel and ran the C reproducer as root inside an x86_64 QEMU guest. The crash log we observed, the delay patch and the reproducer are all included below. The following kernel config options are required to reproduce the issue: CONFIG_VT=y CONFIG_VT_CONSOLE=y CONFIG_FB=y CONFIG_FB_CORE=y CONFIG_FRAMEBUFFER_CONSOLE=y CONFIG_FRAMEBUFFER_CONSOLE_ROTATION=y CONFIG_FONT_SUPPORT=y CONFIG_FONT_8x16=y CONFIG_DRM=y CONFIG_DRM_BOCHS=y CONFIG_DRM_FBDEV_EMULATION=y CONFIG_KASAN=y We hope this report is useful. Please let us know if any further information would help. Reported-by: Eulgyu Kim Reported-by: Jaeyoung Chung Kernel delay patch: ================================================================== diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index 8f467b22b799..e4d9233c4f5f 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -86,6 +86,7 @@ #include #include #include +#include #include #include #include @@ -4979,6 +4980,9 @@ static int con_font_set(struct vc_data *vc, const struct console_font_op *op) font.width = op->width; font.height = op->height; + if (!strncmp(current->comm, "syzrepro", 8) && + op->width == 32 && op->height == 30 && op->charcount == 512) + mdelay(100); guard(console_lock)(); if (vc->vc_mode != KD_TEXT) diff --git a/drivers/video/fbdev/core/fbcon_rotate.c b/drivers/video/fbdev/core/fbcon_rotate.c index 6cdbc96eeca6..f4797dc2dd78 100644 --- a/drivers/video/fbdev/core/fbcon_rotate.c +++ b/drivers/video/fbdev/core/fbcon_rotate.c @@ -9,6 +9,8 @@ */ #include +#include +#include #include #include @@ -38,6 +40,9 @@ int fbcon_rotate_font(struct fb_info *info, struct vc_data *vc) ret = PTR_ERR(buf); goto err_kfree; } + if (!strncmp(current->comm, "syzrepro", 8) && + par->rotated.bufsize == 4096) + mdelay(50); par->rotated.buf = buf; ================================================================== C reproducer: ================================================================== #define _GNU_SOURCE #include #include #include #include #include #include #include #include #include #define SYSCHK(x) ({ long __r = (long)(x); if (__r == -1L) { perror(#x); exit(1); } __r; }) static pthread_barrier_t start; static unsigned char font[4U * 32U * 512U]; static void *rotate_thread(void *arg) { int fd; prctl(PR_SET_NAME, "syzrepro0", 0, 0, 0); pthread_barrier_wait(&start); fd = SYSCHK(open("/sys/class/graphics/fbcon/rotate_all", O_WRONLY)); SYSCHK(write(fd, "1", 1)); close(fd); return NULL; } static void *font_thread(void *arg) { struct console_font_op op = { .op = KD_FONT_OP_SET, .width = 32, .height = 30, .charcount = 512, .data = font }; int fd; prctl(PR_SET_NAME, "syzrepro1", 0, 0, 0); pthread_barrier_wait(&start); fd = SYSCHK(open("/dev/tty1", O_RDWR | O_NOCTTY)); ioctl(fd, KDFONTOP, &op); close(fd); return NULL; } int main(void) { pthread_t t[2]; memset(font, 0x5a, sizeof(font)); pthread_barrier_init(&start, NULL, 2); pthread_create(&t[0], NULL, rotate_thread, NULL); pthread_create(&t[1], NULL, font_thread, NULL); pthread_join(t[0], NULL); pthread_join(t[1], NULL); return 0; } ================================================================== Crash log: ================================================================== BUG: KASAN: slab-out-of-bounds in soft_cursor+0x3eb/0xb70 drivers/video/fbdev/core/softcursor.c:70 Read of size 128 at addr ffff88810792d000 by task syzrepro1/401 CPU: 2 UID: 0 PID: 401 Comm: syzrepro1 Not tainted 7.2.0-dirty #1 PREEMPT Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014 Call Trace: dump_stack_lvl+0x5e/0x80 lib/dump_stack.c:120 print_address_description+0x77/0x200 mm/kasan/report.c:378 print_report+0x64/0x70 mm/kasan/report.c:482 kasan_report+0x118/0x150 mm/kasan/report.c:595 check_region_inline mm/kasan/generic.c:-1 [inline] kasan_check_range+0x2b0/0x2c0 mm/kasan/generic.c:200 __asan_memcpy+0x29/0x70 mm/kasan/shadow.c:105 soft_cursor+0x3eb/0xb70 drivers/video/fbdev/core/softcursor.c:70 cw_cursor+0x1322/0x2080 drivers/video/fbdev/core/fbcon_cw.c:324 hide_cursor+0x84/0x350 drivers/tty/vt/vt.c:884 redraw_screen+0x3b9/0xcb0 drivers/tty/vt/vt.c:986 vc_do_resize+0xe4f/0x1490 drivers/tty/vt/vt.c:1310 vc_resize include/linux/vt_kern.h:49 [inline] fbcon_do_set_font+0x666/0x1980 drivers/video/fbdev/core/fbcon.c:2435 fbcon_set_font+0x50c/0x750 drivers/video/fbdev/core/fbcon.c:2518 con_font_set drivers/tty/vt/vt.c:4996 [inline] con_font_op+0x91b/0xdc0 drivers/tty/vt/vt.c:5036 vt_k_ioctl drivers/tty/vt/vt_ioctl.c:474 [inline] vt_ioctl+0x719/0x1660 drivers/tty/vt/vt_ioctl.c:745 tty_ioctl+0x89c/0xa40 drivers/tty/tty_io.c:2792 vfs_ioctl fs/ioctl.c:51 [inline] __do_sys_ioctl fs/ioctl.c:597 [inline] __se_sys_ioctl+0xb6/0x100 fs/ioctl.c:583 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xf7/0x370 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x76/0x7e RIP: 0033:0x7fa577d5ad6b Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1c 48 8b 44 24 18 64 48 2b 04 25 28 00 00 RSP: 002b:00007fa577457e40 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fa577d5ad6b RDX: 00007fa577457ea0 RSI: 0000000000004b72 RDI: 0000000000000003 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000002 R10: 0000000000000000 R11: 0000000000000246 R12: ffffffffffffff80 R13: 0000000000000000 R14: 00007ffdf1422270 R15: 00007fa576c58000 Allocated by task 400: kasan_save_stack mm/kasan/common.c:57 [inline] kasan_save_track+0x3e/0x80 mm/kasan/common.c:78 poison_kmalloc_redzone mm/kasan/common.c:398 [inline] __kasan_kmalloc+0x72/0x90 mm/kasan/common.c:415 kasan_kmalloc include/linux/kasan.h:263 [inline] __do_kmalloc_node mm/slub.c:5334 [inline] __kmalloc_noprof+0x20f/0x4b0 mm/slub.c:5359 _kmalloc_noprof include/linux/slab.h:992 [inline] _kmalloc_array_noprof include/linux/slab.h:1208 [inline] font_data_rotate+0xc1/0x6f0 lib/fonts/font_rotate.c:231 fbcon_rotate_font+0x2d5/0x520 drivers/video/fbdev/core/fbcon_rotate.c:35 fbcon_modechanged+0x280/0x11f0 drivers/video/fbdev/core/fbcon.c:2664 fbcon_rotate_all drivers/video/fbdev/core/fbcon.c:254 [inline] rotate_all_store+0x140/0x4c0 drivers/video/fbdev/core/fbcon.c:3218 kernfs_fop_write_iter+0x289/0x3c0 fs/kernfs/file.c:345 new_sync_write fs/read_write.c:595 [inline] vfs_write+0x7a2/0xa10 fs/read_write.c:687 ksys_write+0xb0/0x170 fs/read_write.c:739 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xf7/0x370 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x76/0x7e The buggy address belongs to the object at ffff88810792c000 which belongs to the cache kmalloc-4k of size 4096 The buggy address is located 0 bytes to the right of allocated 4096-byte region [ffff88810792c000, ffff88810792d000) The buggy address belongs to the physical page: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x107928 head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 flags: 0x200000000000040(head|node=0|zone=2) page_type: f5(slab) raw: 0200000000000040 ffff888100043040 dead000000000100 dead000000000122 raw: 0000000000000000 0000000800040004 00000000f5000000 0000000000000000 head: 0200000000000040 ffff888100043040 dead000000000100 dead000000000122 head: 0000000000000000 0000000800040004 00000000f5000000 0000000000000000 head: 0200000000000003 fffffffffffffe01 00000000ffffffff 00000000ffffffff head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008 page dumped because: kasan: bad access detected Memory state around the buggy address: ffff88810792cf00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ffff88810792cf80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >ffff88810792d000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ^ ffff88810792d080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff88810792d100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ==================================================================