Hello.
While trying to debug https://syzkaller.appspot.com/bug?extid=017265e8553724e514e8 , I noticed that a crash can happen without opening /dev/ttyXX .
For example, while a driver which syzbot is reporting accepts screen with var.xres = var.yres = 0 (and a crash is not visible until trying to write to /dev/ttyXX ), a driver for VMware environment which I'm using (dmesg says "fbcon: svgadrmfb (fb0) is primary device") rejects screen with var.xres = var.yres = 0. However, specifying var.xres = var.yres = 1 like below reproducer causes a crash in my VMware environment.
---------- #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <sys/ioctl.h> #include <linux/fb.h>
int main(int argc, char *argv[]) { const int fd = open("/dev/fb0", O_ACCMODE); struct fb_var_screeninfo var = { }; ioctl(fd, FBIOGET_VSCREENINFO, &var); var.xres = var.yres = 1; ioctl(fd, FBIOPUT_VSCREENINFO, &var); return 0; } ----------
---------- [ 20.102222] BUG: unable to handle page fault for address: ffffb80500d7b000 [ 20.102225] #PF: supervisor write access in kernel mode [ 20.102226] #PF: error_code(0x0002) - not-present page [ 20.102227] PGD 13a48c067 P4D 13a48c067 PUD 13a48d067 PMD 132525067 PTE 0 [ 20.102230] Oops: 0002 [#1] SMP [ 20.102232] CPU: 3 PID: 2786 Comm: a.out Not tainted 5.8.0-rc4+ #749 [ 20.102233] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 02/27/2020 [ 20.102237] RIP: 0010:bitfill_aligned+0x87/0x120 [cfbfillrect] [ 20.102238] Code: c3 45 85 db 0f 85 85 00 00 00 44 89 c0 31 d2 41 f7 f1 89 c2 83 f8 07 76 41 8d 48 f8 c1 e9 03 48 83 c1 01 48 c1 e1 06 48 01 f1 <48> 89 3e 48 89 7e 08 48 89 7e 10 48 89 7e 18 48 89 7e 20 48 89 7e [ 20.102239] RSP: 0018:ffffb805012939a8 EFLAGS: 00010206 [ 20.102240] RAX: 0000000003fffe70 RBX: 00000000ffff9c20 RCX: ffffb80520982000 [ 20.102241] RDX: 0000000003fffe70 RSI: ffffb80500d7b000 RDI: 0000000000000000 [ 20.102242] RBP: ffffb805012939b8 R08: 00000000ffff9c20 R09: ffffb80500d7aff8 [ 20.102242] R10: 00000000ffffffff R11: 0000000000000000 R12: ffffffffffffffff [ 20.102243] R13: ffff976734c0c000 R14: 0000000000000000 R15: ffffb80500982c80 [ 20.102244] FS: 00007f0c9589e740(0000) GS:ffff97673aec0000(0000) knlGS:0000000000000000 [ 20.102265] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 20.102265] CR2: ffffb80500d7b000 CR3: 0000000136cdf004 CR4: 00000000001606e0 [ 20.102277] Call Trace: [ 20.102281] cfb_fillrect+0x159/0x340 [cfbfillrect] [ 20.102385] ? __mutex_unlock_slowpath+0x158/0x2d0 [ 20.102493] ? cfb_fillrect+0x340/0x340 [cfbfillrect] [ 20.102747] vmw_fb_fillrect+0x12/0x30 [vmwgfx] [ 20.102755] bit_clear_margins+0x92/0xf0 [fb] [ 20.102760] fbcon_clear_margins+0x4c/0x50 [fb] [ 20.102763] fbcon_switch+0x321/0x570 [fb] [ 20.102771] redraw_screen+0xe0/0x250 [ 20.102775] fbcon_modechanged+0x164/0x1b0 [fb] [ 20.102779] fbcon_update_vcs+0x15/0x20 [fb] [ 20.102781] fb_set_var+0x364/0x3c0 [fb] [ 20.102817] do_fb_ioctl+0x2ff/0x3f0 [fb] [ 20.102894] ? find_held_lock+0x35/0xa0 [ 20.103126] ? __audit_syscall_entry+0xd8/0x120 [ 20.103135] ? kfree+0x25a/0x2b0 [ 20.103139] fb_ioctl+0x2e/0x40 [fb] [ 20.103141] ksys_ioctl+0x86/0xc0 [ 20.103144] ? do_syscall_64+0x20/0xa0 [ 20.103146] __x64_sys_ioctl+0x15/0x20 [ 20.103148] do_syscall_64+0x54/0xa0 [ 20.103151] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [ 20.103152] RIP: 0033:0x7f0c953b8307 [ 20.103153] Code: Bad RIP value. [ 20.103154] RSP: 002b:00007ffecbdce0f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 [ 20.103155] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f0c953b8307 [ 20.103156] RDX: 00007ffecbdce100 RSI: 0000000000004601 RDI: 0000000000000003 [ 20.103156] RBP: 0000000000000000 R08: 00007f0c9568be80 R09: 0000000000000000 [ 20.103157] R10: 00007ffecbdcdb60 R11: 0000000000000246 R12: 00000000004004f2 [ 20.103158] R13: 00007ffecbdce280 R14: 0000000000000000 R15: 0000000000000000 [ 20.103162] Modules linked in: mousedev rapl evdev input_leds led_class mac_hid psmouse pcspkr xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack sg ebtable_nat af_packet ip6table_nat ip6table_mangle ip6table_raw iptable_nat nf_nat iptable_mangle iptable_raw nf_conntrack rtc_cmos nf_defrag_ipv4 ip_set nfnetlink ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter bpfilter i2c_piix4 vmw_vmci ac intel_agp button intel_gtt ip_tables x_tables ata_generic pata_acpi serio_raw atkbd libps2 vmwgfx drm_kms_helper cfbfillrect syscopyarea cfbimgblt sysfillrect sysimgblt fb_sys_fops cfbcopyarea fb fbdev ttm drm i2c_core ahci drm_panel_orientation_quirks libahci backlight e1000 agpgart ata_piix libata i8042 serio unix ipv6 nf_defrag_ipv6 [ 20.103194] CR2: ffffb80500d7b000 [ 20.103196] ---[ end trace b2348f839f6524f9 ]--- [ 20.103198] RIP: 0010:bitfill_aligned+0x87/0x120 [cfbfillrect] [ 20.103200] Code: c3 45 85 db 0f 85 85 00 00 00 44 89 c0 31 d2 41 f7 f1 89 c2 83 f8 07 76 41 8d 48 f8 c1 e9 03 48 83 c1 01 48 c1 e1 06 48 01 f1 <48> 89 3e 48 89 7e 08 48 89 7e 10 48 89 7e 18 48 89 7e 20 48 89 7e [ 20.103201] RSP: 0018:ffffb805012939a8 EFLAGS: 00010206 [ 20.103202] RAX: 0000000003fffe70 RBX: 00000000ffff9c20 RCX: ffffb80520982000 [ 20.103202] RDX: 0000000003fffe70 RSI: ffffb80500d7b000 RDI: 0000000000000000 [ 20.103203] RBP: ffffb805012939b8 R08: 00000000ffff9c20 R09: ffffb80500d7aff8 [ 20.103204] R10: 00000000ffffffff R11: 0000000000000000 R12: ffffffffffffffff [ 20.103204] R13: ffff976734c0c000 R14: 0000000000000000 R15: ffffb80500982c80 [ 20.103205] FS: 00007f0c9589e740(0000) GS:ffff97673aec0000(0000) knlGS:0000000000000000 [ 20.103213] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 20.103214] CR2: ffffb80500d7b000 CR3: 0000000136cdf004 CR4: 00000000001606e0 ----------
A debug printk() patch
---------- diff --git a/drivers/video/fbdev/core/cfbfillrect.c b/drivers/video/fbdev/core/cfbfillrect.c index ba9f58b2a5e8..57e4c2d1bcc0 100644 --- a/drivers/video/fbdev/core/cfbfillrect.c +++ b/drivers/video/fbdev/core/cfbfillrect.c @@ -321,6 +321,9 @@ void cfb_fillrect(struct fb_info *p, const struct fb_fillrect *rect) fill_op32 = bitfill_aligned; break; } + if (fill_op32 == bitfill_aligned) + printk(KERN_DEBUG "height=%lu width=%lu bpp=%u bits=%u bytes=%u dst=%px dst_idx=%u p->fix.line_length=%u\n", + height, width, bpp, bits, bytes, dst, dst_idx, p->fix.line_length); while (height--) { dst += dst_idx >> (ffs(bits) - 1); dst_idx &= (bits - 1); ----------
says that width * bpp was a sane value for normal boot
[ 9.993434] height=16 width=800 bpp=32 bits=64 bytes=8 dst=ffff9864409c2000 dst_idx=21676032 p->fix.line_length=4704 [ 15.494941] height=8 width=800 bpp=32 bits=64 bytes=8 dst=ffff9864409c2000 dst_idx=22278144 p->fix.line_length=4704
but width * bpp was overflowing when the reproducer shown above was executed.
[ 28.164111] height=885 width=4294966497 bpp=32 bits=64 bytes=8 dst=ffff9864409c2000 dst_idx=25600 p->fix.line_length=4704
syzbot has several bug reports which are stalling inside filling functions. I guess that these reports are unexpectedly longer loops caused by integer overflow/underflow.
Thus, I consider that we need more sanity/constraints checks. I don't have other devices to test. Please check your drivers.