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Subject[RFC PATCH] fbcon: fix ypos over boundary issue
Date
From: Feng Tiantian <fengtiantian@huawei.com>

While using "top" on a CentOS guest's VNC-client, then continuously press
"Shift+PgUp", the guest kernel will get panic! Backtrace is attached below.
We tested it on 5.2.0, and the issue remains.

[ 66.946362] Unable to handle kernel paging request at virtual address ffff00000e240840
[ 66.946363] Mem abort info:
[ 66.946364] Exception class = DABT (current EL), IL = 32 bits
[ 66.946365] SET = 0, FnV = 0
[ 66.946366] EA = 0, S1PTW = 0
[ 66.946367] Data abort info:
[ 66.946368] ISV = 0, ISS = 0x00000047
[ 66.946368] CM = 0, WnR = 1
[ 66.946370] swapper pgtable: 64k pages, 48-bit VAs, pgd = ffff000009660000
[ 66.946372] [ffff00000e240840] *pgd=000000023ffe0003, *pud=000000023ffe0003, *pmd=000000023ffd0003, *pte=0000000000000000
[ 66.946378] Internal error: Oops: 96000047 [#1] SMP
[ 66.946379] Modules linked in: vfat fat crc32_ce ghash_ce sg sha2_ce sha256_arm64 virtio_balloon virtio_net sha1_ce ip_tables ext4 mbcache jbd2 virtio_gpu drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm drm i2c_core virtio_scsi virtio_pci virtio_mmio virtio_ring virtio
[ 66.946403] CPU: 0 PID: 1035 Comm: top Not tainted 4.14.0-49.el7a.aarch64 #1
[ 66.946404] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
[ 66.946405] task: ffff8001c18fdc00 task.stack: ffff00000d4e0000
[ 66.946409] PC is at sys_imageblit+0x40c/0x10000 [sysimgblt]
[ 66.946431] LR is at drm_fb_helper_sys_imageblit+0x28/0x4c [drm_kms_helper]
[ 66.946433] pc : [<ffff0000020a040c>] lr : [<ffff000002202e74>] pstate: 00000005
[ 66.946433] sp : ffff00000d4ef7f0
[ 66.946434] x29: ffff00000d4ef7f0 x28: 00000000000001ff
[ 66.946436] x27: ffff8001c1c88100 x26: 0000000000000001
[ 66.946438] x25: 00000000000001f0 x24: 0000000000000018
[ 66.946440] x23: 0000000000000000 x22: ffff00000d4ef978
[ 66.946442] x21: ffff00000e240840 x20: 0000000000000000
[ 66.946444] x19: ffff8001c98c9000 x18: 0000fffff9d56670
[ 66.946445] x17: 0000000000000000 x16: 0000000000000000
[ 66.946447] x15: 0000000000000008 x14: 1b20202020202020
[ 66.946449] x13: 00000000000001f0 x12: 000000000000003e
[ 66.946450] x11: 000000000000000f x10: ffff8001c8400000
[ 66.946452] x9 : 0000000000aaaaaa x8 : 0000000000000001
[ 66.946454] x7 : ffff0000020b0090 x6 : 0000000000000001
[ 66.946456] x5 : 0000000000000000 x4 : 0000000000000000
[ 66.946457] x3 : ffff8001c8400000 x2 : ffff00000e240840
[ 66.946459] x1 : 00000000000001ef x0 : 0000000000000007
[ 66.946461] Process top (pid: 1035, stack limit = 0xffff00000d4e0000)
[ 66.946462] Call trace:
[ 66.946464] Exception stack(0xffff00000d4ef6b0 to 0xffff00000d4ef7f0)
[ 66.946465] f6a0: 0000000000000007 00000000000001ef
[ 66.946467] f6c0: ffff00000e240840 ffff8001c8400000 0000000000000000 0000000000000000
[ 66.946468] f6e0: 0000000000000001 ffff0000020b0090 0000000000000001 0000000000aaaaaa
[ 66.946470] f700: ffff8001c8400000 000000000000000f 000000000000003e 00000000000001f0
[ 66.946471] f720: 1b20202020202020 0000000000000008 0000000000000000 0000000000000000
[ 66.946472] f740: 0000fffff9d56670 ffff8001c98c9000 0000000000000000 ffff00000e240840
[ 66.946474] f760: ffff00000d4ef978 0000000000000000 0000000000000018 00000000000001f0
[ 66.946475] f780: 0000000000000001 ffff8001c1c88100 00000000000001ff ffff00000d4ef7f0
[ 66.946476] f7a0: ffff000002202e74 ffff00000d4ef7f0 ffff0000020a040c 0000000000000005
[ 66.946478] f7c0: ffff00000d4ef7e0 ffff0000080ea614 0001000000000000 ffff000008152f08
[ 66.946479] f7e0: ffff00000d4ef7f0 ffff0000020a040c
[ 66.946481] [<ffff0000020a040c>] sys_imageblit+0x40c/0x10000 [sysimgblt]
[ 66.946501] [<ffff000002202e74>] drm_fb_helper_sys_imageblit+0x28/0x4c [drm_kms_helper]
[ 66.946510] [<ffff0000022a12dc>] virtio_gpu_3d_imageblit+0x2c/0x78 [virtio_gpu]
[ 66.946515] [<ffff00000847f458>] bit_putcs+0x288/0x49c
[ 66.946517] [<ffff00000847ad24>] fbcon_putcs+0x114/0x148
[ 66.946519] [<ffff0000084fe92c>] do_update_region+0x118/0x19c
[ 66.946521] [<ffff00000850413c>] do_con_trol+0x114c/0x1314
[ 66.946523] [<ffff0000085044dc>] do_con_write.part.22+0x1d8/0x890
[ 66.946525] [<ffff000008504c88>] con_write+0x84/0x8c
[ 66.946527] [<ffff0000084ec7f0>] n_tty_write+0x19c/0x408
[ 66.946529] [<ffff0000084e9120>] tty_write+0x150/0x270
[ 66.946532] [<ffff00000829d558>] __vfs_write+0x58/0x180
[ 66.946534] [<ffff00000829d880>] vfs_write+0xa8/0x1a0
[ 66.946536] [<ffff00000829db40>] SyS_write+0x60/0xc0
[ 66.946537] Exception stack(0xffff00000d4efec0 to 0xffff00000d4f0000)
[ 66.946539] fec0: 0000000000000001 0000000000457958 0000000000000800 0000000000000000
[ 66.946540] fee0: 00000000fbad2885 0000000000000bd0 0000ffff8556add4 0000000000000000
[ 66.946541] ff00: 0000000000000040 0000000000000000 0000000000434a88 0000000000000012
[ 66.946543] ff20: 0000000100000000 0000fffff9d564f0 0000fffff9d564a0 0000000000000008
[ 66.946544] ff40: 0000000000000000 0000ffff85593b1c 0000fffff9d56670 0000000000000800
[ 66.946546] ff60: 0000000000457958 0000ffff856a1158 0000000000000800 0000ffff85720000
[ 66.946547] ff80: 0000000000000000 0000ffff856f604c 0000000000000000 0000000000436000
[ 66.946548] ffa0: 000000001c90a160 0000fffff9d56f20 0000ffff855965f4 0000fffff9d56f20
[ 66.946549] ffc0: 0000ffff855f12c8 0000000020000000 0000000000000001 0000000000000040
[ 66.946551] ffe0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[ 66.946554] [<ffff00000808359c>] __sys_trace_return+0x0/0x4
[ 66.946556] Code: 0a080084 b86478e4 0a040124 4a050084 (b9000044)
[ 66.946561] ---[ end trace 32d49c68b19c4796 ]---
[ 66.946562] Kernel panic - not syncing: Fatal exception
[ 66.946564] SMP: stopping secondary CPUs
[ 66.946596] Kernel Offset: disabled
[ 66.946598] CPU features: 0x1802008
[ 66.946598] Memory Limit: none
[ 67.092353] ---[ end Kernel panic - not syncing: Fatal exception

From our non-expert analysis, fbcon ypos will sometimes over boundary and
then fbcon_putcs() access invalid VGA framebuffer address. We modify the
real_y() to make sure fbcon ypos is always less than rows.

Reported-by: Zengruan Ye <yezengruan@huawei.com>
Signed-off-by: Feng Tiantian <fengtiantian@huawei.com>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
---

Hi Bartlomiej,

Zengruan had reported this issue [1] but received no reply. Does it make
sense to fix this issue? Could you please take a look into this patch?

Thanks,
zenghui

[1] https://lkml.org/lkml/2018/11/27/639

drivers/video/fbdev/core/fbcon.h | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbcon.h b/drivers/video/fbdev/core/fbcon.h
index 20dea85..b1aa00e 100644
--- a/drivers/video/fbdev/core/fbcon.h
+++ b/drivers/video/fbdev/core/fbcon.h
@@ -230,7 +230,10 @@ static inline int real_y(struct fbcon_display *p, int ypos)
int rows = p->vrows;

ypos += p->yscroll;
- return ypos < rows ? ypos : ypos - rows;
+ if (rows == 0)
+ return ypos;
+ else
+ return ypos < rows ? ypos : ypos % rows;
}


--
1.8.3.1

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