KMSAN: uninit-value in is_idle_task

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    3b3ea602 x86: add failure injection to get/put/clear_user
git tree:       https://github.com/google/kmsan.git master
console output: https://syzkaller.appspot.com/x/log.txt?x=144b8965900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ee5f7a0b2e48ed66
dashboard link: https://syzkaller.appspot.com/bug?extid=a4b29e75da1c2c8bd34f
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a4b29e75da1c2c8bd34f@xxxxxxxxxxxxxxxxxxxxxxxxx

=====================================================
BUG: KMSAN: uninit-value in is_idle_task+0x65/0x70 include/linux/sched.h:1672
CPU: 0 PID: 5351 Comm: syz-executor.3 Not tainted 5.8.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x21c/0x280 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:121
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 is_idle_task+0x65/0x70 include/linux/sched.h:1672
 sysvec_apic_timer_interrupt+0x2e/0x130 arch/x86/kernel/apic/apic.c:1091
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:593
RIP: 0010:kmsan_get_metadata+0x136/0x180 mm/kmsan/kmsan_shadow.c:201
Code: 75 5f 48 89 df e8 6a 00 00 00 48 85 c0 74 50 48 8b 48 40 48 85 c9 74 47 48 8b 40 48 48 85 c0 74 3e 81 e3 ff 0f 00 00 45 84 f6 <48> 0f 45 c8 48 b8 00 00 00 00 00 16 00 00 48 01 c8 48 c1 e8 04 48
RSP: 0018:ffff8881cde9f460 EFLAGS: 00000246
RAX: ffffea00090451b0 RBX: 0000000000000608 RCX: ffffea00090311b0
RDX: 00000001cde9f608 RSI: 0000000000000720 RDI: ffff8881cde9f608
RBP: ffff8881cde9f478 R08: ffffea000000000f R09: ffff88812fffa000
R10: 0000000000000004 R11: 00000000ffffffff R12: 000000000000006a
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
 kmsan_get_shadow_origin_ptr+0x6c/0xb0 mm/kmsan/kmsan_shadow.c:149
 __msan_metadata_ptr_for_load_4+0x10/0x20 mm/kmsan/kmsan_instr.c:54
 vga_8planes_fillrect drivers/video/fbdev/vga16fb.c:855 [inline]
 vga16fb_fillrect+0xbbc/0x1870 drivers/video/fbdev/vga16fb.c:947
 bit_clear_margins+0x582/0x740 drivers/video/fbdev/core/bitblit.c:232
 fbcon_clear_margins drivers/video/fbdev/core/fbcon.c:1381 [inline]
 fbcon_switch+0x2e79/0x3830 drivers/video/fbdev/core/fbcon.c:2363
 redraw_screen+0x9b2/0x2980 drivers/tty/vt/vt.c:1015
 fbcon_modechanged+0x110d/0x1320 drivers/video/fbdev/core/fbcon.c:3001
 fbcon_update_vcs+0x86/0xa0 drivers/video/fbdev/core/fbcon.c:3048
 fb_set_var+0x1420/0x1850 drivers/video/fbdev/core/fbmem.c:1056
 do_fb_ioctl+0xc00/0x1150 drivers/video/fbdev/core/fbmem.c:1109
 fb_ioctl+0x1e4/0x210 drivers/video/fbdev/core/fbmem.c:1185
 vfs_ioctl fs/ioctl.c:48 [inline]
 ksys_ioctl fs/ioctl.c:753 [inline]
 __do_sys_ioctl fs/ioctl.c:762 [inline]
 __se_sys_ioctl+0x319/0x4d0 fs/ioctl.c:760
 __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:760
 do_syscall_64+0xad/0x160 arch/x86/entry/common.c:386
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d5f9
Code: Bad RIP value.
RSP: 002b:00007fdcda761c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 000000000000e280 RCX: 000000000045d5f9
RDX: 00000000200000c0 RSI: 0000000000004601 RDI: 0000000000000003
RBP: 000000000118cf80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118cf4c
R13: 000000000169fb6f R14: 00007fdcda7629c0 R15: 000000000118cf4c

------------[ cut here ]------------
slab index 32768 out of bounds (558) for stack id 262b8000
WARNING: CPU: 0 PID: 5351 at lib/stackdepot.c:235 stack_depot_fetch+0x2d/0x60 lib/stackdepot.c:234


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