KMSAN: kernel-infoleak in fb_cmap_to_user

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    29ad81a1 arch/x86: add missing include to sparsemem.h
git tree:       https://github.com/google/kmsan.git master
console output: https://syzkaller.appspot.com/x/log.txt?x=1001ac60d00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c8e3b38ca92283e
dashboard link: https://syzkaller.appspot.com/bug?extid=47fa9c9c648b765305b9
compiler:       Debian clang version 11.0.1-2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17ffe738d00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11ca2914d00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+47fa9c9c648b765305b9@xxxxxxxxxxxxxxxxxxxxxxxxx

=====================================================
BUG: KMSAN: kernel-infoleak in kmsan_copy_to_user+0x9c/0xb0 mm/kmsan/kmsan_hooks.c:249
CPU: 1 PID: 8225 Comm: syz-executor269 Not tainted 5.11.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x21c/0x280 lib/dump_stack.c:120
 kmsan_report+0xfb/0x1e0 mm/kmsan/kmsan_report.c:118
 kmsan_internal_check_memory+0x484/0x520 mm/kmsan/kmsan.c:437
 kmsan_copy_to_user+0x9c/0xb0 mm/kmsan/kmsan_hooks.c:249
 instrument_copy_to_user include/linux/instrumented.h:121 [inline]
 _copy_to_user+0x1ac/0x270 lib/usercopy.c:33
 copy_to_user include/linux/uaccess.h:209 [inline]
 fb_cmap_to_user+0x40a/0x990 drivers/video/fbdev/core/fbcmap.c:208
 do_fb_ioctl+0xc53/0x1090 drivers/video/fbdev/core/fbmem.c:1136
 fb_ioctl+0x1e4/0x210 drivers/video/fbdev/core/fbmem.c:1185
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0x311/0x4d0 fs/ioctl.c:739
 __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:739
 do_syscall_64+0x9f/0x140 arch/x86/entry/common.c:48
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x43fbd9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 14 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc68acbf98 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000400488 RCX: 000000000043fbd9
RDX: 0000000020007400 RSI: 0000000000004604 RDI: 0000000000000003
RBP: 0000000000000000 R08: 00007ffc68acc138 R09: 00007ffc68acc138
R10: 00007ffc68acba10 R11: 0000000000000246 R12: 0000000000403460
R13: 431bde82d7b634db R14: 00000000004ad018 R15: 0000000000400488

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:121 [inline]
 kmsan_internal_poison_shadow+0x5c/0xf0 mm/kmsan/kmsan.c:104
 kmsan_slab_alloc+0x8d/0xe0 mm/kmsan/kmsan_hooks.c:76
 slab_alloc_node mm/slub.c:2907 [inline]
 slab_alloc mm/slub.c:2916 [inline]
 __kmalloc+0x378/0x560 mm/slub.c:3998
 kmalloc include/linux/slab.h:557 [inline]
 fb_alloc_cmap_gfp+0x39b/0xa70 drivers/video/fbdev/core/fbcmap.c:104
 fb_alloc_cmap+0x95/0xb0 drivers/video/fbdev/core/fbcmap.c:135
 drm_fb_helper_alloc_fbi+0x106/0x3f0 drivers/gpu/drm/drm_fb_helper.c:563
 drm_fb_helper_generic_probe+0x4f3/0xc70 drivers/gpu/drm/drm_fb_helper.c:2320
 drm_fb_helper_single_fb_probe drivers/gpu/drm/drm_fb_helper.c:1658 [inline]
 __drm_fb_helper_initial_config_and_unlock+0x1cac/0x26c0 drivers/gpu/drm/drm_fb_helper.c:1816
 drm_fb_helper_initial_config drivers/gpu/drm/drm_fb_helper.c:1911 [inline]
 drm_fbdev_client_hotplug+0xbb8/0xd70 drivers/gpu/drm/drm_fb_helper.c:2413
 drm_fbdev_generic_setup+0x39d/0xa00 drivers/gpu/drm/drm_fb_helper.c:2495
 vkms_init+0x880/0xa02 drivers/gpu/drm/vkms/vkms_drv.c:168
 do_one_initcall+0x362/0x8d0 init/main.c:1226
 do_initcall_level+0x1e7/0x35a init/main.c:1299
 do_initcalls+0x127/0x1cb init/main.c:1315
 do_basic_setup+0x33/0x36 init/main.c:1335
 kernel_init_freeable+0x23d/0x390 init/main.c:1536
 kernel_init+0x1f/0x840 init/main.c:1424
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Bytes 0-1 of 2 are uninitialized
Memory access of size 2 starts at ffff8881455651c0
Data copied to user address 0000000020007300
=====================================================


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