KASAN: slab-out-of-bounds Read in fbcon_get_font

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 76bb8b05 Merge tag 'kbuild-v5.5' of git://git.kernel.org/p..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10bfe282e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
dashboard link: https://syzkaller.appspot.com/bug?extid=4455ca3b3291de891abc
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11181edae00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=105cbb7ae00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+4455ca...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-out-of-bounds in memcpy include/linux/string.h:380 [inline]
BUG: KASAN: slab-out-of-bounds in fbcon_get_font+0x2b2/0x5e0
drivers/video/fbdev/core/fbcon.c:2465
Read of size 16 at addr ffff888094b0aa10 by task syz-executor414/9999

CPU: 0 PID: 9999 Comm: syz-executor414 Not tainted 5.4.0-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+0x197/0x210 lib/dump_stack.c:118
print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
__kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
kasan_report+0x12/0x20 mm/kasan/common.c:638
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
memcpy+0x24/0x50 mm/kasan/common.c:124
memcpy include/linux/string.h:380 [inline]
fbcon_get_font+0x2b2/0x5e0 drivers/video/fbdev/core/fbcon.c:2465
con_font_get drivers/tty/vt/vt.c:4446 [inline]
con_font_op+0x20b/0x1250 drivers/tty/vt/vt.c:4605
vt_ioctl+0x181a/0x26d0 drivers/tty/vt/vt_ioctl.c:965
tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2658
vfs_ioctl fs/ioctl.c:47 [inline]
file_ioctl fs/ioctl.c:545 [inline]
do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
__do_sys_ioctl fs/ioctl.c:756 [inline]
__se_sys_ioctl fs/ioctl.c:754 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4444d9
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 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 0f 83 7b d8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff6f4393b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fff6f4393c0 RCX: 00000000004444d9
RDX: 0000000020000440 RSI: 0000000000004b72 RDI: 0000000000000005
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000400da0
R10: 00007fff6f438f00 R11: 0000000000000246 R12: 00000000004021e0
R13: 0000000000402270 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 9999:
save_stack+0x23/0x90 mm/kasan/common.c:71
set_track mm/kasan/common.c:79 [inline]
__kasan_kmalloc mm/kasan/common.c:512 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:485
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:526
__do_kmalloc mm/slab.c:3656 [inline]
__kmalloc+0x163/0x770 mm/slab.c:3665
kmalloc include/linux/slab.h:561 [inline]
fbcon_set_font+0x32d/0x860 drivers/video/fbdev/core/fbcon.c:2663
con_font_set drivers/tty/vt/vt.c:4538 [inline]
con_font_op+0xe18/0x1250 drivers/tty/vt/vt.c:4603
vt_ioctl+0xd2e/0x26d0 drivers/tty/vt/vt_ioctl.c:913
tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2658
vfs_ioctl fs/ioctl.c:47 [inline]
file_ioctl fs/ioctl.c:545 [inline]
do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
__do_sys_ioctl fs/ioctl.c:756 [inline]
__se_sys_ioctl fs/ioctl.c:754 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 9771:
save_stack+0x23/0x90 mm/kasan/common.c:71
set_track mm/kasan/common.c:79 [inline]
kasan_set_free_info mm/kasan/common.c:334 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:473
kasan_slab_free+0xe/0x10 mm/kasan/common.c:482
__cache_free mm/slab.c:3426 [inline]
kfree+0x10a/0x2c0 mm/slab.c:3757
tomoyo_init_log+0x15c1/0x2070 security/tomoyo/audit.c:294
tomoyo_supervisor+0x33f/0xef0 security/tomoyo/common.c:2095
tomoyo_audit_env_log security/tomoyo/environ.c:36 [inline]
tomoyo_env_perm+0x18e/0x210 security/tomoyo/environ.c:63
tomoyo_environ security/tomoyo/domain.c:670 [inline]
tomoyo_find_next_domain+0x1354/0x1f6c security/tomoyo/domain.c:876
tomoyo_bprm_check_security security/tomoyo/tomoyo.c:107 [inline]
tomoyo_bprm_check_security+0x124/0x1a0 security/tomoyo/tomoyo.c:97
security_bprm_check+0x63/0xb0 security/security.c:784
search_binary_handler+0x71/0x570 fs/exec.c:1645
exec_binprm fs/exec.c:1701 [inline]
__do_execve_file.isra.0+0x1329/0x22b0 fs/exec.c:1821
do_execveat_common fs/exec.c:1867 [inline]
do_execve fs/exec.c:1884 [inline]
__do_sys_execve fs/exec.c:1960 [inline]
__se_sys_execve fs/exec.c:1955 [inline]
__x64_sys_execve+0x8f/0xc0 fs/exec.c:1955
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff888094b0a000
which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 2576 bytes inside of
4096-byte region [ffff888094b0a000, ffff888094b0b000)
The buggy address belongs to the page:
page:ffffea000252c280 refcount:1 mapcount:0 mapping:ffff8880aa402000
index:0x0 compound_mapcount: 0
raw: 00fffe0000010200 ffffea0002a3ae08 ffffea0002a6aa88 ffff8880aa402000
raw: 0000000000000000 ffff888094b0a000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888094b0a900: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff888094b0a980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffff888094b0aa00: 00 00 fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff888094b0aa80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888094b0ab00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

[Daniel Vetter]

On Tue, Dec 3, 2019 at 11:25 PM syzbot
<syzbot+4455ca...@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit: 76bb8b05 Merge tag 'kbuild-v5.5' of git://git.kernel.org/p..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=10bfe282e00000
> kernel config: https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
> dashboard link: https://syzkaller.appspot.com/bug?extid=4455ca3b3291de891abc
> compiler: gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11181edae00000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=105cbb7ae00000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+4455ca...@syzkaller.appspotmail.com
>
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in memcpy include/linux/string.h:380 [inline]
> BUG: KASAN: slab-out-of-bounds in fbcon_get_font+0x2b2/0x5e0
> drivers/video/fbdev/core/fbcon.c:2465
> Read of size 16 at addr ffff888094b0aa10 by task syz-executor414/9999

So fbcon allocates some memory, security/tomoyo goes around and frees
it, fbcon goes boom because the memory is gone. I'm kinda leaning
towards "not an fbcon bug". Adding relevant security folks and mailing
lists.

But from a very quick look in tomoyo it loosk more like "machine on
fire, random corruption all over". No idea what's going on here.
-Daniel

--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

[Dmitry Vyukov]

Hi Daniel,

This is an out-of-bounds access, not use-after-free.
I don't know why we print the free stack at all (maybe +Andrey knows),
but that's what KASAN did from day one. I filed
https://bugzilla.kernel.org/show_bug.cgi?id=198425 which I think is a
good idea, I will add your confusion as a data point :)
Re this bug, free stack is irrelevant, I guess it's when the heap
block was freed before it was reallocated by console. So it's plain
out-of-bounds in fbcon_get_font, which looks sane and consistent to me
and reproducible on top.

[Daniel Vetter]

Ugh, that's indeed very confusing, thanks for explaining.
-Daniel

[Andrey Ryabinin]

Because we have that information (free stack) and it usually better to provide
all the information we have rather than hide it. You never known what information
might be needed to fix the bug.
Free memory might be reused and what we report as OOB might be an UAF and free stack
could be useful in such case.

[syzbot]

syzbot has bisected this bug to:

commit 2de50e9674fc4ca3c6174b04477f69eb26b4ee31
Author: Russell Currey <rus...@russell.cc>
Date: Mon Feb 8 04:08:20 2016 +0000

powerpc/powernv: Remove support for p5ioc2

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=127a042ae00000
start commit: 76bb8b05 Merge tag 'kbuild-v5.5' of git://git.kernel.org/p..
git tree: upstream
final crash: https://syzkaller.appspot.com/x/report.txt?x=117a042ae00000
console output: https://syzkaller.appspot.com/x/log.txt?x=167a042ae00000

kernel config: https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
dashboard link: https://syzkaller.appspot.com/bug?extid=4455ca3b3291de891abc

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11181edae00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=105cbb7ae00000

Reported-by: syzbot+4455ca...@syzkaller.appspotmail.com
Fixes: 2de50e9674fc ("powerpc/powernv: Remove support for p5ioc2")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

[Tetsuo Handa]

Hello.

syzbot is reporting that memory allocation size at fbcon_set_font() is too small
because font's height is rounded up from 10 to 16 after memory allocation.

----------
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index c9235a2f42f8..68fe66e435d3 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2461,6 +2461,7 @@ static int fbcon_get_font(struct vc_data *vc, struct console_font *font)

if (font->width <= 8) {
j = vc->vc_font.height;
+ printk("ksize(fontdata)=%lu font->charcount=%d vc->vc_font.height=%d font->width=%u\n", ksize(fontdata), font->charcount, j, font->width);
for (i = 0; i < font->charcount; i++) {
memcpy(data, fontdata, j);
memset(data + j, 0, 32 - j);
@@ -2661,6 +2662,8 @@ static int fbcon_set_font(struct vc_data *vc, struct console_font *font,
size = h * pitch * charcount;

new_data = kmalloc(FONT_EXTRA_WORDS * sizeof(int) + size, GFP_USER);
+ if (new_data)
+ printk("ksize(new_data)=%lu h=%u pitch=%u charcount=%u font->width=%u\n", ksize(new_data), h, pitch, charcount, font->width);

if (!new_data)
return -ENOMEM;
----------

Normal usage:

[ 27.305293] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.328527] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.362551] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.385084] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.387653] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.417562] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.437808] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.440738] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.461157] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.495346] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.607372] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.655674] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.675310] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8
[ 27.702193] ksize(new_data)=8192 h=16 pitch=1 charcount=256 font->width=8

syzbot's testcase:

[ 115.784893] ksize(new_data)=4096 h=10 pitch=1 charcount=256 font->width=8
[ 115.790269] ksize(fontdata)=4096 font->charcount=256 vc->vc_font.height=16 font->width=8

[Paolo Bonzini]

Why is everybody being CC'd, even if the bug has nothing to do with the
person's subsystem?

Thanks,

Paolo

[Dmitry Vyukov]

The To list should be intersection of 2 groups of emails: result of
get_maintainers.pl on the file identified as culprit in the crash
message + emails extracted from the bisected to commit.

[Paolo Bonzini]

Ah, and because the machine is a KVM guest, kvm_wait appears in a lot of
backtrace and I get to share syzkaller's joy every time. :)

This bisect result is bogus, though Tetsuo found the bug anyway.
Perhaps you can exclude commits that only touch architectures other than
x86?

Paolo

[Tetsuo Handa]

There is "#syz uncc" command but it is too hard to utilize?

[Dmitry Vyukov]

I don't see any mention of "kvm" in the crash report. And it's only 1
file, not all of them, in this case I would expect it to be
drivers/video/fbdev/core/fbcon.c. So it should be something different.

> This bisect result is bogus, though Tetsuo found the bug anyway.
> Perhaps you can exclude commits that only touch architectures other than
> x86?

We do this. It work sometimes. But sometimes it hits non-deterministic
kernel build bugs:
https://github.com/google/syzkaller/issues/1271#issuecomment-559093018
And in this case it hit some git bisect weirdness which I can't explain yet:
https://github.com/google/syzkaller/issues/1527

[Tetsuo Handa]

On 2019/12/05 19:22, Paolo Bonzini wrote:
> Ah, and because the machine is a KVM guest, kvm_wait appears in a lot of
> backtrace and I get to share syzkaller's joy every time. :)
>
> This bisect result is bogus, though Tetsuo found the bug anyway.
> Perhaps you can exclude commits that only touch architectures other than
> x86?
>

It would be nice if coverage functionality can extract filenames in the source
code and supply the list of filenames as arguments for bisect operation.

Also, (unrelated but) it would be nice if we can have "make yes2modconfig"
target which converts CONFIG_FOO=y to CONFIG_FOO=m if FOO is tristate.
syzbot is testing kernel configs close to "make allyesconfig" but I want to
save kernel rebuild time by disabling unrelated functionality when manually
"debug printk()ing" kernels.

[Paolo Bonzini]

On 05/12/19 11:31, Dmitry Vyukov wrote:
>> Ah, and because the machine is a KVM guest, kvm_wait appears in a lot of
>> backtrace and I get to share syzkaller's joy every time. :)
> I don't see any mention of "kvm" in the crash report.

It's there in the stack trace, not sure if this is what triggered my Cc:

[<ffffffff810c7c3a>] kvm_wait+0xca/0xe0 arch/x86/kernel/kvm.c:612

Paolo

[Dmitry Vyukov]

Oh, you mean the final bisection crash. Indeed it contains a kvm frame
and it turns out to be a bug in syzkaller code that indeed
misattributed it to kvm instead of netfilter.
Should be fixed now, you may read the commit message for details:
https://github.com/google/syzkaller/commit/4fb74474cf0af2126be3a8989d770c3947ae9478

Overall this "making sense out of kernel output" task is the ultimate
insanity, you may skim through this file to get a taste of amount of
hardcoding and special corner cases that need to be handled:
https://github.com/google/syzkaller/blob/master/pkg/report/linux.go
And this is never done, such "exception from exception corner case"
things pop up every week. There is always something to shuffle and
tune. It only keeps functioning due to 500+ test cases for all
possible insane kernel outputs:
https://github.com/google/syzkaller/tree/master/pkg/report/testdata/linux/report
https://github.com/google/syzkaller/tree/master/pkg/report/testdata/linux/guilty

So thanks for persisting and questioning! We are getting better with
each new test.

[Paolo Bonzini]

On 05/12/19 12:27, Dmitry Vyukov wrote:
> Oh, you mean the final bisection crash. Indeed it contains a kvm frame
> and it turns out to be a bug in syzkaller code that indeed
> misattributed it to kvm instead of netfilter.
> Should be fixed now, you may read the commit message for details:
> https://github.com/google/syzkaller/commit/4fb74474cf0af2126be3a8989d770c3947ae9478
>
> Overall this "making sense out of kernel output" task is the ultimate
> insanity, you may skim through this file to get a taste of amount of
> hardcoding and special corner cases that need to be handled:
> https://github.com/google/syzkaller/blob/master/pkg/report/linux.go
> And this is never done, such "exception from exception corner case"
> things pop up every week. There is always something to shuffle and
> tune. It only keeps functioning due to 500+ test cases for all
> possible insane kernel outputs:
> https://github.com/google/syzkaller/tree/master/pkg/report/testdata/linux/report
> https://github.com/google/syzkaller/tree/master/pkg/report/testdata/linux/guilty
>
> So thanks for persisting and questioning! We are getting better with
> each new test.

Thanks to you! I "complain" because I know you're so responsive. :)

Paolo

[Dmitry Vyukov]

On Thu, Dec 5, 2019 at 11:41 AM Tetsuo Handa
<penguin...@i-love.sakura.ne.jp> wrote:
>
> On 2019/12/05 19:22, Paolo Bonzini wrote:
> > Ah, and because the machine is a KVM guest, kvm_wait appears in a lot of
> > backtrace and I get to share syzkaller's joy every time. :)
> >
> > This bisect result is bogus, though Tetsuo found the bug anyway.
> > Perhaps you can exclude commits that only touch architectures other than
> > x86?
> >
>
> It would be nice if coverage functionality can extract filenames in the source
> code and supply the list of filenames as arguments for bisect operation.

What is the criteria for file name extraction? What will bisect
operation do with the set of files?
If you have a feature/improvement request, please file it at:
https://github.com/google/syzkaller/issues/new

[Dmitry Vyukov]

On Thu, Dec 5, 2019 at 11:41 AM Tetsuo Handa
<penguin...@i-love.sakura.ne.jp> wrote:
>

I thought that maybe sed "s#=y#=m#g" && make olddefconfig will do, but
unfortunately, it turns off non-tristate configs...

$ egrep "CONFIG_MEMORY_HOTPLUG|CONFIG_TCP_CONG_DCTCP" .config
CONFIG_MEMORY_HOTPLUG=y
CONFIG_TCP_CONG_DCTCP=y
# sed -i "s/CONFIG_MEMORY_HOTPLUG=y/CONFIG_MEMORY_HOTPLUG=m/g" .config
# sed -i "s/CONFIG_TCP_CONG_DCTCP=y/CONFIG_TCP_CONG_DCTCP=m/g" .config
# egrep "CONFIG_MEMORY_HOTPLUG|CONFIG_TCP_CONG_DCTCP" .config
CONFIG_MEMORY_HOTPLUG=m
CONFIG_TCP_CONG_DCTCP=m
# make olddefconfig
# egrep "CONFIG_MEMORY_HOTPLUG|CONFIG_TCP_CONG_DCTCP" .config
# CONFIG_MEMORY_HOTPLUG is not set
CONFIG_TCP_CONG_DCTCP=m

[syzbot]

Auto-closing this bug as obsolete.
No recent activity, existing reproducers are no longer triggering the issue.