[PATCH] drm: assure aux_dev is nonzero before using it

From: tcamuso
Date: Thu May 23 2019 - 07:12:04 EST

>From Daniel Kwon <dkwon@xxxxxxxxxx>

The system was crashed due to invalid memory access while trying to access
auxiliary device.

crash> bt
PID: 9863 TASK: ffff89d1bdf11040 CPU: 1 COMMAND: "ipmitool"
#0 [ffff89cedd7f3868] machine_kexec at ffffffffb0663674
#1 [ffff89cedd7f38c8] __crash_kexec at ffffffffb071cf62
#2 [ffff89cedd7f3998] crash_kexec at ffffffffb071d050
#3 [ffff89cedd7f39b0] oops_end at ffffffffb0d6d758
#4 [ffff89cedd7f39d8] no_context at ffffffffb0d5bcde
#5 [ffff89cedd7f3a28] __bad_area_nosemaphore at ffffffffb0d5bd75
#6 [ffff89cedd7f3a78] bad_area at ffffffffb0d5c085
#7 [ffff89cedd7f3aa0] __do_page_fault at ffffffffb0d7080c
#8 [ffff89cedd7f3b10] do_page_fault at ffffffffb0d70905
#9 [ffff89cedd7f3b40] page_fault at ffffffffb0d6c758
[exception RIP: drm_dp_aux_dev_get_by_minor+0x3d]
RIP: ffffffffc0a589bd RSP: ffff89cedd7f3bf0 RFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff89cedd7f3fd8
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffc0a613e0
RBP: ffff89cedd7f3bf8 R8: ffff89f1bcbabbd0 R9: 0000000000000000
R10: ffff89f1be7a1cc0 R11: 0000000000000000 R12: 0000000000000000
R13: ffff89f1b32a2830 R14: ffff89d18fadfa00 R15: 0000000000000000
ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
RIP: 00002b45f0d80d30 RSP: 00007ffc416066a0 RFLAGS: 00010246
RAX: 0000000000000002 RBX: 000056062e212d80 RCX: 00007ffc41606810
RDX: 0000000000000000 RSI: 0000000000000002 RDI: 00007ffc41606ec0
RBP: 0000000000000000 R8: 000056062dfed229 R9: 00002b45f0cdf14d
R10: 0000000000000002 R11: 0000000000000246 R12: 00007ffc41606ec0
R13: 00007ffc41606ed0 R14: 00007ffc41606ee0 R15: 0000000000000000
ORIG_RAX: 0000000000000002 CS: 0033 SS: 002b

----------------------------------------------------------------------------

It was trying to open '/dev/ipmi0', but as no entry in aux_dir, it returned
NULL from 'idr_find()'. This drm_dp_aux_dev_get_by_minor() should have done a
check on this, but had failed to do it.

----------------------------------------------------------------------------
/usr/src/debug/kernel-3.10.0-957.12.1.el7/linux-3.10.0-957.12.1.el7.x86_64/include/linux/idr.h: 114
114 struct idr_layer *hint = rcu_dereference_raw(idr->hint);
0xffffffffc0a58998 <drm_dp_aux_dev_get_by_minor+0x18>: mov 0x8a41(%rip),%rax # 0xffffffffc0a613e0 <aux_idr>
/usr/src/debug/kernel-3.10.0-957.12.1.el7/linux-3.10.0-957.12.1.el7.x86_64/include/linux/idr.h: 116
116 if (hint && (id & ~IDR_MASK) == hint->prefix)
117 return rcu_dereference_raw(hint->ary[id & IDR_MASK]);
0xffffffffc0a5899f <drm_dp_aux_dev_get_by_minor+0x1f>: test %rax,%rax
0xffffffffc0a589a2 <drm_dp_aux_dev_get_by_minor+0x22>: je 0xffffffffc0a589ac <drm_dp_aux_dev_get_by_minor+0x2c>
0xffffffffc0a589a4 <drm_dp_aux_dev_get_by_minor+0x24>: mov %ebx,%edx
0xffffffffc0a589a6 <drm_dp_aux_dev_get_by_minor+0x26>: xor %dl,%dl
0xffffffffc0a589a8 <drm_dp_aux_dev_get_by_minor+0x28>: cmp (%rax),%edx
0xffffffffc0a589aa <drm_dp_aux_dev_get_by_minor+0x2a>: je 0xffffffffc0a589f0 <drm_dp_aux_dev_get_by_minor+0x70>
/usr/src/debug/kernel-3.10.0-957.12.1.el7/linux-3.10.0-957.12.1.el7.x86_64/include/linux/idr.h: 119
119 return idr_find_slowpath(idr, id);
0xffffffffc0a589ac <drm_dp_aux_dev_get_by_minor+0x2c>: mov %ebx,%esi
0xffffffffc0a589ae <drm_dp_aux_dev_get_by_minor+0x2e>: mov $0xffffffffc0a613e0,%rdi
0xffffffffc0a589b5 <drm_dp_aux_dev_get_by_minor+0x35>: callq 0xffffffffb09771b0 <idr_find_slowpath>
0xffffffffc0a589ba <drm_dp_aux_dev_get_by_minor+0x3a>: mov %rax,%rbx
/usr/src/debug/kernel-3.10.0-957.12.1.el7/linux-3.10.0-957.12.1.el7.x86_64/arch/x86/include/asm/atomic.h: 25
25 return ACCESS_ONCE((v)->counter);
0xffffffffc0a589bd <drm_dp_aux_dev_get_by_minor+0x3d>: mov 0x18(%rbx),%edx

crash> struct file.f_path 0xffff89d18fadfa00
f_path = {
mnt = 0xffff89f23feaa620,
dentry = 0xffff89f1be7a1cc0
}
crash> files -d 0xffff89f1be7a1cc0
DENTRY INODE SUPERBLK TYPE PATH
ffff89f1be7a1cc0 ffff89f1b32a2830 ffff89d293aa8800 CHR /dev/ipmi0

crash> struct inode.i_rdev ffff89f1b32a2830
i_rdev = 0xf200000
crash> eval (0xfffff & 0xf200000)
hexadecimal: 0
decimal: 0
octal: 0
binary: 0000000000000000000000000000000000000000000000000000000000000000
----------------------------------------------------------------------------

As the index value was 0 and aux_idr had value 0 for all, it can have value
NULL from idr_find() function, but the below function doesn't check and just
tries to use it.

----------------------------------------------------------------------------
crash> aux_idr
aux_idr = $8 = {
hint = 0x0,
top = 0x0,
id_free = 0x0,
layers = 0x0,
id_free_cnt = 0x0,
cur = 0x0,
lock = {
{
rlock = {
raw_lock = {
val = {
counter = 0x0
}
}
}
}
}
}

crash> edis -f drm_dp_aux_dev_get_by_minor
/usr/src/debug/kernel-3.10.0-957.12.1.el7/linux-3.10.0-957.12.1.el7.x86_64/drivers/gpu/drm/drm_dp_aux_dev.c: 57

56 static struct drm_dp_aux_dev *drm_dp_aux_dev_get_by_minor(unsigned index)
57 {
58 struct drm_dp_aux_dev *aux_dev = NULL;
59
60 mutex_lock(&aux_idr_mutex);
61 aux_dev = idr_find(&aux_idr, index);
62 if (!kref_get_unless_zero(&aux_dev->refcount))
63 aux_dev = NULL;
64 mutex_unlock(&aux_idr_mutex);
65
66 return aux_dev;
67 }
----------------------------------------------------------------------------

To avoid this kinds of situation, we should make a safeguard for the returned
value. Changing the line 62 with the below would do.

62 if (aux_dev && !kref_get_unless_zero(&aux_dev->refcount))
^^^^^^^^^^
>From Tony Camuso <tcamuso@xxxxxxxxxx>
I built a patched kernel for several architectures.
Booted the kernel, and ran the following for 100 iterations.
rmmod ipmi kmods to remove /dev/ipmi0.
Invoked ipmitool
insmod ipmi kmods
Did not see any crashes or call traces.

Suggested-by: Daniel Kwon <dkwon@xxxxxxxxxx>
Signed-off-by: Tony Camuso <tcamuso@xxxxxxxxxx>
---
drivers/gpu/drm/drm_dp_aux_dev.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_aux_dev.c b/drivers/gpu/drm/drm_dp_aux_dev.c
index 0e4f25d63fd2d..0b11210c882ee 100644
--- a/drivers/gpu/drm/drm_dp_aux_dev.c
+++ b/drivers/gpu/drm/drm_dp_aux_dev.c
@@ -60,7 +60,7 @@ static struct drm_dp_aux_dev *drm_dp_aux_dev_get_by_minor(unsigned index)

mutex_lock(&aux_idr_mutex);
aux_dev = idr_find(&aux_idr, index);
- if (!kref_get_unless_zero(&aux_dev->refcount))
+ if (aux_dev && !kref_get_unless_zero(&aux_dev->refcount))
aux_dev = NULL;
mutex_unlock(&aux_idr_mutex);

--
2.20.1