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From:   tcamuso <tcamuso@redhat.com>
To:     dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Cc:     airlied@linux.ie, daniel@ffwll.ch, tcamuso@redhat.com,
        dkwon@redhat.com
Subject: [PATCH] drm: assure aux_dev is nonzero before using it
Date:   Thu, 23 May 2019 07:09:05 -0400
Message-Id: <20190523110905.22445-1-tcamuso@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From Daniel Kwon <dkwon@redhat.com>

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@redhat.com>
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@redhat.com>
Signed-off-by: Tony Camuso <tcamuso@redhat.com>
---
 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