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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Jul 10, 2019 at 09:47:11AM -0400, Tony Camuso wrote:
> On 5/24/19 4:36 AM, Jani Nikula wrote:
> > On Thu, 23 May 2019, tcamuso <tcamuso@redhat.com> wrote:
> >>  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.
> > 
> > I think the better question is, *why* does the idr_find() return NULL? I
> > don't think it should, under any circumstances. I fear adding the check
> > here papers over some other problem, taking us further away from the
> > root cause.
> > 
> > Also, can you reproduce this on a recent upstream kernel? The aux device
> > nodes were introduced in kernel v4.6. Whatever you reproduced on v3.10
> > is pretty much irrelevant for upstream.
> > 
> > 
> > BR,
> > Jani.
> 
> I have not been able to reproduce this problem.

mknod /dev/foo c <drm_dp_aux major> 255
cat /dev/foo

should do it.

> 
> However, whatever the reason idr_find() returns NULL, isn't it good form to
> check it before using it? What would be the software engineering reason not
> to do this?
> 
> > 
> > 
> > 
> > 
> >>
> >> ----------------------------------------------------------------------------
> >> /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);
> > 
> 
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-- 
Ville Syrjälä
Intel