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

From: Ville Syrjälä
Date: Wed Jul 10 2019 - 09:56:24 EST

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@xxxxxxxxxx> wrote:
> >> 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.
> >
> > 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@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);
> >
>
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--
Ville Syrjälä
Intel