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.
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);