From: Zwane Mwaikambo zwane@yosper.io
commit 73b62cdb93b68d7e2c1d373c6a411bc00c53e702 upstream.
I observed this when unplugging a DP monitor whilst a computer is asleep and then waking it up. This left DP chardev nodes still being present on the filesystem and accessing these device nodes caused an oops because drm_dp_aux_dev_get_by_minor() assumes a device exists if it is opened. This can also be reproduced by creating a device node with mknod(1) and issuing an open(2)
[166164.933198] BUG: kernel NULL pointer dereference, address: 0000000000000018 [166164.933202] #PF: supervisor read access in kernel mode [166164.933204] #PF: error_code(0x0000) - not-present page [166164.933205] PGD 0 P4D 0 [166164.933208] Oops: 0000 [#1] PREEMPT SMP NOPTI [166164.933211] CPU: 4 PID: 99071 Comm: fwupd Tainted: G W 5.8.0-rc6+ #1 [166164.933213] Hardware name: LENOVO 20RD002VUS/20RD002VUS, BIOS R16ET25W (1.11 ) 04/21/2020 [166164.933232] RIP: 0010:drm_dp_aux_dev_get_by_minor+0x29/0x70 [drm_kms_helper] [166164.933234] Code: 00 0f 1f 44 00 00 55 48 89 e5 41 54 41 89 fc 48 c7 c7 60 01 a4 c0 e8 26 ab 30 d7 44 89 e6 48 c7 c7 80 01 a4 c0 e8 47 94 d6 d6 <8b> 50 18 49 89 c4 48 8d 78 18 85 d2 74 33 8d 4a 01 89 d0 f0 0f b1 [166164.933236] RSP: 0018:ffffb7d7c41cbbf0 EFLAGS: 00010246 [166164.933237] RAX: 0000000000000000 RBX: ffff8a90001fe900 RCX: 0000000000000000 [166164.933238] RDX: 0000000000000000 RSI: 0000000000000003 RDI: ffffffffc0a40180 [166164.933239] RBP: ffffb7d7c41cbbf8 R08: 0000000000000000 R09: ffff8a93e157d6d0 [166164.933240] R10: 0000000000000000 R11: ffffffffc0a40188 R12: 0000000000000003 [166164.933241] R13: ffff8a9402200e80 R14: ffff8a90001fe900 R15: 0000000000000000 [166164.933244] FS: 00007f7fb041eb00(0000) GS:ffff8a9411500000(0000) knlGS:0000000000000000 [166164.933245] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [166164.933246] CR2: 0000000000000018 CR3: 00000000352c2003 CR4: 00000000003606e0 [166164.933247] Call Trace: [166164.933264] auxdev_open+0x1b/0x40 [drm_kms_helper] [166164.933278] chrdev_open+0xa7/0x1c0 [166164.933282] ? cdev_put.part.0+0x20/0x20 [166164.933287] do_dentry_open+0x161/0x3c0 [166164.933291] vfs_open+0x2d/0x30 [166164.933297] path_openat+0xb27/0x10e0 [166164.933306] ? atime_needs_update+0x73/0xd0 [166164.933309] do_filp_open+0x91/0x100 [166164.933313] ? __alloc_fd+0xb2/0x150 [166164.933316] do_sys_openat2+0x210/0x2d0 [166164.933318] do_sys_open+0x46/0x80 [166164.933320] __x64_sys_openat+0x20/0x30 [166164.933328] do_syscall_64+0x52/0xc0 [166164.933336] entry_SYSCALL_64_after_hwframe+0x44/0xa9
(gdb) disassemble drm_dp_aux_dev_get_by_minor+0x29 Dump of assembler code for function drm_dp_aux_dev_get_by_minor: 0x0000000000017b10 <+0>: callq 0x17b15 <drm_dp_aux_dev_get_by_minor+5> 0x0000000000017b15 <+5>: push %rbp 0x0000000000017b16 <+6>: mov %rsp,%rbp 0x0000000000017b19 <+9>: push %r12 0x0000000000017b1b <+11>: mov %edi,%r12d 0x0000000000017b1e <+14>: mov $0x0,%rdi 0x0000000000017b25 <+21>: callq 0x17b2a <drm_dp_aux_dev_get_by_minor+26> 0x0000000000017b2a <+26>: mov %r12d,%esi 0x0000000000017b2d <+29>: mov $0x0,%rdi 0x0000000000017b34 <+36>: callq 0x17b39 <drm_dp_aux_dev_get_by_minor+41> 0x0000000000017b39 <+41>: mov 0x18(%rax),%edx <========= 0x0000000000017b3c <+44>: mov %rax,%r12 0x0000000000017b3f <+47>: lea 0x18(%rax),%rdi 0x0000000000017b43 <+51>: test %edx,%edx 0x0000000000017b45 <+53>: je 0x17b7a <drm_dp_aux_dev_get_by_minor+106> 0x0000000000017b47 <+55>: lea 0x1(%rdx),%ecx 0x0000000000017b4a <+58>: mov %edx,%eax 0x0000000000017b4c <+60>: lock cmpxchg %ecx,(%rdi) 0x0000000000017b50 <+64>: jne 0x17b76 <drm_dp_aux_dev_get_by_minor+102> 0x0000000000017b52 <+66>: test %edx,%edx 0x0000000000017b54 <+68>: js 0x17b6d <drm_dp_aux_dev_get_by_minor+93> 0x0000000000017b56 <+70>: test %ecx,%ecx 0x0000000000017b58 <+72>: js 0x17b6d <drm_dp_aux_dev_get_by_minor+93> 0x0000000000017b5a <+74>: mov $0x0,%rdi 0x0000000000017b61 <+81>: callq 0x17b66 <drm_dp_aux_dev_get_by_minor+86> 0x0000000000017b66 <+86>: mov %r12,%rax 0x0000000000017b69 <+89>: pop %r12 0x0000000000017b6b <+91>: pop %rbp 0x0000000000017b6c <+92>: retq 0x0000000000017b6d <+93>: xor %esi,%esi 0x0000000000017b6f <+95>: callq 0x17b74 <drm_dp_aux_dev_get_by_minor+100> 0x0000000000017b74 <+100>: jmp 0x17b5a <drm_dp_aux_dev_get_by_minor+74> 0x0000000000017b76 <+102>: mov %eax,%edx 0x0000000000017b78 <+104>: jmp 0x17b43 <drm_dp_aux_dev_get_by_minor+51> 0x0000000000017b7a <+106>: xor %r12d,%r12d 0x0000000000017b7d <+109>: jmp 0x17b5a <drm_dp_aux_dev_get_by_minor+74> End of assembler dump.
(gdb) list *drm_dp_aux_dev_get_by_minor+0x29 0x17b39 is in drm_dp_aux_dev_get_by_minor (drivers/gpu/drm/drm_dp_aux_dev.c:65). 60 static struct drm_dp_aux_dev *drm_dp_aux_dev_get_by_minor(unsigned index) 61 { 62 struct drm_dp_aux_dev *aux_dev = NULL; 63 64 mutex_lock(&aux_idr_mutex); 65 aux_dev = idr_find(&aux_idr, index); 66 if (!kref_get_unless_zero(&aux_dev->refcount)) 67 aux_dev = NULL; 68 mutex_unlock(&aux_idr_mutex); 69 (gdb) p/x &((struct drm_dp_aux_dev *)(0x0))->refcount $8 = 0x18
Looking at the caller, checks on the minor are pushed down to drm_dp_aux_dev_get_by_minor()
static int auxdev_open(struct inode *inode, struct file *file) { unsigned int minor = iminor(inode); struct drm_dp_aux_dev *aux_dev;
aux_dev = drm_dp_aux_dev_get_by_minor(minor); <==== if (!aux_dev) return -ENODEV;
file->private_data = aux_dev; return 0; }
Fixes: e94cb37b34eb ("drm/dp: Add a drm_aux-dev module for reading/writing dpcd registers.") Cc: stable@vger.kernel.org # v4.6+ Signed-off-by: Zwane Mwaikambo zwane@yosper.io Reviewed-by: Lyude Paul lyude@redhat.com [added Cc to stable] Signed-off-by: Lyude Paul lyude@redhat.com Link: https://patchwork.freedesktop.org/patch/msgid/alpine.DEB.2.21.2010122231070.... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/gpu/drm/drm_dp_aux_dev.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/drivers/gpu/drm/drm_dp_aux_dev.c +++ b/drivers/gpu/drm/drm_dp_aux_dev.c @@ -63,7 +63,7 @@ static struct drm_dp_aux_dev *drm_dp_aux
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);