Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx> JFYI - since the commit this fixes has been in the kernel for a while, we should definitely Cc this to stable@xxxxxxxxxxxxxxx so that it gets backported. In the future, the dim tools have a command called "fixes" that can figure this out for you: https://drm.pages.freedesktop.org/maintainer-tools/dim.html I'll make sure to do that when I push your patch to drm-misc-next today, thanks! On Mon, 2020-10-12 at 22:59 -0700, zwane@xxxxxxxxx wrote: > 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: e94cb37b34eb8 ("Add a drm_aux-dev module for reading/writing dpcd > registers") > Cc: Lyude Paul <lyude@xxxxxxxxxx> > Signed-off-by: Zwane Mwaikambo <zwane@xxxxxxxxx> > --- > v2 patch re-generated from drm-next branch > 0d2e90f47c4f80a5dcf10f5fe57b5e692e62b48f > > diff --git a/drivers/gpu/drm/drm_dp_aux_dev.c > b/drivers/gpu/drm/drm_dp_aux_dev.c > index 2510717d5a08..e25181bf2c48 100644 > --- 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_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); > > -- Sincerely, Lyude Paul (she/her) Software Engineer at Red Hat Note: I deal with a lot of emails and have a lot of bugs on my plate. If you've asked me a question, are waiting for a review/merge on a patch, etc. and I haven't responded in a while, please feel free to send me another email to check on my status. I don't bite! _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
