Re: [PATCH] drm/crtc: Fix uninit-value bug in drm_mode_setcrtc

[Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>]

Hello,

On 21/07/23 9:44 pm, Ziqi Zhao wrote:
> The connector_set contains uninitialized values when allocated with
> kmalloc_array. However, in the "out" branch, the logic assumes that any
> element in connector_set would be equal to NULL if failed to
> initialize, which causes the bug reported by Syzbot. The fix is to use
> an extra variable to keep track of how many connectors are initialized
> indeed, and use that variable to decrease any refcounts in the "out"
> branch.
> 

This bug is reproducible on 6.7-rc3 on KASAN enabled kernel as wild 
memory access.

[  424.699429] general protection fault, probably for non-canonical 
address 0xfbf7c8b63d84d2a6: 0000 [#1] PREEMPT SMP KASAN PTI
[  424.727952] KASAN: maybe wild-memory-access in range 
[0xdfbe65b1ec269530-0xdfbe65b1ec269537]
[  424.743794] CPU: 3 PID: 9040 Comm: r Not tainted 6.7.0-rc3+ #1
[  424.758855] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), 
BIOS 1.11.0-2.el7 04/01/2014
[  424.774845] RIP: 0010:drm_mode_object_put+0x27/0x50
[  424.782854] Code: 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 55 48 89 fd e8 
ae 92 0b fd 48 8d 7d 18 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 
03 <80> 3c 02 00 75 1a 48 83 7d 18 00 74 0d e8 87 92 0b fd 48 89 ef e8
[  424.816805] RSP: 0018:ffff8881199b7ad0 EFLAGS: 00010a06
[  424.830847] RAX: dffffc0000000000 RBX: ffffed1023336fc3 RCX: 
0000000000000000
[  424.844180] RDX: 1bf7ccb63d84d2a6 RSI: 0000000000000000 RDI: 
dfbe65b1ec269530
[  424.854860] RBP: dfbe65b1ec269518 R08: 0000000000000000 R09: 
0000000000000000
[  424.870833] R10: 0000000000000000 R11: 0000000000000000 R12: 
dfbe65b1ec2694d8
[  424.886846] R13: dffffc0000000000 R14: ffff8881060731c0 R15: 
0000000000000001
[  424.901889] FS:  00007fecfc1ec740(0000) GS:ffff8881f3f80000(0000) 
knlGS:0000000000000000
[  424.910833] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  424.918929] CR2: 0000000000000000 CR3: 0000000117e7c000 CR4: 
00000000000006f0
[  424.936058] Call Trace:
[  424.936058]  <TASK>
[  424.936058]  ? show_regs+0x9b/0xb0
[  424.950853]  ? die_addr+0x55/0xe0
[  424.950853]  ? exc_general_protection+0x1a4/0x320
[  424.965905]  ? asm_exc_general_protection+0x26/0x30
[  424.974878]  ? drm_mode_object_put+0x27/0x50
[  424.982866]  drm_mode_setcrtc+0x7ec/0x1630
[  424.990875]  ? __pfx_drm_mode_setcrtc+0x10/0x10
[  424.998877]  ? ww_mutex_lock+0x9a/0x1c0
[  425.006852]  ? __pfx_ww_mutex_lock+0x10/0x10
[  425.014875]  ? __drm_dev_dbg+0xbd/0x1a0
[  425.014875]  ? __pfx___drm_dev_dbg+0x10/0x10
[  425.030321]  ? drm_lease_owner+0x44/0x60
[  425.030981]  drm_ioctl_kernel+0x2a0/0x500
[  425.040058]  ? __pfx_drm_mode_setcrtc+0x10/0x10
[  425.048128]  ? __pfx_drm_ioctl_kernel+0x10/0x10
[  425.055809]  drm_ioctl+0x58a/0xb60
[  425.062876]  ? __pfx_drm_mode_setcrtc+0x10/0x10
[  425.070875]  ? __pfx_drm_ioctl+0x10/0x10
[  425.078875]  ? __pfx_do_sys_openat2+0x10/0x10
[  425.086875]  ? selinux_file_ioctl+0x184/0x270
[  425.099093]  ? selinux_file_ioctl+0xba/0x270
[  425.102865]  ? __pfx_drm_ioctl+0x10/0x10
[  425.111092]  __x64_sys_ioctl+0x1b1/0x220
[  425.119055]  do_syscall_64+0x45/0x100
[  425.127106]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
[  425.135102] RIP: 0033:0x7fecfb6f8289


After applying this patch, the bug is not reproducible.


Thanks,
Harshit




> Reported-by: syzbot+4fad2e57beb6397ab2fc@syzkaller.appspotmail.com
> Signed-off-by: Ziqi Zhao <astrajoan@yahoo.com>
> ---
>   drivers/gpu/drm/drm_crtc.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index df9bf3c9206e..d718c17ab1e9 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -715,8 +715,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>   	struct drm_mode_set set;
>   	uint32_t __user *set_connectors_ptr;
>   	struct drm_modeset_acquire_ctx ctx;
> -	int ret;
> -	int i;
> +	int ret, i, num_connectors;
>   
>   	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>   		return -EOPNOTSUPP;
> @@ -851,6 +850,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>   			goto out;
>   		}
>   
> +		num_connectors = 0;
>   		for (i = 0; i < crtc_req->count_connectors; i++) {
>   			connector_set[i] = NULL;
>   			set_connectors_ptr = (uint32_t __user *)(unsigned long)crtc_req->set_connectors_ptr;
> @@ -871,6 +871,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>   					connector->name);
>   
>   			connector_set[i] = connector;
> +			num_connectors++;
>   		}
>   	}
>   
> @@ -879,7 +880,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>   	set.y = crtc_req->y;
>   	set.mode = mode;
>   	set.connectors = connector_set;
> -	set.num_connectors = crtc_req->count_connectors;
> +	set.num_connectors = num_connectors;
>   	set.fb = fb;
>   
>   	if (drm_drv_uses_atomic_modeset(dev))
> @@ -892,7 +893,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
>   		drm_framebuffer_put(fb);
>   
>   	if (connector_set) {
> -		for (i = 0; i < crtc_req->count_connectors; i++) {
> +		for (i = 0; i < num_connectors; i++) {
>   			if (connector_set[i])
>   				drm_connector_put(connector_set[i]);
>   		}