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Subject[PATCH v2] drm/nouveau/kms: Fix NULL pointer dereference in nouveau_connector_detect_depth
Date
This oops manifests itself on the following hardware:
01:00.0 VGA compatible controller: NVIDIA Corporation G98M [GeForce G 103M] (rev a1)

Oct 09 14:17:46 lp-sasha kernel: BUG: kernel NULL pointer dereference, address: 0000000000000000
Oct 09 14:17:46 lp-sasha kernel: #PF: supervisor read access in kernel mode
Oct 09 14:17:46 lp-sasha kernel: #PF: error_code(0x0000) - not-present page
Oct 09 14:17:46 lp-sasha kernel: PGD 0 P4D 0
Oct 09 14:17:46 lp-sasha kernel: Oops: 0000 [#1] SMP PTI
Oct 09 14:17:46 lp-sasha kernel: CPU: 1 PID: 191 Comm: systemd-udevd Not tainted 5.9.0-rc8-next-20201009 #38
Oct 09 14:17:46 lp-sasha kernel: Hardware name: Hewlett-Packard Compaq Presario CQ61 Notebook PC/306A, BIOS F.03 03/23/2009
Oct 09 14:17:46 lp-sasha kernel: RIP: 0010:nouveau_connector_detect_depth+0x71/0xc0 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: Code: 0a 00 00 48 8b 49 48 c7 87 b8 00 00 00 06 00 00 00 80 b9 4d 0a 00 00 00 75 1e 83 fa 41 75 05 48 85 c0 75 29 8b 81 10 0d 00 00 <39> 06 7c 25 f6 81 14 0d 00 00 02 75 b7 c3 80 b9 0c 0d 00 00 00 75
Oct 09 14:17:46 lp-sasha kernel: RSP: 0018:ffffc9000028f8c0 EFLAGS: 00010297
Oct 09 14:17:46 lp-sasha kernel: RAX: 0000000000014c08 RBX: ffff8880369d4000 RCX: ffff8880369d3000
Oct 09 14:17:46 lp-sasha kernel: RDX: 0000000000000040 RSI: 0000000000000000 RDI: ffff8880369d4000
Oct 09 14:17:46 lp-sasha kernel: RBP: ffff88800601cc00 R08: ffff8880051da298 R09: ffffffff8226201a
Oct 09 14:17:46 lp-sasha kernel: R10: ffff88800469aa80 R11: ffff888004c84ff8 R12: 0000000000000000
Oct 09 14:17:46 lp-sasha kernel: R13: ffff8880051da000 R14: 0000000000002000 R15: 0000000000000003
Oct 09 14:17:46 lp-sasha kernel: FS: 00007fd0192b3440(0000) GS:ffff8880bc900000(0000) knlGS:0000000000000000
Oct 09 14:17:46 lp-sasha kernel: CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Oct 09 14:17:46 lp-sasha kernel: CR2: 0000000000000000 CR3: 0000000004976000 CR4: 00000000000006e0
Oct 09 14:17:46 lp-sasha kernel: Call Trace:
Oct 09 14:17:46 lp-sasha kernel: nouveau_connector_get_modes+0x1e6/0x240 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: ? kfree+0xb9/0x240
Oct 09 14:17:46 lp-sasha kernel: ? drm_connector_list_iter_next+0x7c/0xa0
Oct 09 14:17:46 lp-sasha kernel: drm_helper_probe_single_connector_modes+0x1ba/0x7c0
Oct 09 14:17:46 lp-sasha kernel: drm_client_modeset_probe+0x27e/0x1360
Oct 09 14:17:46 lp-sasha kernel: ? nvif_object_sclass_put+0xc/0x20 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: ? nouveau_cli_init+0x3cc/0x440 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: ? ktime_get_mono_fast_ns+0x49/0xa0
Oct 09 14:17:46 lp-sasha kernel: ? nouveau_drm_open+0x4e/0x180 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: __drm_fb_helper_initial_config_and_unlock+0x3f/0x4a0
Oct 09 14:17:46 lp-sasha kernel: ? drm_file_alloc+0x18f/0x260
Oct 09 14:17:46 lp-sasha kernel: ? mutex_lock+0x9/0x40
Oct 09 14:17:46 lp-sasha kernel: ? drm_client_init+0x110/0x160
Oct 09 14:17:46 lp-sasha kernel: nouveau_fbcon_init+0x14d/0x1c0 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: nouveau_drm_device_init+0x1c0/0x880 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: nouveau_drm_probe+0x11a/0x1e0 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: pci_device_probe+0xcd/0x140
Oct 09 14:17:46 lp-sasha kernel: really_probe+0xd8/0x400
Oct 09 14:17:46 lp-sasha kernel: driver_probe_device+0x4a/0xa0
Oct 09 14:17:46 lp-sasha kernel: device_driver_attach+0x9c/0xc0
Oct 09 14:17:46 lp-sasha kernel: __driver_attach+0x6f/0x100
Oct 09 14:17:46 lp-sasha kernel: ? device_driver_attach+0xc0/0xc0
Oct 09 14:17:46 lp-sasha kernel: bus_for_each_dev+0x75/0xc0
Oct 09 14:17:46 lp-sasha kernel: bus_add_driver+0x106/0x1c0
Oct 09 14:17:46 lp-sasha kernel: driver_register+0x86/0xe0
Oct 09 14:17:46 lp-sasha kernel: ? 0xffffffffa044e000
Oct 09 14:17:46 lp-sasha kernel: do_one_initcall+0x48/0x1e0
Oct 09 14:17:46 lp-sasha kernel: ? _cond_resched+0x11/0x60
Oct 09 14:17:46 lp-sasha kernel: ? kmem_cache_alloc_trace+0x19c/0x1e0
Oct 09 14:17:46 lp-sasha kernel: do_init_module+0x57/0x220
Oct 09 14:17:46 lp-sasha kernel: __do_sys_finit_module+0xa0/0xe0
Oct 09 14:17:46 lp-sasha kernel: do_syscall_64+0x33/0x40
Oct 09 14:17:46 lp-sasha kernel: entry_SYSCALL_64_after_hwframe+0x44/0xa9
Oct 09 14:17:46 lp-sasha kernel: RIP: 0033:0x7fd01a060d5d
Oct 09 14:17:46 lp-sasha kernel: Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d e3 70 0c 00 f7 d8 64 89 01 48
Oct 09 14:17:46 lp-sasha kernel: RSP: 002b:00007ffc8ad38a98 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
Oct 09 14:17:46 lp-sasha kernel: RAX: ffffffffffffffda RBX: 0000563f6e7fd530 RCX: 00007fd01a060d5d
Oct 09 14:17:46 lp-sasha kernel: RDX: 0000000000000000 RSI: 00007fd01a19f95d RDI: 000000000000000f
Oct 09 14:17:46 lp-sasha kernel: RBP: 0000000000020000 R08: 0000000000000000 R09: 0000000000000007
Oct 09 14:17:46 lp-sasha kernel: R10: 000000000000000f R11: 0000000000000246 R12: 00007fd01a19f95d
Oct 09 14:17:46 lp-sasha kernel: R13: 0000000000000000 R14: 0000563f6e7fbc10 R15: 0000563f6e7fd530
Oct 09 14:17:46 lp-sasha kernel: Modules linked in: nouveau(+) ttm xt_string xt_mark xt_LOG vgem v4l2_dv_timings uvcvideo ulpi udf ts_kmp ts_fsm ts_bm snd_aloop sil164 qat_dh895xccvf nf_nat_sip nf_nat_irc nf_nat_ftp nf_nat nf_log_ipv6 nf_log_ipv4 nf_log_common ltc2990 lcd intel_qat input_leds i2c_mux gspca_main videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_common videodev mc drivetemp cuse fuse crc_itu_t coretemp ch7006 ath5k ath algif_hash
Oct 09 14:17:46 lp-sasha kernel: CR2: 0000000000000000
Oct 09 14:17:46 lp-sasha kernel: ---[ end trace 0ddafe218ad30017 ]---
Oct 09 14:17:46 lp-sasha kernel: RIP: 0010:nouveau_connector_detect_depth+0x71/0xc0 [nouveau]
Oct 09 14:17:46 lp-sasha kernel: Code: 0a 00 00 48 8b 49 48 c7 87 b8 00 00 00 06 00 00 00 80 b9 4d 0a 00 00 00 75 1e 83 fa 41 75 05 48 85 c0 75 29 8b 81 10 0d 00 00 <39> 06 7c 25 f6 81 14 0d 00 00 02 75 b7 c3 80 b9 0c 0d 00 00 00 75
Oct 09 14:17:46 lp-sasha kernel: RSP: 0018:ffffc9000028f8c0 EFLAGS: 00010297
Oct 09 14:17:46 lp-sasha kernel: RAX: 0000000000014c08 RBX: ffff8880369d4000 RCX: ffff8880369d3000
Oct 09 14:17:46 lp-sasha kernel: RDX: 0000000000000040 RSI: 0000000000000000 RDI: ffff8880369d4000
Oct 09 14:17:46 lp-sasha kernel: RBP: ffff88800601cc00 R08: ffff8880051da298 R09: ffffffff8226201a
Oct 09 14:17:46 lp-sasha kernel: R10: ffff88800469aa80 R11: ffff888004c84ff8 R12: 0000000000000000
Oct 09 14:17:46 lp-sasha kernel: R13: ffff8880051da000 R14: 0000000000002000 R15: 0000000000000003
Oct 09 14:17:46 lp-sasha kernel: FS: 00007fd0192b3440(0000) GS:ffff8880bc900000(0000) knlGS:0000000000000000
Oct 09 14:17:46 lp-sasha kernel: CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Oct 09 14:17:46 lp-sasha kernel: CR2: 0000000000000000 CR3: 0000000004976000 CR4: 00000000000006e0

The disassembly:
Code: 0a 00 00 48 8b 49 48 c7 87 b8 00 00 00 06 00 00 00 80 b9 4d 0a 00 00 00 75 1e 83 fa 41 75 05 48 85 c0 75 29 8b 81 10 0d 00 00 <39> 06 7c 25 f6 81 14 0d 00 00 02 75 b7 c3 80 b9 0c 0d 00 00 00 75
All code
========
0: 0a 00 or (%rax),%al
2: 00 48 8b add %cl,-0x75(%rax)
5: 49 rex.WB
6: 48 c7 87 b8 00 00 00 movq $0x6,0xb8(%rdi)
d: 06 00 00 00
11: 80 b9 4d 0a 00 00 00 cmpb $0x0,0xa4d(%rcx)
18: 75 1e jne 0x38
1a: 83 fa 41 cmp $0x41,%edx
1d: 75 05 jne 0x24
1f: 48 85 c0 test %rax,%rax
22: 75 29 jne 0x4d
24: 8b 81 10 0d 00 00 mov 0xd10(%rcx),%eax
2a:* 39 06 cmp %eax,(%rsi) <-- trapping instruction
2c: 7c 25 jl 0x53
2e: f6 81 14 0d 00 00 02 testb $0x2,0xd14(%rcx)
35: 75 b7 jne 0xffffffffffffffee
37: c3 retq
38: 80 b9 0c 0d 00 00 00 cmpb $0x0,0xd0c(%rcx)
3f: 75 .byte 0x75

Code starting with the faulting instruction
===========================================
0: 39 06 cmp %eax,(%rsi)
2: 7c 25 jl 0x29
4: f6 81 14 0d 00 00 02 testb $0x2,0xd14(%rcx)
b: 75 b7 jne 0xffffffffffffffc4
d: c3 retq
e: 80 b9 0c 0d 00 00 00 cmpb $0x0,0xd0c(%rcx)
15: 75 .byte 0x75

objdump -SF --disassemble=nouveau_connector_detect_depth
[...]
if (nv_connector->edid &&
c85e1: 83 fa 41 cmp $0x41,%edx
c85e4: 75 05 jne c85eb <nouveau_connector_detect_depth+0x6b> (File Offset: 0xc866b)
c85e6: 48 85 c0 test %rax,%rax
c85e9: 75 29 jne c8614 <nouveau_connector_detect_depth+0x94> (File Offset: 0xc8694)
nv_connector->type == DCB_CONNECTOR_LVDS_SPWG)
duallink = ((u8 *)nv_connector->edid)[121] == 2;
else
duallink = mode->clock >= bios->fp.duallink_transition_clk;

if ((!duallink && (bios->fp.strapless_is_24bit & 1)) ||
c85eb: 8b 81 10 0d 00 00 mov 0xd10(%rcx),%eax
c85f1: 39 06 cmp %eax,(%rsi)
c85f3: 7c 25 jl c861a <nouveau_connector_detect_depth+0x9a> (File Offset: 0xc869a)
( duallink && (bios->fp.strapless_is_24bit & 2)))
c85f5: f6 81 14 0d 00 00 02 testb $0x2,0xd14(%rcx)
c85fc: 75 b7 jne c85b5 <nouveau_connector_detect_depth+0x35> (File Offset: 0xc8635)
connector->display_info.bpc = 8;
[...]

% scripts/faddr2line /lib/modules/5.9.0-rc8-next-20201009/kernel/drivers/gpu/drm/nouveau/nouveau.ko nouveau_connector_detect_depth+0x71/0xc0
nouveau_connector_detect_depth+0x71/0xc0:
nouveau_connector_detect_depth at /home/sasha/linux-next/drivers/gpu/drm/nouveau/nouveau_connector.c:891

It is actually line 889. See the disassembly below.
889 duallink = mode->clock >= bios->fp.duallink_transition_clk;

The NULL pointer being dereferenced is mode.

Git bisect has identified the following commit as bad:
f28e32d3906e drm/nouveau/kms: Don't change EDID when it hasn't actually changed

Here is the chain of events that causes the oops.
On entry to nouveau_connector_detect_lvds, edid is set to NULL. The call
to nouveau_connector_detect sets nv_connector->edid to valid memory,
with status set to connector_status_connected and the flow of execution
branching to the out label.

The subsequent call to nouveau_connector_set_edid erronously clears
nv_connector->edid, via the local edid pointer which remains set to NULL.

Fix this by setting edid to the value of the just acquired
nv_connector->edid and executing the body of nouveau_connector_set_edid
only if nv_connector->edid and edid point to different memory addresses
thus preventing nv_connector->edid from being turned into a dangling
pointer.

Fixes: f28e32d3906e ("drm/nouveau/kms: Don't change EDID when it hasn't actually changed")
Signed-off-by: Alexander Kapshuk <alexander.kapshuk@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
---
v2:
-----
- nouveau_connector_set_edid updated to do the (nv_connector->edid
!= edid) check instead of open coding it in nouveau_connector_detect_lvds
- added Reviewed-by: from Lyude Paul

drivers/gpu/drm/nouveau/nouveau_connector.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_connector.c b/drivers/gpu/drm/nouveau/nouveau_connector.c
index 49dd0cbc332f..5eb322276be7 100644
--- a/drivers/gpu/drm/nouveau/nouveau_connector.c
+++ b/drivers/gpu/drm/nouveau/nouveau_connector.c
@@ -532,11 +532,13 @@ static void
nouveau_connector_set_edid(struct nouveau_connector *nv_connector,
struct edid *edid)
{
- struct edid *old_edid = nv_connector->edid;
+ if (nv_connector->edid != edid) {
+ struct edid *old_edid = nv_connector->edid;

- drm_connector_update_edid_property(&nv_connector->base, edid);
- kfree(old_edid);
- nv_connector->edid = edid;
+ drm_connector_update_edid_property(&nv_connector->base, edid);
+ kfree(old_edid);
+ nv_connector->edid = edid;
+ }
}

static enum drm_connector_status
@@ -669,8 +671,10 @@ nouveau_connector_detect_lvds(struct drm_connector *connector, bool force)
/* Try retrieving EDID via DDC */
if (!drm->vbios.fp_no_ddc) {
status = nouveau_connector_detect(connector, force);
- if (status == connector_status_connected)
+ if (status == connector_status_connected) {
+ edid = nv_connector->edid;
goto out;
+ }
}

/* On some laptops (Sony, i'm looking at you) there appears to
--
2.28.0
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