Re: [PATCH net 2/2] ax25: fix potential reference counting leak in ax25_addr_ax25dev

[Lars Kellogg-Stedman]

On Thu, May 02, 2024 at 10:43:38PM +0800, Duoming Zhou wrote:
> The reference counting of ax25_dev potentially increase more
> than once in ax25_addr_ax25dev(), which will cause memory leak.

With this patch applied, I see a kernel panic as soon as something binds
an ax.25 socket (e.g., starting ax25d):

BUG: kernel NULL pointer dereference, address: 0000000000000098
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] SMP PTI
CPU: 0 PID: 111 Comm: ax25d Not tainted 6.9.0-rc6-ax25+ #69
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-2.fc40 04/01/2014
RIP: 0010:ax25_addr_ax25dev+0x5b/0xb0
Code: 8b 43 08 4c 89 ef 48 8b b0 d0 03 00 00 e8 6d fb ff ff 85 c0 4c 0f 44 e3 48 8b 1b 48 85 db 75 df 4d 85 e4 74 19 b8 01 00 00 00 <f0> 0f c1 04 25 98 00 00 00 85 c0 74 21 8d 50 01 09 c2 78 2b 48 c7
RSP: 0018:ffffc90000463e00 EFLAGS: 00010282
RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff888101a5f728 RDI: ffffc90000463e6a
RBP: ffffc90000463e18 R08: ffffc90000463e68 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888101523e40
R13: ffffc90000463e6a R14: ffff88810111f200 R15: ffff8881004e4e00
FS:  00007fb6a6d93b08(0000) GS:ffff88813bc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000098 CR3: 0000000101bb4000 CR4: 00000000000006b0
Call Trace:
 <TASK>
 ? show_regs.part.0+0x22/0x30
 ? __die+0x5b/0x99
 ? page_fault_oops+0xae/0x220
 ? search_extable+0x2e/0x40
 ? ax25_addr_ax25dev+0x5b/0xb0
 ? kernelmode_fixup_or_oops+0x9f/0x120
 ? __bad_area_nosemaphore+0x15f/0x1a0
 ? bad_area_nosemaphore+0x16/0x20
 ? exc_page_fault+0x2a8/0x6e0
 ? asm_exc_page_fault+0x2b/0x30
 ? ax25_addr_ax25dev+0x5b/0xb0
 ax25_bind+0x14c/0x260
 __sys_bind+0xc0/0xf0
 ? alloc_file_pseudo+0xae/0xe0
 __x64_sys_bind+0x1c/0x30
 x64_sys_call+0xfe3/0x1d00
 do_syscall_64+0x55/0x120
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fb6a6d2cfa4
Code: 00 00 00 ba 01 00 00 00 0f 05 80 e7 08 74 c3 eb b0 48 83 ec 08 89 d2 48 63 ff 45 31 d2 45 31 c0 45 31 c9 b8 31 00 00 00 0f 05 <48> 89 c7 e8 da 3f fe ff 48 83 c4 08 c3 48 83 ec 10 48 89 f0 89 d1
RSP: 002b:00007ffc1a5c7670 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
RAX: ffffffffffffffda RBX: 00007ffc1a5c7799 RCX: 00007fb6a6d2cfa4
RDX: 0000000000000048 RSI: 00007ffc1a5c7750 RDI: 0000000000000005
RBP: 00007fb6a6d94a80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb6a6cebca0
R13: 0000000000000048 R14: 0000561ff9f9e776 R15: 00007ffc1a5c8b00
 </TASK>
CR2: 0000000000000098
---[ end trace 0000000000000000 ]---
RIP: 0010:ax25_addr_ax25dev+0x5b/0xb0
Code: 8b 43 08 4c 89 ef 48 8b b0 d0 03 00 00 e8 6d fb ff ff 85 c0 4c 0f 44 e3 48 8b 1b 48 85 db 75 df 4d 85 e4 74 19 b8 01 00 00 00 <f0> 0f c1 04 25 98 00 00 00 85 c0 74 21 8d 50 01 09 c2 78 2b 48 c7
RSP: 0018:ffffc90000463e00 EFLAGS: 00010282
RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff888101a5f728 RDI: ffffc90000463e6a
RBP: ffffc90000463e18 R08: ffffc90000463e68 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888101523e40
R13: ffffc90000463e6a R14: ffff88810111f200 R15: ffff8881004e4e00
FS:  00007fb6a6d93b08(0000) GS:ffff88813bc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000098 CR3: 0000000101bb4000 CR4: 00000000000006b0
Kernel panic - not syncing: Fatal exception in interrupt
Kernel Offset: disabled
---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

My kernel tree looks like this:

d32d77d2b4a (HEAD -> ham-patches) ax25: fix potential reference counting leak in ax25_addr_ax25dev
742ab0da09d ax25: change kfree in ax25_dev_free to ax25_dev_free
e6357cafe3e ax25: fix reference counting issue of ax25_dev
0106679839f (origin/master, origin/HEAD, master) Merge tag 'regulator-fix-v6.9-rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/broonieer

I don't see the kernel panic if I discard the top patch.

-- 
Lars Kellogg-Stedman <lars@xxxxxxxxxx> | larsks @ {irc,twitter,github}
http://blog.oddbit.com/                | N1LKS