Re: [PATCH] io_uring/msg_ring: Fix NULL pointer dereference in io_msg_send_fd()

[Jens Axboe]

On 10/19/22 10:12 AM, Harshit Mogalapalli wrote:
> Syzkaller produced the below call trace:
> 
>  BUG: KASAN: null-ptr-deref in io_msg_ring+0x3cb/0x9f0
>  Write of size 8 at addr 0000000000000070 by task repro/16399
> 
>  CPU: 0 PID: 16399 Comm: repro Not tainted 6.1.0-rc1 #28
>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7
>  Call Trace:
>   <TASK>
>   dump_stack_lvl+0xcd/0x134
>   ? io_msg_ring+0x3cb/0x9f0
>   kasan_report+0xbc/0xf0
>   ? io_msg_ring+0x3cb/0x9f0
>   kasan_check_range+0x140/0x190
>   io_msg_ring+0x3cb/0x9f0
>   ? io_msg_ring_prep+0x300/0x300
>   io_issue_sqe+0x698/0xca0
>   io_submit_sqes+0x92f/0x1c30
>   __do_sys_io_uring_enter+0xae4/0x24b0
> ....
>  RIP: 0033:0x7f2eaf8f8289
>  RSP: 002b:00007fff40939718 EFLAGS: 00000246 ORIG_RAX: 00000000000001aa
>  RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2eaf8f8289
>  RDX: 0000000000000000 RSI: 0000000000006f71 RDI: 0000000000000004
>  RBP: 00007fff409397a0 R08: 0000000000000000 R09: 0000000000000039
>  R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004006d0
>  R13: 00007fff40939880 R14: 0000000000000000 R15: 0000000000000000
>   </TASK>
>  Kernel panic - not syncing: panic_on_warn set ...
> 
> We don't have a NULL check on file_ptr in io_msg_send_fd() function,
> so when file_ptr is NUL src_file is also NULL and get_file()
> dereferences a NULL pointer and leads to above crash.
> 
> Add a NULL check to fix this issue.
> 
> Fixes: e6130eba8a84 ("io_uring: add support for passing fixed file descriptors")
> Reported-by: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
> Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
> ---
> I am not completely sure whether to place the NULL check on file_ptr
> which i did in this case as file_ptr is NULL, or the masked src_file.
> 
> Similar checks are present in other files, io_uring/filetable.c has NULL
> check before masking and io_uring/cancel.c has NULL check after masking
> with FFS_MASK.

Doesn't really matter when it's done, but we should arguably have a
helper for this to avoid having differences in this pattern.

-- 
Jens Axboe