linux-kernel - Re: [PATCH] signal: allow the null signal in rt

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Date:   Mon, 7 Jan 2019 20:33:20 -0500
From:   Qian Cai <cai@....pw>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] signal: allow the null signal in rt_sigqueueinfo()



On 1/7/19 6:03 PM, Andrew Morton wrote:
> On Sat,  5 Jan 2019 00:47:29 -0500 Qian Cai <cai@....pw> wrote:
> 
>> Running the trinity fuzzer triggered this,
>>
>> UBSAN: Undefined behaviour in kernel/signal.c:2946:7
>> shift exponent 4294967295 is too large for 64-bit type 'long unsigned
>> int'
>> [ 3752.406618]  dump_stack+0xe0/0x17a
>> [ 3752.419817]  ubsan_epilogue+0xd/0x4e
>> [ 3752.423429]  __ubsan_handle_shift_out_of_bounds+0x1d6/0x227
>> [ 3752.447269]  known_siginfo_layout.cold.9+0x16/0x1b
>> [ 3752.452105]  __copy_siginfo_from_user+0x4b/0x70
>> [ 3752.466620]  do_syscall_64+0x164/0x7ea
>> [ 3752.565030]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> This is because signo is 0 from userspace, and then it ends up calling
>> (1UL << -1) in sig_specific_sicodes(). Since the null signal (0) is
>> allowed in the spec, just deal with it accordingly.
>>
>> ...
>>
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -2943,7 +2943,7 @@ static bool known_siginfo_layout(unsigned sig, int si_code)
>>  	if (si_code == SI_KERNEL)
>>  		return true;
>>  	else if ((si_code > SI_USER)) {
>> -		if (sig_specific_sicodes(sig)) {
>> +		if (sig && sig_specific_sicodes(sig)) {
>>  			if (si_code <= sig_sicodes[sig].limit)
>>  				return true;
>>  		}
> 
> Maybe.
> 
> - What happens if userspace passes in si_code == -1?  

I suppose you meant sig (signo) instead of si_code which is this patch is for.
Sig can never be -1 because it is unsigned int. si_code is an int which is fine
to be -1.

in /include/uapi/asm-generic/siginfo.h,

/*
 * si_code values
 * Digital reserves positive values for kernel-generated signals.
 */
#define SI_USER		0
#define SI_KERNEL	0x80
#define SI_QUEUE	-1
#define SI_TIMER	-2
#define SI_MESGQ	-3
#define SI_ASYNCIO	-4
#define SI_SIGIO	-5
#define SI_TKILL	-6
#define SI_DETHREAD	-7
#define SI_ASYNCNL	-60

> 
> - If we are to check the validity of the userspace-provided input
>   then it would be better to do that up-front, right at the point where
>   the data is copied in from userspace.  That's better than checking it
>   several layers deep in one particular place which hit an issue.
> 

Well, the thing here is that signo 0 is a valid input, so it has to process as
further as possible for error checking if I read it correctly.

in man rt_sigqueueinfo,

"As with kill(2), the null signal (0) can be used to check if the specified
process or thread exists."

Then, in man 2 kill

"If sig is 0, then no signal is sent, but error checking is still performed;
this can be used to check for the existence of a process ID or process group ID."

Later, it will will be dealt with properly in group_send_sig_info()

if (!ret && sig)
	ret = do_send_sig_info(sig, info, p, type);

return ret;

Hence the only problem here is that sig_specific_sicodes(sig) forgot to deal
with sig 0 in the first place.