UBSAN reports issue in ip_idents_reserve

UBSAN reports issue in ip_idents_reserve

[Jiri Pirko]

Hi.

I'm consistently getting following UBSAN warning on every bootup:

[   47.545820] ================================================================================
[   47.554340] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
[   47.561808] signed integer overflow:
[   47.565420] -2117905507 + -695755206 cannot be represented in type 'int'
[   47.572226] CPU: 0 PID: 389 Comm: ntpd Not tainted 4.8.0-rc6jiri+ #1
[   47.578636] Hardware name: Mellanox Technologies Ltd. Mellanox switch/Mellanox switch, BIOS 4.6.5 05/21/2015
[   47.588586]  ffffffff847bf8c0 00000000987b8f47 ffff8803829af5a8 ffffffff818354e3
[   47.596165]  0000000041b58ab3 ffffffff8277e711 ffffffff81835431 ffff8803829af5d0
[   47.603722]  ffff8803829af580 ffffffffd6879e3a 1ffffffff08f8214 ffffed0070535e6c
[   47.611298] Call Trace:
[   47.613795]  [<ffffffff818354e3>] dump_stack+0xb2/0x10f
[   47.619077]  [<ffffffff81835431>] ? _atomic_dec_and_lock+0xa1/0xa1
[   47.625327]  [<ffffffff818a884f>] ubsan_epilogue+0xd/0x4e
[   47.630811]  [<ffffffff818a9821>] handle_overflow+0x190/0x1de
[   47.636627]  [<ffffffff818a9691>] ? __ubsan_handle_negate_overflow+0x140/0x140
[   47.643914]  [<ffffffff81863130>] ? iov_iter_copy_from_user_atomic+0x6e0/0x6e0
[   47.651219]  [<ffffffff811e6f79>] ? __lock_acquire.isra.17+0xb79/0xe50
[   47.657832]  [<ffffffff81e581f2>] ? ip_generic_getfrag+0xd2/0x190
[   47.664011]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.669827]  [<ffffffff818a987d>] __ubsan_handle_add_overflow+0xe/0x10
[   47.676444]  [<ffffffff81e41d52>] ip_idents_reserve+0xb2/0xe0
[   47.682254]  [<ffffffff81e443e9>] __ip_select_ident+0x159/0x1b0
[   47.688248]  [<ffffffff81e44290>] ? update_or_create_fnhe+0x850/0x850
[   47.694782]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.700624]  [<ffffffff81e5ef40>] __ip_make_skb+0x8a0/0xab0
[   47.706259]  [<ffffffff81e5f3fd>] ip_make_skb+0x17d/0x1d0
[   47.711717]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.717526]  [<ffffffff81e5f280>] ? ip_flush_pending_frames+0x20/0x20
[   47.724032]  [<ffffffff81e46ef0>] ? ip_rt_update_pmtu+0x4f0/0x4f0
[   47.730231]  [<ffffffff81f35291>] ? xfrm_lookup_route+0x21/0xe0
[   47.736216]  [<ffffffff81ec0cdb>] udp_sendmsg+0x9db/0xf60
[   47.741668]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
[   47.747472]  [<ffffffff81ec0300>] ? udp_abort+0x70/0x70
[   47.752763]  [<ffffffff81ede3d8>] inet_sendmsg+0x198/0x220
[   47.758324]  [<ffffffff81ede292>] ? inet_sendmsg+0x52/0x220
[   47.763982]  [<ffffffff81ede240>] ? inet_recvmsg+0x300/0x300
[   47.769728]  [<ffffffff81d6fd25>] sock_sendmsg+0xa5/0xd0
[   47.775100]  [<ffffffff81d72f70>] SYSC_sendto+0x1d0/0x280
[   47.780551]  [<ffffffff81d72da0>] ? SYSC_connect+0x200/0x200
[   47.786283]  [<ffffffff814f66df>] ? poll_select_copy_remaining+0x2af/0x310
[   47.793265]  [<ffffffff814f6430>] ? set_fd_set+0x60/0x60
[   47.798665]  [<ffffffff811ee360>] ? do_raw_spin_trylock+0x90/0x90
[   47.804853]  [<ffffffff814f80e3>] ? SyS_select+0x1a3/0x200
[   47.810399]  [<ffffffff814f7f40>] ? core_sys_select+0x570/0x570
[   47.816415]  [<ffffffff8100467c>] ? exit_to_usermode_loop+0xec/0x110
[   47.822842]  [<ffffffff811e8abd>] ? lockdep_sys_exit+0x2d/0xb0
[   47.828769]  [<ffffffff81004016>] ? lockdep_sys_exit_thunk+0x16/0x30
[   47.835199]  [<ffffffff81d7433e>] SyS_sendto+0xe/0x10
[   47.840321]  [<ffffffff820381f2>] entry_SYSCALL_64_fastpath+0x1a/0xa9
[   47.846826] ================================================================================

Looks like this might be result of following commit:

commit 04ca6973f7c1a0d8537f2d9906a0cf8e69886d75
Author: Eric Dumazet <edumazet@google.com>
Date:   Sat Jul 26 08:58:10 2014 +0200

    ip: make IP identifiers less predictable

Eric, could you please take look at that?

Thanks.

Jiri

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 14:00 +0200, Jiri Pirko wrote:
> Hi.
> 
> I'm consistently getting following UBSAN warning on every bootup:
> 
> [   47.545820] ================================================================================
> [   47.554340] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
> [   47.561808] signed integer overflow:
> [   47.565420] -2117905507 + -695755206 cannot be represented in type 'int'
> [   47.572226] CPU: 0 PID: 389 Comm: ntpd Not tainted 4.8.0-rc6jiri+ #1
> [   47.578636] Hardware name: Mellanox Technologies Ltd. Mellanox switch/Mellanox switch, BIOS 4.6.5 05/21/2015
> [   47.588586]  ffffffff847bf8c0 00000000987b8f47 ffff8803829af5a8 ffffffff818354e3
> [   47.596165]  0000000041b58ab3 ffffffff8277e711 ffffffff81835431 ffff8803829af5d0
> [   47.603722]  ffff8803829af580 ffffffffd6879e3a 1ffffffff08f8214 ffffed0070535e6c
> [   47.611298] Call Trace:
> [   47.613795]  [<ffffffff818354e3>] dump_stack+0xb2/0x10f
> [   47.619077]  [<ffffffff81835431>] ? _atomic_dec_and_lock+0xa1/0xa1
> [   47.625327]  [<ffffffff818a884f>] ubsan_epilogue+0xd/0x4e
> [   47.630811]  [<ffffffff818a9821>] handle_overflow+0x190/0x1de
> [   47.636627]  [<ffffffff818a9691>] ? __ubsan_handle_negate_overflow+0x140/0x140
> [   47.643914]  [<ffffffff81863130>] ? iov_iter_copy_from_user_atomic+0x6e0/0x6e0
> [   47.651219]  [<ffffffff811e6f79>] ? __lock_acquire.isra.17+0xb79/0xe50
> [   47.657832]  [<ffffffff81e581f2>] ? ip_generic_getfrag+0xd2/0x190
> [   47.664011]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
> [   47.669827]  [<ffffffff818a987d>] __ubsan_handle_add_overflow+0xe/0x10
> [   47.676444]  [<ffffffff81e41d52>] ip_idents_reserve+0xb2/0xe0
> [   47.682254]  [<ffffffff81e443e9>] __ip_select_ident+0x159/0x1b0
> [   47.688248]  [<ffffffff81e44290>] ? update_or_create_fnhe+0x850/0x850
> [   47.694782]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
> [   47.700624]  [<ffffffff81e5ef40>] __ip_make_skb+0x8a0/0xab0
> [   47.706259]  [<ffffffff81e5f3fd>] ip_make_skb+0x17d/0x1d0
> [   47.711717]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
> [   47.717526]  [<ffffffff81e5f280>] ? ip_flush_pending_frames+0x20/0x20
> [   47.724032]  [<ffffffff81e46ef0>] ? ip_rt_update_pmtu+0x4f0/0x4f0
> [   47.730231]  [<ffffffff81f35291>] ? xfrm_lookup_route+0x21/0xe0
> [   47.736216]  [<ffffffff81ec0cdb>] udp_sendmsg+0x9db/0xf60
> [   47.741668]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
> [   47.747472]  [<ffffffff81ec0300>] ? udp_abort+0x70/0x70
> [   47.752763]  [<ffffffff81ede3d8>] inet_sendmsg+0x198/0x220
> [   47.758324]  [<ffffffff81ede292>] ? inet_sendmsg+0x52/0x220
> [   47.763982]  [<ffffffff81ede240>] ? inet_recvmsg+0x300/0x300
> [   47.769728]  [<ffffffff81d6fd25>] sock_sendmsg+0xa5/0xd0
> [   47.775100]  [<ffffffff81d72f70>] SYSC_sendto+0x1d0/0x280
> [   47.780551]  [<ffffffff81d72da0>] ? SYSC_connect+0x200/0x200
> [   47.786283]  [<ffffffff814f66df>] ? poll_select_copy_remaining+0x2af/0x310
> [   47.793265]  [<ffffffff814f6430>] ? set_fd_set+0x60/0x60
> [   47.798665]  [<ffffffff811ee360>] ? do_raw_spin_trylock+0x90/0x90
> [   47.804853]  [<ffffffff814f80e3>] ? SyS_select+0x1a3/0x200
> [   47.810399]  [<ffffffff814f7f40>] ? core_sys_select+0x570/0x570
> [   47.816415]  [<ffffffff8100467c>] ? exit_to_usermode_loop+0xec/0x110
> [   47.822842]  [<ffffffff811e8abd>] ? lockdep_sys_exit+0x2d/0xb0
> [   47.828769]  [<ffffffff81004016>] ? lockdep_sys_exit_thunk+0x16/0x30
> [   47.835199]  [<ffffffff81d7433e>] SyS_sendto+0xe/0x10
> [   47.840321]  [<ffffffff820381f2>] entry_SYSCALL_64_fastpath+0x1a/0xa9
> [   47.846826] ================================================================================
> 
> Looks like this might be result of following commit:
> 
> commit 04ca6973f7c1a0d8537f2d9906a0cf8e69886d75
> Author: Eric Dumazet <edumazet@google.com>
> Date:   Sat Jul 26 08:58:10 2014 +0200
> 
>     ip: make IP identifiers less predictable
> 
> Eric, could you please take look at that?

Sure

I do not think we have to worry here.

These is best effort, and unfortunately atomic_t are int.

Adding uatomic_t helpers in the kernel with unsigned int would be a huge
effort, given this would touch all arches.

RE: UBSAN reports issue in ip_idents_reserve

[David Laight]

From: Eric Dumazet
> Sent: 20 September 2016 14:29
...
> > [   47.565420] -2117905507 + -695755206 cannot be represented in type 'int'
...
> I do not think we have to worry here.
>
> These is best effort, and unfortunately atomic_t are int.

Not until we compile on a cpu where int arithmetic doesn't wrap.

While I expect that various other parts of the kernel (and userspace)
wouldn't like such cpu, they do exist.

	David

Re: UBSAN reports issue in ip_idents_reserve

[Jiri Pirko]

Tue, Sep 20, 2016 at 03:28:35PM CEST, eric.dumazet@gmail.com wrote:
>On Tue, 2016-09-20 at 14:00 +0200, Jiri Pirko wrote:
>> Hi.
>> 
>> I'm consistently getting following UBSAN warning on every bootup:
>> 
>> [   47.545820] ================================================================================
>> [   47.554340] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
>> [   47.561808] signed integer overflow:
>> [   47.565420] -2117905507 + -695755206 cannot be represented in type 'int'
>> [   47.572226] CPU: 0 PID: 389 Comm: ntpd Not tainted 4.8.0-rc6jiri+ #1
>> [   47.578636] Hardware name: Mellanox Technologies Ltd. Mellanox switch/Mellanox switch, BIOS 4.6.5 05/21/2015
>> [   47.588586]  ffffffff847bf8c0 00000000987b8f47 ffff8803829af5a8 ffffffff818354e3
>> [   47.596165]  0000000041b58ab3 ffffffff8277e711 ffffffff81835431 ffff8803829af5d0
>> [   47.603722]  ffff8803829af580 ffffffffd6879e3a 1ffffffff08f8214 ffffed0070535e6c
>> [   47.611298] Call Trace:
>> [   47.613795]  [<ffffffff818354e3>] dump_stack+0xb2/0x10f
>> [   47.619077]  [<ffffffff81835431>] ? _atomic_dec_and_lock+0xa1/0xa1
>> [   47.625327]  [<ffffffff818a884f>] ubsan_epilogue+0xd/0x4e
>> [   47.630811]  [<ffffffff818a9821>] handle_overflow+0x190/0x1de
>> [   47.636627]  [<ffffffff818a9691>] ? __ubsan_handle_negate_overflow+0x140/0x140
>> [   47.643914]  [<ffffffff81863130>] ? iov_iter_copy_from_user_atomic+0x6e0/0x6e0
>> [   47.651219]  [<ffffffff811e6f79>] ? __lock_acquire.isra.17+0xb79/0xe50
>> [   47.657832]  [<ffffffff81e581f2>] ? ip_generic_getfrag+0xd2/0x190
>> [   47.664011]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
>> [   47.669827]  [<ffffffff818a987d>] __ubsan_handle_add_overflow+0xe/0x10
>> [   47.676444]  [<ffffffff81e41d52>] ip_idents_reserve+0xb2/0xe0
>> [   47.682254]  [<ffffffff81e443e9>] __ip_select_ident+0x159/0x1b0
>> [   47.688248]  [<ffffffff81e44290>] ? update_or_create_fnhe+0x850/0x850
>> [   47.694782]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
>> [   47.700624]  [<ffffffff81e5ef40>] __ip_make_skb+0x8a0/0xab0
>> [   47.706259]  [<ffffffff81e5f3fd>] ip_make_skb+0x17d/0x1d0
>> [   47.711717]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
>> [   47.717526]  [<ffffffff81e5f280>] ? ip_flush_pending_frames+0x20/0x20
>> [   47.724032]  [<ffffffff81e46ef0>] ? ip_rt_update_pmtu+0x4f0/0x4f0
>> [   47.730231]  [<ffffffff81f35291>] ? xfrm_lookup_route+0x21/0xe0
>> [   47.736216]  [<ffffffff81ec0cdb>] udp_sendmsg+0x9db/0xf60
>> [   47.741668]  [<ffffffff81e58120>] ? ip_setup_cork+0x320/0x320
>> [   47.747472]  [<ffffffff81ec0300>] ? udp_abort+0x70/0x70
>> [   47.752763]  [<ffffffff81ede3d8>] inet_sendmsg+0x198/0x220
>> [   47.758324]  [<ffffffff81ede292>] ? inet_sendmsg+0x52/0x220
>> [   47.763982]  [<ffffffff81ede240>] ? inet_recvmsg+0x300/0x300
>> [   47.769728]  [<ffffffff81d6fd25>] sock_sendmsg+0xa5/0xd0
>> [   47.775100]  [<ffffffff81d72f70>] SYSC_sendto+0x1d0/0x280
>> [   47.780551]  [<ffffffff81d72da0>] ? SYSC_connect+0x200/0x200
>> [   47.786283]  [<ffffffff814f66df>] ? poll_select_copy_remaining+0x2af/0x310
>> [   47.793265]  [<ffffffff814f6430>] ? set_fd_set+0x60/0x60
>> [   47.798665]  [<ffffffff811ee360>] ? do_raw_spin_trylock+0x90/0x90
>> [   47.804853]  [<ffffffff814f80e3>] ? SyS_select+0x1a3/0x200
>> [   47.810399]  [<ffffffff814f7f40>] ? core_sys_select+0x570/0x570
>> [   47.816415]  [<ffffffff8100467c>] ? exit_to_usermode_loop+0xec/0x110
>> [   47.822842]  [<ffffffff811e8abd>] ? lockdep_sys_exit+0x2d/0xb0
>> [   47.828769]  [<ffffffff81004016>] ? lockdep_sys_exit_thunk+0x16/0x30
>> [   47.835199]  [<ffffffff81d7433e>] SyS_sendto+0xe/0x10
>> [   47.840321]  [<ffffffff820381f2>] entry_SYSCALL_64_fastpath+0x1a/0xa9
>> [   47.846826] ================================================================================
>> 
>> Looks like this might be result of following commit:
>> 
>> commit 04ca6973f7c1a0d8537f2d9906a0cf8e69886d75
>> Author: Eric Dumazet <edumazet@google.com>
>> Date:   Sat Jul 26 08:58:10 2014 +0200
>> 
>>     ip: make IP identifiers less predictable
>> 
>> Eric, could you please take look at that?
>
>Sure
>
>I do not think we have to worry here.
>
>These is best effort, and unfortunately atomic_t are int.

I see. So how to silent the warning?

>
>Adding uatomic_t helpers in the kernel with unsigned int would be a huge
>effort, given this would touch all arches.
>
>
>

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 13:36 +0000, David Laight wrote:
> From: Eric Dumazet
> > Sent: 20 September 2016 14:29
> ...
> > > [   47.565420] -2117905507 + -695755206 cannot be represented in type 'int'
> ...
> > I do not think we have to worry here.
> >
> > These is best effort, and unfortunately atomic_t are int.
> 
> Not until we compile on a cpu where int arithmetic doesn't wrap.

Then I guess the guy adding this kind of arches in the kernel will have
to add all the core kernel infra.

If you have an idea, I will happily review a patch.

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 15:39 +0200, Jiri Pirko wrote:

> I see. So how to silent the warning?
> 

We can replace the atomic_add_return() and use a loop around
atomic_read() and atomic_cmpxhg()

This would change the nice property of x86 xadd into a loop.

Or we also could fallback to random generation if the atomic_cmpxchg()
fails.

I'll provide a patch, thanks.

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 07:11 -0700, Eric Dumazet wrote:
> On Tue, 2016-09-20 at 15:39 +0200, Jiri Pirko wrote:
> 
> > I see. So how to silent the warning?
> > 
> 
> We can replace the atomic_add_return() and use a loop around
> atomic_read() and atomic_cmpxhg()
> 
> This would change the nice property of x86 xadd into a loop.
> 
> Or we also could fallback to random generation if the atomic_cmpxchg()
> fails.
> 
> I'll provide a patch, thanks.
> 

Could you try the following ?

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index
b52496fd51075821c39435f50ac62f813967aecc..91dc108ef6dc75df80f0e73b6fa062d98dc9a58a 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -476,12 +476,19 @@ u32 ip_idents_reserve(u32 hash, int segs)
 	atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;
 	u32 old = ACCESS_ONCE(*p_tstamp);
 	u32 now = (u32)jiffies;
-	u32 delta = 0;
+	u32 new, delta = 0;
 
 	if (old != now && cmpxchg(p_tstamp, old, now) == old)
 		delta = prandom_u32_max(now - old);
 
-	return atomic_add_return(segs + delta, p_id) - segs;
+	old = (u32)atomic_read(p_id);
+	new = old + delta + segs;
+	/* Do not try too hard, if multiple cpus are there,
+	 * just fallback to pseudo random number.
+	 */
+	if (unlikely(atomic_cmpxchg(p_id, old, new) != old))
+		new = prandom_u32();
+	return new;
 }
 EXPORT_SYMBOL(ip_idents_reserve);
 

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 07:18 -0700, Eric Dumazet wrote:

> +	 */
> +	if (unlikely(atomic_cmpxchg(p_id, old, new) != old))
> +		new = prandom_u32();
> +	return new;

Looks like we should return new - segs;

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 07:11 -0700, Eric Dumazet wrote:
> On Tue, 2016-09-20 at 15:39 +0200, Jiri Pirko wrote:
> 
> > I see. So how to silent the warning?
> > 
> 
> We can replace the atomic_add_return() and use a loop around
> atomic_read() and atomic_cmpxhg()
> 
> This would change the nice property of x86 xadd into a loop.
> 
> Or we also could fallback to random generation if the atomic_cmpxchg()
> fails.
> 
> I'll provide a patch, thanks.
> 

I looks at other places, I am surprised you do not see other UBSAN
issues in networking :)

netdev_refcnt_read() can potentially gives errors as well.

Re: UBSAN reports issue in ip_idents_reserve

[Jiri Pirko]

Tue, Sep 20, 2016 at 05:25:16PM CEST, eric.dumazet@gmail.com wrote:
>On Tue, 2016-09-20 at 07:11 -0700, Eric Dumazet wrote:
>> On Tue, 2016-09-20 at 15:39 +0200, Jiri Pirko wrote:
>> 
>> > I see. So how to silent the warning?
>> > 
>> 
>> We can replace the atomic_add_return() and use a loop around
>> atomic_read() and atomic_cmpxhg()
>> 
>> This would change the nice property of x86 xadd into a loop.
>> 
>> Or we also could fallback to random generation if the atomic_cmpxchg()
>> fails.
>> 
>> I'll provide a patch, thanks.
>> 

I'm going to test your patch now.

>
>I looks at other places, I am surprised you do not see other UBSAN
>issues in networking :)

Not yet :)

>
>netdev_refcnt_read() can potentially gives errors as well.
>
>
>

Re: UBSAN reports issue in ip_idents_reserve

[Jiri Pirko]

Tue, Sep 20, 2016 at 04:18:12PM CEST, eric.dumazet@gmail.com wrote:
>On Tue, 2016-09-20 at 07:11 -0700, Eric Dumazet wrote:
>> On Tue, 2016-09-20 at 15:39 +0200, Jiri Pirko wrote:
>> 
>> > I see. So how to silent the warning?
>> > 
>> 
>> We can replace the atomic_add_return() and use a loop around
>> atomic_read() and atomic_cmpxhg()
>> 
>> This would change the nice property of x86 xadd into a loop.
>> 
>> Or we also could fallback to random generation if the atomic_cmpxchg()
>> fails.
>> 
>> I'll provide a patch, thanks.
>> 
>
>Could you try the following ?
>
>diff --git a/net/ipv4/route.c b/net/ipv4/route.c
>index
>b52496fd51075821c39435f50ac62f813967aecc..91dc108ef6dc75df80f0e73b6fa062d98dc9a58a 100644
>--- a/net/ipv4/route.c
>+++ b/net/ipv4/route.c
>@@ -476,12 +476,19 @@ u32 ip_idents_reserve(u32 hash, int segs)
> 	atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;
> 	u32 old = ACCESS_ONCE(*p_tstamp);
> 	u32 now = (u32)jiffies;
>-	u32 delta = 0;
>+	u32 new, delta = 0;
> 
> 	if (old != now && cmpxchg(p_tstamp, old, now) == old)
> 		delta = prandom_u32_max(now - old);
> 
>-	return atomic_add_return(segs + delta, p_id) - segs;
>+	old = (u32)atomic_read(p_id);
>+	new = old + delta + segs;
>+	/* Do not try too hard, if multiple cpus are there,
>+	 * just fallback to pseudo random number.
>+	 */
>+	if (unlikely(atomic_cmpxchg(p_id, old, new) != old))
>+		new = prandom_u32();
>+	return new;
> }
> EXPORT_SYMBOL(ip_idents_reserve);
> 

This patch makes ubsan silent.


>
>

Re: UBSAN reports issue in ip_idents_reserve

[Eric Dumazet]

On Tue, 2016-09-20 at 19:46 +0200, Jiri Pirko wrote:

> 
> This patch makes ubsan silent.

Thanks Jiri, I will post an official patch then ;)

[PATCH net] net: get rid of an signed integer overflow in ip_idents_reserve()

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

Jiri Pirko reported an UBSAN warning happening in ip_idents_reserve()

[] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
[] signed integer overflow:
[] -2117905507 + -695755206 cannot be represented in type 'int'

Since we do not have uatomic_add_return() yet, use atomic_cmpxchg()
so that the arithmetics can be done using unsigned int.

Fixes: 04ca6973f7c1 ("ip: make IP identifiers less predictable")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jiri Pirko <jiri@resnulli.us>
---
David, Jiri, I removed the prandom_u32() stuff in favor of a traditional
loop to meet stable requirements. Thanks !

 net/ipv4/route.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index b52496fd51075821c39435f50ac62f813967aecc..654a9af201366887652a4e19a6f1261e5e747056 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -476,12 +476,18 @@ u32 ip_idents_reserve(u32 hash, int segs)
 	atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;
 	u32 old = ACCESS_ONCE(*p_tstamp);
 	u32 now = (u32)jiffies;
-	u32 delta = 0;
+	u32 new, delta = 0;
 
 	if (old != now && cmpxchg(p_tstamp, old, now) == old)
 		delta = prandom_u32_max(now - old);
 
-	return atomic_add_return(segs + delta, p_id) - segs;
+	/* Do not use atomic_add_return() as it makes UBSAN unhappy */
+	do {
+		old = (u32)atomic_read(p_id);
+		new = old + delta + segs;
+	} while (atomic_cmpxchg(p_id, old, new) != old);
+
+	return new - segs;
 }
 EXPORT_SYMBOL(ip_idents_reserve);
 

Re: [PATCH net] net: get rid of an signed integer overflow in ip_idents_reserve()

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 20 Sep 2016 18:06:17 -0700

> From: Eric Dumazet <edumazet@google.com>
> 
> Jiri Pirko reported an UBSAN warning happening in ip_idents_reserve()
> 
> [] UBSAN: Undefined behaviour in ./arch/x86/include/asm/atomic.h:156:11
> [] signed integer overflow:
> [] -2117905507 + -695755206 cannot be represented in type 'int'
> 
> Since we do not have uatomic_add_return() yet, use atomic_cmpxchg()
> so that the arithmetics can be done using unsigned int.
> 
> Fixes: 04ca6973f7c1 ("ip: make IP identifiers less predictable")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Jiri Pirko <jiri@resnulli.us>
> ---
> David, Jiri, I removed the prandom_u32() stuff in favor of a traditional
> loop to meet stable requirements. Thanks !

Applied.