Subject: Re: [PATCH bpf 1/6] xdp: produce a warning when calculated tailroom is negative

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 03 Feb 2026 14:37:54 +0100

Larysa Zaremba <[email-masked]> writes:

> On Tue, Feb 03, 2026 at 01:38:47PM +0100, Toke Høiland-Jørgensen wrote:
>> Larysa Zaremba <[email-masked]> writes:
>> 
>> > On Tue, Feb 03, 2026 at 01:26:21PM +0100, Toke Høiland-Jørgensen wrote:
>> >> Larysa Zaremba <[email-masked]> writes:
>> >> 
>> >> > Many ethernet drivers report xdp Rx queue frag size as being the same as
>> >> > DMA write size. However, the only user of this field, namely
>> >> > bpf_xdp_frags_increase_tail(), clearly expects a truesize.
>> >> >
>> >> > Such difference leads to unspecific memory corruption issues under certain
>> >> > circumstances, e.g. in ixgbevf maximum DMA write size is 3 KB, so when
>> >> > running xskxceiver's XDP_ADJUST_TAIL_GROW_MULTI_BUFF, 6K packet fully uses
>> >> > all DMA-writable space in 2 buffers. This would be fine, if only
>> >> > rxq->frag_size was properly set to 4K, but value of 3K results in a
>> >> > negative tailroom, because there is a non-zero page offset.
>> >> >
>> >> > We could return -EINVAL and be done with it in such case, but due to
>> >> > tailroom being stored as an unsigned int, it is reported to be somewhere
>> >> > near UINT_MAX, resulting in a tail being grown, even if the requested
>> >> > offset is too much (it is around 2K in the abovementioned test). This later
>> >> > leads to all kinds of unspecific calltraces.
>> >> >
>> >> > [ 7340.337579] xskxceiver[1440]: segfault at 1da718 ip 00007f4161aeac9d sp 00007f41615a6a00 error 6
>> >> > [ 7340.338040] xskxceiver[1441]: segfault at 7f410000000b ip 00000000004042b5 sp 00007f415bffecf0 error 4
>> >> > [ 7340.338179]  in libc.so.6[61c9d,7f4161aaf000+160000]
>> >> > [ 7340.339230]  in xskxceiver[42b5,400000+69000]
>> >> > [ 7340.340300]  likely on CPU 6 (core 0, socket 6)
>> >> > [ 7340.340302] Code: ff ff 01 e9 f4 fe ff ff 0f 1f 44 00 00 4c 39 f0 74 73 31 c0 ba 01 00 00 00 f0 0f b1 17 0f 85 ba 00 00 00 49 8b 87 88 00 00 00 <4c> 89 70 08 eb cc 0f 1f 44 00 00 48 8d bd f0 fe ff ff 89 85 ec fe
>> >> > [ 7340.340888]  likely on CPU 3 (core 0, socket 3)
>> >> > [ 7340.345088] Code: 00 00 00 ba 00 00 00 00 be 00 00 00 00 89 c7 e8 31 ca ff ff 89 45 ec 8b 45 ec 85 c0 78 07 b8 00 00 00 00 eb 46 e8 0b c8 ff ff <8b> 00 83 f8 69 74 24 e8 ff c7 ff ff 8b 00 83 f8 0b 74 18 e8 f3 c7
>> >> > [ 7340.404334] Oops: general protection fault, probably for non-canonical address 0x6d255010bdffc: 0000 [#1] SMP NOPTI
>> >> > [ 7340.405972] CPU: 7 UID: 0 PID: 1439 Comm: xskxceiver Not tainted 6.19.0-rc1+ #21 PREEMPT(lazy)
>> >> > [ 7340.408006] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-5.fc42 04/01/2014
>> >> > [ 7340.409716] RIP: 0010:lookup_swap_cgroup_id+0x44/0x80
>> >> > [ 7340.410455] Code: 83 f8 1c 73 39 48 ba ff ff ff ff ff ff ff 03 48 8b 04 c5 20 55 fa bd 48 21 d1 48 89 ca 83 e1 01 48 d1 ea c1 e1 04 48 8d 04 90 <8b> 00 48 83 c4 10 d3 e8 c3 cc cc cc cc 31 c0 e9 98 b7 dd 00 48 89
>> >> > [ 7340.412787] RSP: 0018:ffffcc5c04f7f6d0 EFLAGS: 00010202
>> >> > [ 7340.413494] RAX: 0006d255010bdffc RBX: ffff891f477895a8 RCX: 0000000000000010
>> >> > [ 7340.414431] RDX: 0001c17e3fffffff RSI: 00fa070000000000 RDI: 000382fc7fffffff
>> >> > [ 7340.415354] RBP: 00fa070000000000 R08: ffffcc5c04f7f8f8 R09: ffffcc5c04f7f7d0
>> >> > [ 7340.416283] R10: ffff891f4c1a7000 R11: ffffcc5c04f7f9c8 R12: ffffcc5c04f7f7d0
>> >> > [ 7340.417218] R13: 03ffffffffffffff R14: 00fa06fffffffe00 R15: ffff891f47789500
>> >> > [ 7340.418229] FS:  0000000000000000(0000) GS:ffff891ffdfaa000(0000) knlGS:0000000000000000
>> >> > [ 7340.419489] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> >> > [ 7340.420286] CR2: 00007f415bfffd58 CR3: 0000000103f03002 CR4: 0000000000772ef0
>> >> > [ 7340.421237] PKRU: 55555554
>> >> > [ 7340.421623] Call Trace:
>> >> > [ 7340.421987]  <TASK>
>> >> > [ 7340.422309]  ? softleaf_from_pte+0x77/0xa0
>> >> > [ 7340.422855]  swap_pte_batch+0xa7/0x290
>> >> > [ 7340.423363]  zap_nonpresent_ptes.constprop.0.isra.0+0xd1/0x270
>> >> > [ 7340.424102]  zap_pte_range+0x281/0x580
>> >> > [ 7340.424607]  zap_pmd_range.isra.0+0xc9/0x240
>> >> > [ 7340.425177]  unmap_page_range+0x24d/0x420
>> >> > [ 7340.425714]  unmap_vmas+0xa1/0x180
>> >> > [ 7340.426185]  exit_mmap+0xe1/0x3b0
>> >> > [ 7340.426644]  __mmput+0x41/0x150
>> >> > [ 7340.427098]  exit_mm+0xb1/0x110
>> >> > [ 7340.427539]  do_exit+0x1b2/0x460
>> >> > [ 7340.427992]  do_group_exit+0x2d/0xc0
>> >> > [ 7340.428477]  get_signal+0x79d/0x7e0
>> >> > [ 7340.428957]  arch_do_signal_or_restart+0x34/0x100
>> >> > [ 7340.429571]  exit_to_user_mode_loop+0x8e/0x4c0
>> >> > [ 7340.430159]  do_syscall_64+0x188/0x6b0
>> >> > [ 7340.430672]  ? __do_sys_clone3+0xd9/0x120
>> >> > [ 7340.431212]  ? switch_fpu_return+0x4e/0xd0
>> >> > [ 7340.431761]  ? arch_exit_to_user_mode_prepare.isra.0+0xa1/0xc0
>> >> > [ 7340.432498]  ? do_syscall_64+0xbb/0x6b0
>> >> > [ 7340.433015]  ? __handle_mm_fault+0x445/0x690
>> >> > [ 7340.433582]  ? count_memcg_events+0xd6/0x210
>> >> > [ 7340.434151]  ? handle_mm_fault+0x212/0x340
>> >> > [ 7340.434697]  ? do_user_addr_fault+0x2b4/0x7b0
>> >> > [ 7340.435271]  ? clear_bhb_loop+0x30/0x80
>> >> > [ 7340.435788]  ? clear_bhb_loop+0x30/0x80
>> >> > [ 7340.436299]  ? clear_bhb_loop+0x30/0x80
>> >> > [ 7340.436812]  ? clear_bhb_loop+0x30/0x80
>> >> > [ 7340.437323]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>> >> > [ 7340.437973] RIP: 0033:0x7f4161b14169
>> >> > [ 7340.438468] Code: Unable to access opcode bytes at 0x7f4161b1413f.
>> >> > [ 7340.439242] RSP: 002b:00007ffc6ebfa770 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
>> >> > [ 7340.440173] RAX: fffffffffffffe00 RBX: 00000000000005a1 RCX: 00007f4161b14169
>> >> > [ 7340.441061] RDX: 00000000000005a1 RSI: 0000000000000109 RDI: 00007f415bfff990
>> >> > [ 7340.441943] RBP: 00007ffc6ebfa7a0 R08: 0000000000000000 R09: 00000000ffffffff
>> >> > [ 7340.442824] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>> >> > [ 7340.443707] R13: 0000000000000000 R14: 00007f415bfff990 R15: 00007f415bfff6c0
>> >> > [ 7340.444586]  </TASK>
>> >> > [ 7340.444922] Modules linked in: rfkill intel_rapl_msr intel_rapl_common intel_uncore_frequency_common skx_edac_common nfit libnvdimm kvm_intel vfat fat kvm snd_pcm irqbypass rapl iTCO_wdt snd_timer intel_pmc_bxt iTCO_vendor_support snd ixgbevf virtio_net soundcore i2c_i801 pcspkr libeth_xdp net_failover i2c_smbus lpc_ich failover libeth virtio_balloon joydev 9p fuse loop zram lz4hc_compress lz4_compress 9pnet_virtio 9pnet netfs ghash_clmulni_intel serio_raw qemu_fw_cfg
>> >> > [ 7340.449650] ---[ end trace 0000000000000000 ]---
>> >> >
>> >> > The issue can be fixed in all in-tree drivers, but we cannot just trust OOT
>> >> > drivers to not do this. Therefore, make tailroom a signed int and produce a
>> >> > warning when it is negative to prevent such mistakes in the future.
>> >> >
>> >> > Fixes: bf25146a5595 ("bpf: add frags support to the bpf_xdp_adjust_tail() API")
>> >> > Reviewed-by: Aleksandr Loktionov <[email-masked]>
>> >> > Signed-off-by: Larysa Zaremba <[email-masked]>
>> >> > ---
>> >> >  net/core/filter.c | 3 ++-
>> >> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >> >
>> >> > diff --git a/net/core/filter.c b/net/core/filter.c
>> >> > index 616e0520a0bb..9715d957e3c5 100644
>> >> > --- a/net/core/filter.c
>> >> > +++ b/net/core/filter.c
>> >> > @@ -4149,12 +4149,13 @@ static int bpf_xdp_frags_increase_tail(struct xdp_buff *xdp, int offset)
>> >> >  	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>> >> >  	skb_frag_t *frag = &sinfo->frags[sinfo->nr_frags - 1];
>> >> >  	struct xdp_rxq_info *rxq = xdp->rxq;
>> >> > -	unsigned int tailroom;
>> >> > +	int tailroom;
>> >> >  
>> >> >  	if (!rxq->frag_size || rxq->frag_size > xdp->frame_sz)
>> >> >  		return -EOPNOTSUPP;
>> >> >  
>> >> >  	tailroom = rxq->frag_size - skb_frag_size(frag) - skb_frag_off(frag);
>> >> > +	WARN_ON_ONCE(tailroom < 0);
>> >> >  	if (unlikely(offset > tailroom))
>> >> >  		return -EINVAL;
>> >> >  
>> >> 
>> >> Why can't we do both? I.e., WARN_ON_ONCE() *and* return -EINVAL?
>> >> 
>> >> -Toke
>> >> 
>> >
>> > It would be redundant, offset is always >= 0 here, so with tailroom now being a 
>> > signed int, offset is always bigger and -EINVAL is returned.
>> 
>> Oh, I see. OK, may be worth calling out; I read this paragraph in your
>> commit message to mean "we don't bother returning EINVAL in this case,
>> we just warn":
>
> Worth changing 'could' to 'are supposed to', if there will be v2.

Yeah, that would be better, thanks!

Other than that,

Reviewed-by: Toke Høiland-Jørgensen <[email-masked]>