[PATCH 6.6 182/567] xdp: produce a warning when calculated tailroom is negative

Greg Kroah-Hartman
2026-03-23T15:00:47Z

6.6-stable review patch. If anyone has any objections, please let me know.

------------------

From: Larysa Zaremba <larysa.zaremba@intel.com>

[ Upstream commit 8821e857759be9db3cde337ad328b71fe5c8a55f ]

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 are supposed to 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 <aleksandr.loktionov@intel.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://patch.msgid.link/20260305111253.2317394-10-larysa.zaremba@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 58109f6201b76..b1e9abb3891cc 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4132,13 +4132,14 @@ 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) % rxq->frag_size;
+ WARN_ON_ONCE(tailroom < 0);
if (unlikely(offset > tailroom))
return -EINVAL;

--
2.51.0