net: allow small head cache usage with large MAX_SKB_FRAGS values

In the Linux kernel, the following vulnerability has been resolved:

net: allow small head cache usage with large MAX_SKB_FRAGS values

Sabrina reported the following splat:

WARNING: CPU: 0 PID: 1 at net/core/dev.c:6935 netif_napi_add_weight_locked+0x8f2/0xba0

Modules linked in:

CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.14.0-rc1-net-00092-g011b03359038 #996

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014

RIP: 0010:netif_napi_add_weight_locked+0x8f2/0xba0

Code: e8 c3 e6 6a fe 48 83 c4 28 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc c7 44 24 10 ff ff ff ff e9 8f fb ff ff e8 9e e6 6a fe <0f> 0b e9 d3 fe ff ff e8 92 e6 6a fe 48 8b 04 24 be ff ff ff ff 48

RSP: 0000:ffffc9000001fc60 EFLAGS: 00010293

RAX: 0000000000000000 RBX: ffff88806ce48128 RCX: 1ffff11001664b9e

RDX: ffff888008f00040 RSI: ffffffff8317ca42 RDI: ffff88800b325cb6

RBP: ffff88800b325c40 R08: 0000000000000001 R09: ffffed100167502c

R10: ffff88800b3a8163 R11: 0000000000000000 R12: ffff88800ac1c168

R13: ffff88800ac1c168 R14: ffff88800ac1c168 R15: 0000000000000007

FS: 0000000000000000(0000) GS:ffff88806ce00000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: ffff888008201000 CR3: 0000000004c94001 CR4: 0000000000370ef0

DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000

DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Call Trace:

<TASK>

gro_cells_init+0x1ba/0x270

xfrm_input_init+0x4b/0x2a0

xfrm_init+0x38/0x50

ip_rt_init+0x2d7/0x350

ip_init+0xf/0x20

inet_init+0x406/0x590

do_one_initcall+0x9d/0x2e0

do_initcalls+0x23b/0x280

kernel_init_freeable+0x445/0x490

kernel_init+0x20/0x1d0

ret_from_fork+0x46/0x80

ret_from_fork_asm+0x1a/0x30

</TASK>

irq event stamp: 584330

hardirqs last enabled at (584338): [<ffffffff8168bf87>] __up_console_sem+0x77/0xb0

hardirqs last disabled at (584345): [<ffffffff8168bf6c>] __up_console_sem+0x5c/0xb0

softirqs last enabled at (583242): [<ffffffff833ee96d>] netlink_insert+0x14d/0x470

softirqs last disabled at (583754): [<ffffffff8317c8cd>] netif_napi_add_weight_locked+0x77d/0xba0

on kernel built with MAX_SKB_FRAGS=45, where SKB_WITH_OVERHEAD(1024)

is smaller than GRO_MAX_HEAD.

Such built additionally contains the revert of the single page frag cache

so that napi_get_frags() ends up using the page frag allocator, triggering

the splat.

Note that the underlying issue is independent from the mentioned

revert; address it ensuring that the small head cache will fit either TCP

and GRO allocation and updating napi_alloc_skb() and __netdev_alloc_skb()

to select kmalloc() usage for any allocation fitting such cache.