bpf, sockmap: Reject max_entries > INT_MAX in sock_map_alloc

Overview

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

bpf, sockmap: Reject max_entries > INT_MAX in sock_map_alloc

sock_map_alloc() only rejects max_entries == 0 and otherwise allows any u32 value. sock_map_free() then walks the sks[] array with a signed int iterator:

int i;
for (i = 0; i < stab->map.max_entries; i++)
	struct sock **psk = &stab->sks[i];

When a SOCKMAP is created with max_entries = 0xffffffff (UINT_MAX), the allocation of 32 GiB can succeed on large-memory hosts. During free the counter reaches 0x80000000, wraps to INT_MIN, is sign-extended by movslq and turned into a ~16 GiB negative offset from stab->sks, pointing far below the allocation.

The faulting access is an xchg() write in sock_map_free(). Without KASAN, the same out-of-bounds write can fault on an unmapped vmalloc page or corrupt an unrelated allocation if that vmalloc address is populated. On a KASAN kernel with CONFIG_KASAN_VMALLOC=y, the shadow check for that address hits an unmapped shadow page and oopses first:

BUG: unable to handle page fault for address: fffff521b59c5a00 RIP: 0010:kasan_check_range+0x107/0x190 Call Trace: sock_map_free+0x93/0x190 map_create+0x68d/0xb30 __sys_bpf+0x21e/0x2e70

Vmcore confirmed stab->map.max_entries == 0xffffffff, stab->sks == 0xffffc911ace2d000, and the faulting address sks + (s64)INT_MIN * 8 exactly at 0xffffc90dace2d000. The same buggy path is reached on the normal close()/bpf_map_free_deferred() path whenever such a map is destroyed.

sock_map_alloc() used to bound its allocation size through bpf_map_charge_init(), but the bound was dropped when rlimit-based memory accounting was removed. Reject max_entries > INT_MAX at creation time so the signed iterator in sock_map_free() never sees a value that would overflow.

Triggered by syzkaller and reproduced on both a 6.6-based KASAN kernel and the upstream v7.3-rc2 kernel.

Affected

• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < f4a73f49e256bd835368504462be3b57b03febff
• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < c56191af3a578b9cc265fa3bd5fc0752c59c1781
• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < 909f761868fda329dc69130ef9eb5b5795bcdf00
• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < cc70972ddbf0e3398f2f32624d31db78594d50d8
• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < cea597519b9ca6d691c98d92460ead8ece2a07af
• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < ba7d627dbc67d62df2fa66d7b41e743c3ace0ec8
• Linux >= 0d2c4f9640501ff57ba0be1f5644a02c29a02fa1 < 814a81c842bd88f6bd8a4ce550d560df071a5d03
• Linux 5.11

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.