bpf: Fix kmemleak warning for percpu hashmap

Overview

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

bpf: Fix kmemleak warning for percpu hashmap

Vlad Poenaru reported the following kmemleak issue:

unreferenced object 0x606fd7c44ac8 (size 32): backtrace (crc 0): pcpu_alloc_noprof+0x730/0xeb0 bpf_map_alloc_percpu+0x69/0xc0 prealloc_init+0x9d/0x1b0 htab_map_alloc+0x363/0x510 map_create+0x215/0x3a0 __sys_bpf+0x16b/0x3e0 __x64_sys_bpf+0x18/0x20 do_syscall_64+0x7b/0x150 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Further investigation shows the reason is due to not 8-byte aligned store of percpu pointer in htab_elem_set_ptr(): *(void __percpu **)(l->key + key_size) = pptr;

Note that the whole htab_elem alignment is 8 (for x86_64). If the key_size is 4, that means pptr is stored in a location which is 4 byte aligned but not 8 byte aligned. In mm/kmemleak.c, scan_block() scans the memory based on 8 byte stride, so it won't detect above pptr, hence reporting the memory leak.

In htab_map_alloc(), we already have

    htab->elem_size = sizeof(struct htab_elem) +
                      round_up(htab->map.key_size, 8);
    if (percpu)
            htab->elem_size += sizeof(void *);
    else
            htab->elem_size += round_up(htab->map.value_size, 8);

So storing pptr with 8-byte alignment won't cause any problem and can fix kmemleak too.

The issue can be reproduced with bpf selftest as well:

1. Enable CONFIG_DEBUG_KMEMLEAK config
2. Add a getchar() before skel destroy in test_hash_map() in prog_tests/for_each.c. The purpose is to keep map available so kmemleak can be detected.
3. run './test_progs -t for_each/hash_map &' and a kmemleak should be reported.

Affected

• Linux >= 824bd0ce6c7c43a9e1e210abf124958e54d88342 < 25014e3dfa6d2fb95c920b9e6b2f0b303baa4533
• Linux >= 824bd0ce6c7c43a9e1e210abf124958e54d88342 < 7758e308aeda1038aba1944f7302d34161b3effe
• Linux >= 824bd0ce6c7c43a9e1e210abf124958e54d88342 < 1f1c29aa1934177349c17e3c32e68ec38a7a56df
• Linux >= 824bd0ce6c7c43a9e1e210abf124958e54d88342 < 11ba7ce076e5903e7bdc1fd1498979c331b3c286
• Linux 4.6

Remediation

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