RE: [PATCH bpf] bpf: Don't use tnum_range on array range checking for poke descriptors

[John Fastabend <john.fastabend@gmail.com>]

Daniel Borkmann wrote:
> Hsin-Wei reported a KASAN splat triggered by their BPF runtime fuzzer which
> is based on a customized syzkaller:
> 
>   BUG: KASAN: slab-out-of-bounds in bpf_int_jit_compile+0x1257/0x13f0
>   Read of size 8 at addr ffff888004e90b58 by task syz-executor.0/1489
>   CPU: 1 PID: 1489 Comm: syz-executor.0 Not tainted 5.19.0 #1
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
>   1.13.0-1ubuntu1.1 04/01/2014
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x9c/0xc9
>    print_address_description.constprop.0+0x1f/0x1f0
>    ? bpf_int_jit_compile+0x1257/0x13f0
>    kasan_report.cold+0xeb/0x197
>    ? kvmalloc_node+0x170/0x200
>    ? bpf_int_jit_compile+0x1257/0x13f0
>    bpf_int_jit_compile+0x1257/0x13f0
>    ? arch_prepare_bpf_dispatcher+0xd0/0xd0
>    ? rcu_read_lock_sched_held+0x43/0x70
>    bpf_prog_select_runtime+0x3e8/0x640
>    ? bpf_obj_name_cpy+0x149/0x1b0
>    bpf_prog_load+0x102f/0x2220
>    ? __bpf_prog_put.constprop.0+0x220/0x220
>    ? find_held_lock+0x2c/0x110
>    ? __might_fault+0xd6/0x180
>    ? lock_downgrade+0x6e0/0x6e0
>    ? lock_is_held_type+0xa6/0x120
>    ? __might_fault+0x147/0x180
>    __sys_bpf+0x137b/0x6070
>    ? bpf_perf_link_attach+0x530/0x530
>    ? new_sync_read+0x600/0x600
>    ? __fget_files+0x255/0x450
>    ? lock_downgrade+0x6e0/0x6e0
>    ? fput+0x30/0x1a0
>    ? ksys_write+0x1a8/0x260
>    __x64_sys_bpf+0x7a/0xc0
>    ? syscall_enter_from_user_mode+0x21/0x70
>    do_syscall_64+0x3b/0x90
>    entry_SYSCALL_64_after_hwframe+0x63/0xcd
>   RIP: 0033:0x7f917c4e2c2d
> 
> The problem here is that a range of tnum_range(0, map->max_entries - 1) has
> limited ability to represent the concrete tight range with the tnum as the
> set of resulting states from value + mask can result in a superset of the
> actual intended range, and as such a tnum_in(range, reg->var_off) check may
> yield true when it shouldn't, for example tnum_range(0, 2) would result in
> 00XX -> v = 0000, m = 0011 such that the intended set of {0, 1, 2} is here
> represented by a less precise superset of {0, 1, 2, 3}. As the register is
> known const scalar, really just use the concrete reg->var_off.value for the
> upper index check.
> 
> Fixes: d2e4c1e6c294 ("bpf: Constant map key tracking for prog array pokes")
> Reported-by: Hsin-Wei Hung <hsinweih@uci.edu>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Shung-Hsi Yu <shung-hsi.yu@suse.com>
> ---
>  kernel/bpf/verifier.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)

LGTM.

Acked-by: John Fastabend <john.fastabend@gmail.com>