• Daniel Borkmann's avatar
    bpf: prevent leaking pointer via xadd on unpriviledged · cd5de9cb
    Daniel Borkmann authored
    commit 6bdf6abc upstream.
    
    Leaking kernel addresses on unpriviledged is generally disallowed,
    for example, verifier rejects the following:
    
      0: (b7) r0 = 0
      1: (18) r2 = 0xffff897e82304400
      3: (7b) *(u64 *)(r1 +48) = r2
      R2 leaks addr into ctx
    
    Doing pointer arithmetic on them is also forbidden, so that they
    don't turn into unknown value and then get leaked out. However,
    there's xadd as a special case, where we don't check the src reg
    for being a pointer register, e.g. the following will pass:
    
      0: (b7) r0 = 0
      1: (7b) *(u64 *)(r1 +48) = r0
      2: (18) r2 = 0xffff897e82304400 ; map
      4: (db) lock *(u64 *)(r1 +48) += r2
      5: (95) exit
    
    We could store the pointer into skb->cb, loose the type context,
    and then read it out from there again to leak it eventually out
    of a map value. Or more easily in a different variant, too:
    
       0: (bf) r6 = r1
       1: (7a) *(u64 *)(r10 -8) = 0
       2: (bf) r2 = r10
       3: (07) r2 += -8
       4: (18) r1 = 0x0
       6: (85) call bpf_map_lookup_elem#1
       7: (15) if r0 == 0x0 goto pc+3
       R0=map_value(ks=8,vs=8,id=0),min_value=0,max_value=0 R6=ctx R10=fp
       8: (b7) r3 = 0
       9: (7b) *(u64 *)(r0 +0) = r3
      10: (db) lock *(u64 *)(r0 +0) += r6
      11: (b7) r0 = 0
      12: (95) exit
    
      from 7 to 11: R0=inv,min_value=0,max_value=0 R6=ctx R10=fp
      11: (b7) r0 = 0
      12: (95) exit
    
    Prevent this by checking xadd src reg for pointer types. Also
    add a couple of test cases related to this.
    
    Fixes: 1be7f75d ("bpf: enable non-root eBPF programs")
    Fixes: 17a52670 ("bpf: verifier (add verifier core)")
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Acked-by: default avatarAlexei Starovoitov <ast@kernel.org>
    Acked-by: default avatarMartin KaFai Lau <kafai@fb.com>
    Acked-by: default avatarEdward Cree <ecree@solarflare.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    cd5de9cb
verifier.c 92.9 KB