From: Andrey Ignatov rdna@fb.com
commit 107c26a70ca81bfc33657366ad69d02fdc9efc9d upstream.
As discussed in [1] max value of variable offset has to be checked for overflow on stack access otherwise verifier would accept code like this:
0: (b7) r2 = 6 1: (b7) r3 = 28 2: (7a) *(u64 *)(r10 -16) = 0 3: (7a) *(u64 *)(r10 -8) = 0 4: (79) r4 = *(u64 *)(r1 +168) 5: (c5) if r4 s< 0x0 goto pc+4 R1=ctx(id=0,off=0,imm=0) R2=inv6 R3=inv28 R4=inv(id=0,umax_value=9223372036854775807,var_off=(0x0; 0x7fffffffffffffff)) R10=fp0,call_-1 fp-8=mmmmmmmm fp-16=mmmmmmmm 6: (17) r4 -= 16 7: (0f) r4 += r10 8: (b7) r5 = 8 9: (85) call bpf_getsockopt#57 10: (b7) r0 = 0 11: (95) exit
, where R4 obviosly has unbounded max value.
Fix it by checking that reg->smax_value is inside (-BPF_MAX_VAR_OFF; BPF_MAX_VAR_OFF) range.
reg->smax_value is used instead of reg->umax_value because stack pointers are calculated using negative offset from fp. This is opposite to e.g. map access where offset must be non-negative and where umax_value is used.
Also dedicated verbose logs are added for both min and max bound check failures to have diagnostics consistent with variable offset handling in check_map_access().
[1] https://marc.info/?l=linux-netdev&m=155433357510597&w=2
Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers") Reported-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Andrey Ignatov rdna@fb.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Ovidiu Panait ovidiu.panait@windriver.com --- kernel/bpf/verifier.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index d2f3bbff3175..3168f331258e 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1833,16 +1833,28 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno, if (meta && meta->raw_mode) meta = NULL;
+ if (reg->smax_value >= BPF_MAX_VAR_OFF || + reg->smax_value <= -BPF_MAX_VAR_OFF) { + verbose(env, "R%d unbounded indirect variable offset stack access\n", + regno); + return -EACCES; + } min_off = reg->smin_value + reg->off; - max_off = reg->umax_value + reg->off; + max_off = reg->smax_value + reg->off; err = __check_stack_boundary(env, regno, min_off, access_size, zero_size_allowed); - if (err) + if (err) { + verbose(env, "R%d min value is outside of stack bound\n", + regno); return err; + } err = __check_stack_boundary(env, regno, max_off, access_size, zero_size_allowed); - if (err) + if (err) { + verbose(env, "R%d max value is outside of stack bound\n", + regno); return err; + } }
if (meta && meta->raw_mode) {