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Subject[RFC v3 04/22] bpf: Set register type according to is_valid_access()
Date
This fix a pointer leak when an unprivileged eBPF program read a pointer
value from the context. Even if is_valid_access() returns a pointer
type, the eBPF verifier replace it with UNKNOWN_VALUE. The register
value containing an address is then allowed to leak. Moreover, this
prevented unprivileged eBPF programs to use functions with (legitimate)
pointer arguments.

This bug was not a problem until now because the only unprivileged eBPF
program allowed is of type BPF_PROG_TYPE_SOCKET_FILTER and all the types
from its context are UNKNOWN_VALUE.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Fixes: 969bf05eb3ce ("bpf: direct packet access")
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
---
kernel/bpf/verifier.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c0c4a92dae8c..608cbffb0e86 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -794,10 +794,8 @@ static int check_mem_access(struct verifier_env *env, u32 regno, int off,
}
err = check_ctx_access(env, off, size, t, &reg_type);
if (!err && t == BPF_READ && value_regno >= 0) {
- mark_reg_unknown_value(state->regs, value_regno);
- if (env->allow_ptr_leaks)
- /* note that reg.[id|off|range] == 0 */
- state->regs[value_regno].type = reg_type;
+ /* note that reg.[id|off|range] == 0 */
+ state->regs[value_regno].type = reg_type;
}

} else if (reg->type == FRAME_PTR || reg->type == PTR_TO_STACK) {
--
2.9.3
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