bpf: Fix kernel address leakage in atomic fetch

The change in commit 37086bfdc737 ("bpf: Propagate stack bounds to registers
in atomics w/ BPF_FETCH") around check_mem_access() handling is buggy since
this would allow for unprivileged users to leak kernel pointers. For example,
an atomic fetch/and with -1 on a stack destination which holds a spilled
pointer will migrate the spilled register type into a scalar, which can then
be exported out of the program (since scalar != pointer) by dumping it into
a map value.

The original implementation of XADD was preventing this situation by using
a double call to check_mem_access() one with BPF_READ and a subsequent one
with BPF_WRITE, in both cases passing -1 as a placeholder value instead of
register as per XADD semantics since it didn't contain a value fetch. The
BPF_READ also included a check in check_stack_read_fixed_off() which rejects
the program if the stack slot is of __is_pointer_value() if dst_regno < 0.
The latter is to distinguish whether we're dealing with a regular stack spill/
fill or some arithmetical operation which is disallowed on non-scalars, see
also 6e7e63cbb023 ("bpf: Forbid XADD on spilled pointers for unprivileged
users") for more context on check_mem_access() and its handling of placeholder
value -1.

One minimally intrusive option to fix the leak is for the BPF_FETCH case to
initially check the BPF_READ case via check_mem_access() with -1 as register,
followed by the actual load case with non-negative load_reg to propagate
stack bounds to registers.

Fixes: 37086bfdc737 ("bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH")
Reported-by: <n4ke4mry@gmail.com>
Acked-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f2f1ed3..53d39db 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4584,13 +4584,19 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
                load_reg = -1;
        }
 
-       /* check whether we can read the memory */
+       /* Check whether we can read the memory, with second call for fetch
+        * case to simulate the register fill.
+        */
        err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
-                              BPF_SIZE(insn->code), BPF_READ, load_reg, true);
+                              BPF_SIZE(insn->code), BPF_READ, -1, true);
+       if (!err && load_reg >= 0)
+               err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
+                                      BPF_SIZE(insn->code), BPF_READ, load_reg,
+                                      true);
        if (err)
                return err;
 
-       /* check whether we can write into the same memory */
+       /* Check whether we can write into the same memory. */
        err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
                               BPF_SIZE(insn->code), BPF_WRITE, -1, true);
        if (err)