[PATCH bpf-next 1/5] bpf: Reject indirect var_off stack access in raw mode

[Andrey Ignatov]

It's hard to guarantee that whole memory is marked as initialized on
helper return if uninitialized stack is accessed with variable offset
since specific bounds are unknown to verifier. This may cause
uninitialized stack leaking.

Reject such an access in check_stack_boundary to prevent possible
leaking.

There are no known use-cases for indirect uninitialized stack access
with variable offset so it shouldn't break anything.

Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers")
Reported-by: Daniel Borkmann <dan...@iogearbox.net>
Signed-off-by: Andrey Ignatov <r...@fb.com>
---
 kernel/bpf/verifier.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b7a7a9caa82f..12b84307ffa8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2212,7 +2212,26 @@ static int check_stack_boundary(struct bpf_verifier_env 
*env, int regno,
                                             zero_size_allowed);
                if (err)
                        return err;
+               if (meta && meta->raw_mode) {
+                       meta->access_size = access_size;
+                       meta->regno = regno;
+                       return 0;
+               }
        } else {
+               /* Only initialized buffer on stack is allowed to be accessed
+                * with variable offset. With uninitialized buffer it's hard to
+                * guarantee that whole memory is marked as initialized on
+                * helper return since specific bounds are unknown what may
+                * cause uninitialized stack leaking.
+                */
+               if (meta && meta->raw_mode) {
+                       char tn_buf[48];
+
+                       tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+                       verbose(env, "R%d invalid indirect access to 
uninitialized stack var_off=%s\n",
+                               regno, tn_buf);
+                       return -EACCES;
+               }
                min_off = reg->smin_value + reg->off;
                max_off = reg->umax_value + reg->off;
                err = __check_stack_boundary(env, regno, min_off, access_size,
@@ -2225,12 +2244,6 @@ static int check_stack_boundary(struct bpf_verifier_env 
*env, int regno,
                        return err;
        }
 
-       if (meta && meta->raw_mode) {
-               meta->access_size = access_size;
-               meta->regno = regno;
-               return 0;
-       }
-
        for (i = min_off; i < max_off + access_size; i++) {
                u8 *stype;
 
-- 
2.17.1